Merge branch 'for-jeff' of git://electric-eye.fr.zoreil.com/home/romieu/linux-2.6

[sfrench/cifs-2.6.git] / drivers / scsi / scsi_lib.c

/*
 *  scsi_lib.c Copyright (C) 1999 Eric Youngdale
 *
 *  SCSI queueing library.
 *      Initial versions: Eric Youngdale (eric@andante.org).
 *                        Based upon conversations with large numbers
 *                        of people at Linux Expo.
 */

#include <linux/bio.h>
#include <linux/blkdev.h>
#include <linux/completion.h>
#include <linux/kernel.h>
#include <linux/mempool.h>
#include <linux/slab.h>
#include <linux/init.h>
#include <linux/pci.h>
#include <linux/delay.h>

#include <scsi/scsi.h>
#include <scsi/scsi_dbg.h>
#include <scsi/scsi_device.h>
#include <scsi/scsi_driver.h>
#include <scsi/scsi_eh.h>
#include <scsi/scsi_host.h>
#include <scsi/scsi_request.h>

#include "scsi_priv.h"
#include "scsi_logging.h"


#define SG_MEMPOOL_NR           (sizeof(scsi_sg_pools)/sizeof(struct scsi_host_sg_pool))
#define SG_MEMPOOL_SIZE         32

struct scsi_host_sg_pool {
        size_t          size;
        char            *name; 
        kmem_cache_t    *slab;
        mempool_t       *pool;
};

#if (SCSI_MAX_PHYS_SEGMENTS < 32)
#error SCSI_MAX_PHYS_SEGMENTS is too small
#endif

#define SP(x) { x, "sgpool-" #x } 
static struct scsi_host_sg_pool scsi_sg_pools[] = {
        SP(8),
        SP(16),
        SP(32),
#if (SCSI_MAX_PHYS_SEGMENTS > 32)
        SP(64),
#if (SCSI_MAX_PHYS_SEGMENTS > 64)
        SP(128),
#if (SCSI_MAX_PHYS_SEGMENTS > 128)
        SP(256),
#if (SCSI_MAX_PHYS_SEGMENTS > 256)
#error SCSI_MAX_PHYS_SEGMENTS is too large
#endif
#endif
#endif
#endif
};      
#undef SP

static void scsi_run_queue(struct request_queue *q);

/*
 * Function:    scsi_unprep_request()
 *
 * Purpose:     Remove all preparation done for a request, including its
 *              associated scsi_cmnd, so that it can be requeued.
 *
 * Arguments:   req     - request to unprepare
 *
 * Lock status: Assumed that no locks are held upon entry.
 *
 * Returns:     Nothing.
 */
static void scsi_unprep_request(struct request *req)
{
        struct scsi_cmnd *cmd = req->special;

        req->flags &= ~REQ_DONTPREP;
        req->special = (req->flags & REQ_SPECIAL) ? cmd->sc_request : NULL;

        scsi_put_command(cmd);
}

/*
 * Function:    scsi_queue_insert()
 *
 * Purpose:     Insert a command in the midlevel queue.
 *
 * Arguments:   cmd    - command that we are adding to queue.
 *              reason - why we are inserting command to queue.
 *
 * Lock status: Assumed that lock is not held upon entry.
 *
 * Returns:     Nothing.
 *
 * Notes:       We do this for one of two cases.  Either the host is busy
 *              and it cannot accept any more commands for the time being,
 *              or the device returned QUEUE_FULL and can accept no more
 *              commands.
 * Notes:       This could be called either from an interrupt context or a
 *              normal process context.
 */
int scsi_queue_insert(struct scsi_cmnd *cmd, int reason)
{
        struct Scsi_Host *host = cmd->device->host;
        struct scsi_device *device = cmd->device;
        struct request_queue *q = device->request_queue;
        unsigned long flags;

        SCSI_LOG_MLQUEUE(1,
                 printk("Inserting command %p into mlqueue\n", cmd));

        /*
         * Set the appropriate busy bit for the device/host.
         *
         * If the host/device isn't busy, assume that something actually
         * completed, and that we should be able to queue a command now.
         *
         * Note that the prior mid-layer assumption that any host could
         * always queue at least one command is now broken.  The mid-layer
         * will implement a user specifiable stall (see
         * scsi_host.max_host_blocked and scsi_device.max_device_blocked)
         * if a command is requeued with no other commands outstanding
         * either for the device or for the host.
         */
        if (reason == SCSI_MLQUEUE_HOST_BUSY)
                host->host_blocked = host->max_host_blocked;
        else if (reason == SCSI_MLQUEUE_DEVICE_BUSY)
                device->device_blocked = device->max_device_blocked;

        /*
         * Decrement the counters, since these commands are no longer
         * active on the host/device.
         */
        scsi_device_unbusy(device);

        /*
         * Requeue this command.  It will go before all other commands
         * that are already in the queue.
         *
         * NOTE: there is magic here about the way the queue is plugged if
         * we have no outstanding commands.
         * 
         * Although we *don't* plug the queue, we call the request
         * function.  The SCSI request function detects the blocked condition
         * and plugs the queue appropriately.
         */
        spin_lock_irqsave(q->queue_lock, flags);
        blk_requeue_request(q, cmd->request);
        spin_unlock_irqrestore(q->queue_lock, flags);

        scsi_run_queue(q);

        return 0;
}

/*
 * Function:    scsi_do_req
 *
 * Purpose:     Queue a SCSI request
 *
 * Arguments:   sreq      - command descriptor.
 *              cmnd      - actual SCSI command to be performed.
 *              buffer    - data buffer.
 *              bufflen   - size of data buffer.
 *              done      - completion function to be run.
 *              timeout   - how long to let it run before timeout.
 *              retries   - number of retries we allow.
 *
 * Lock status: No locks held upon entry.
 *
 * Returns:     Nothing.
 *
 * Notes:       This function is only used for queueing requests for things
 *              like ioctls and character device requests - this is because
 *              we essentially just inject a request into the queue for the
 *              device.
 *
 *              In order to support the scsi_device_quiesce function, we
 *              now inject requests on the *head* of the device queue
 *              rather than the tail.
 */
void scsi_do_req(struct scsi_request *sreq, const void *cmnd,
                 void *buffer, unsigned bufflen,
                 void (*done)(struct scsi_cmnd *),
                 int timeout, int retries)
{
        /*
         * If the upper level driver is reusing these things, then
         * we should release the low-level block now.  Another one will
         * be allocated later when this request is getting queued.
         */
        __scsi_release_request(sreq);

        /*
         * Our own function scsi_done (which marks the host as not busy,
         * disables the timeout counter, etc) will be called by us or by the
         * scsi_hosts[host].queuecommand() function needs to also call
         * the completion function for the high level driver.
         */
        memcpy(sreq->sr_cmnd, cmnd, sizeof(sreq->sr_cmnd));
        sreq->sr_bufflen = bufflen;
        sreq->sr_buffer = buffer;
        sreq->sr_allowed = retries;
        sreq->sr_done = done;
        sreq->sr_timeout_per_command = timeout;

        if (sreq->sr_cmd_len == 0)
                sreq->sr_cmd_len = COMMAND_SIZE(sreq->sr_cmnd[0]);

        /*
         * head injection *required* here otherwise quiesce won't work
         *
         * Because users of this function are apt to reuse requests with no
         * modification, we have to sanitise the request flags here
         */
        sreq->sr_request->flags &= ~REQ_DONTPREP;
        blk_insert_request(sreq->sr_device->request_queue, sreq->sr_request,
                           1, sreq);
}
EXPORT_SYMBOL(scsi_do_req);

/**
 * scsi_execute - insert request and wait for the result
 * @sdev:       scsi device
 * @cmd:        scsi command
 * @data_direction: data direction
 * @buffer:     data buffer
 * @bufflen:    len of buffer
 * @sense:      optional sense buffer
 * @timeout:    request timeout in seconds
 * @retries:    number of times to retry request
 * @flags:      or into request flags;
 *
 * returns the req->errors value which is the the scsi_cmnd result
 * field.
 **/
int scsi_execute(struct scsi_device *sdev, const unsigned char *cmd,
                 int data_direction, void *buffer, unsigned bufflen,
                 unsigned char *sense, int timeout, int retries, int flags)
{
        struct request *req;
        int write = (data_direction == DMA_TO_DEVICE);
        int ret = DRIVER_ERROR << 24;

        req = blk_get_request(sdev->request_queue, write, __GFP_WAIT);

        if (bufflen &&  blk_rq_map_kern(sdev->request_queue, req,
                                        buffer, bufflen, __GFP_WAIT))
                goto out;

        req->cmd_len = COMMAND_SIZE(cmd[0]);
        memcpy(req->cmd, cmd, req->cmd_len);
        req->sense = sense;
        req->sense_len = 0;
        req->retries = retries;
        req->timeout = timeout;
        req->flags |= flags | REQ_BLOCK_PC | REQ_SPECIAL | REQ_QUIET;

        /*
         * head injection *required* here otherwise quiesce won't work
         */
        blk_execute_rq(req->q, NULL, req, 1);

        ret = req->errors;
 out:
        blk_put_request(req);

        return ret;
}
EXPORT_SYMBOL(scsi_execute);


int scsi_execute_req(struct scsi_device *sdev, const unsigned char *cmd,
                     int data_direction, void *buffer, unsigned bufflen,
                     struct scsi_sense_hdr *sshdr, int timeout, int retries)
{
        char *sense = NULL;
        int result;
        
        if (sshdr) {
                sense = kmalloc(SCSI_SENSE_BUFFERSIZE, GFP_NOIO);
                if (!sense)
                        return DRIVER_ERROR << 24;
                memset(sense, 0, SCSI_SENSE_BUFFERSIZE);
        }
        result = scsi_execute(sdev, cmd, data_direction, buffer, bufflen,
                                  sense, timeout, retries, 0);
        if (sshdr)
                scsi_normalize_sense(sense, SCSI_SENSE_BUFFERSIZE, sshdr);

        kfree(sense);
        return result;
}
EXPORT_SYMBOL(scsi_execute_req);

struct scsi_io_context {
        void *data;
        void (*done)(void *data, char *sense, int result, int resid);
        char sense[SCSI_SENSE_BUFFERSIZE];
};

static kmem_cache_t *scsi_io_context_cache;

static void scsi_end_async(struct request *req, int uptodate)
{
        struct scsi_io_context *sioc = req->end_io_data;

        if (sioc->done)
                sioc->done(sioc->data, sioc->sense, req->errors, req->data_len);

        kmem_cache_free(scsi_io_context_cache, sioc);
        __blk_put_request(req->q, req);
}

static int scsi_merge_bio(struct request *rq, struct bio *bio)
{
        struct request_queue *q = rq->q;

        bio->bi_flags &= ~(1 << BIO_SEG_VALID);
        if (rq_data_dir(rq) == WRITE)
                bio->bi_rw |= (1 << BIO_RW);
        blk_queue_bounce(q, &bio);

        if (!rq->bio)
                blk_rq_bio_prep(q, rq, bio);
        else if (!q->back_merge_fn(q, rq, bio))
                return -EINVAL;
        else {
                rq->biotail->bi_next = bio;
                rq->biotail = bio;
                rq->hard_nr_sectors += bio_sectors(bio);
                rq->nr_sectors = rq->hard_nr_sectors;
        }

        return 0;
}

static int scsi_bi_endio(struct bio *bio, unsigned int bytes_done, int error)
{
        if (bio->bi_size)
                return 1;

        bio_put(bio);
        return 0;
}

/**
 * scsi_req_map_sg - map a scatterlist into a request
 * @rq:         request to fill
 * @sg:         scatterlist
 * @nsegs:      number of elements
 * @bufflen:    len of buffer
 * @gfp:        memory allocation flags
 *
 * scsi_req_map_sg maps a scatterlist into a request so that the
 * request can be sent to the block layer. We do not trust the scatterlist
 * sent to use, as some ULDs use that struct to only organize the pages.
 */
static int scsi_req_map_sg(struct request *rq, struct scatterlist *sgl,
                           int nsegs, unsigned bufflen, gfp_t gfp)
{
        struct request_queue *q = rq->q;
        int nr_pages = (bufflen + PAGE_SIZE - 1) >> PAGE_SHIFT;
        unsigned int data_len = 0, len, bytes, off;
        struct page *page;
        struct bio *bio = NULL;
        int i, err, nr_vecs = 0;

        for (i = 0; i < nsegs; i++) {
                page = sgl[i].page;
                off = sgl[i].offset;
                len = sgl[i].length;
                data_len += len;

                while (len > 0) {
                        bytes = min_t(unsigned int, len, PAGE_SIZE - off);

                        if (!bio) {
                                nr_vecs = min_t(int, BIO_MAX_PAGES, nr_pages);
                                nr_pages -= nr_vecs;

                                bio = bio_alloc(gfp, nr_vecs);
                                if (!bio) {
                                        err = -ENOMEM;
                                        goto free_bios;
                                }
                                bio->bi_end_io = scsi_bi_endio;
                        }

                        if (bio_add_pc_page(q, bio, page, bytes, off) !=
                            bytes) {
                                bio_put(bio);
                                err = -EINVAL;
                                goto free_bios;
                        }

                        if (bio->bi_vcnt >= nr_vecs) {
                                err = scsi_merge_bio(rq, bio);
                                if (err) {
                                        bio_endio(bio, bio->bi_size, 0);
                                        goto free_bios;
                                }
                                bio = NULL;
                        }

                        page++;
                        len -= bytes;
                        off = 0;
                }
        }

        rq->buffer = rq->data = NULL;
        rq->data_len = data_len;
        return 0;

free_bios:
        while ((bio = rq->bio) != NULL) {
                rq->bio = bio->bi_next;
                /*
                 * call endio instead of bio_put incase it was bounced
                 */
                bio_endio(bio, bio->bi_size, 0);
        }

        return err;
}

/**
 * scsi_execute_async - insert request
 * @sdev:       scsi device
 * @cmd:        scsi command
 * @cmd_len:    length of scsi cdb
 * @data_direction: data direction
 * @buffer:     data buffer (this can be a kernel buffer or scatterlist)
 * @bufflen:    len of buffer
 * @use_sg:     if buffer is a scatterlist this is the number of elements
 * @timeout:    request timeout in seconds
 * @retries:    number of times to retry request
 * @flags:      or into request flags
 **/
int scsi_execute_async(struct scsi_device *sdev, const unsigned char *cmd,
                       int cmd_len, int data_direction, void *buffer, unsigned bufflen,
                       int use_sg, int timeout, int retries, void *privdata,
                       void (*done)(void *, char *, int, int), gfp_t gfp)
{
        struct request *req;
        struct scsi_io_context *sioc;
        int err = 0;
        int write = (data_direction == DMA_TO_DEVICE);

        sioc = kmem_cache_alloc(scsi_io_context_cache, gfp);
        if (!sioc)
                return DRIVER_ERROR << 24;
        memset(sioc, 0, sizeof(*sioc));

        req = blk_get_request(sdev->request_queue, write, gfp);
        if (!req)
                goto free_sense;
        req->flags |= REQ_BLOCK_PC | REQ_QUIET;

        if (use_sg)
                err = scsi_req_map_sg(req, buffer, use_sg, bufflen, gfp);
        else if (bufflen)
                err = blk_rq_map_kern(req->q, req, buffer, bufflen, gfp);

        if (err)
                goto free_req;

        req->cmd_len = cmd_len;
        memcpy(req->cmd, cmd, req->cmd_len);
        req->sense = sioc->sense;
        req->sense_len = 0;
        req->timeout = timeout;
        req->retries = retries;
        req->end_io_data = sioc;

        sioc->data = privdata;
        sioc->done = done;

        blk_execute_rq_nowait(req->q, NULL, req, 1, scsi_end_async);
        return 0;

free_req:
        blk_put_request(req);
free_sense:
        kfree(sioc);
        return DRIVER_ERROR << 24;
}
EXPORT_SYMBOL_GPL(scsi_execute_async);

/*
 * Function:    scsi_init_cmd_errh()
 *
 * Purpose:     Initialize cmd fields related to error handling.
 *
 * Arguments:   cmd     - command that is ready to be queued.
 *
 * Returns:     Nothing
 *
 * Notes:       This function has the job of initializing a number of
 *              fields related to error handling.   Typically this will
 *              be called once for each command, as required.
 */
static int scsi_init_cmd_errh(struct scsi_cmnd *cmd)
{
        cmd->serial_number = 0;

        memset(cmd->sense_buffer, 0, sizeof cmd->sense_buffer);

        if (cmd->cmd_len == 0)
                cmd->cmd_len = COMMAND_SIZE(cmd->cmnd[0]);

        /*
         * We need saved copies of a number of fields - this is because
         * error handling may need to overwrite these with different values
         * to run different commands, and once error handling is complete,
         * we will need to restore these values prior to running the actual
         * command.
         */
        cmd->old_use_sg = cmd->use_sg;
        cmd->old_cmd_len = cmd->cmd_len;
        cmd->sc_old_data_direction = cmd->sc_data_direction;
        cmd->old_underflow = cmd->underflow;
        memcpy(cmd->data_cmnd, cmd->cmnd, sizeof(cmd->cmnd));
        cmd->buffer = cmd->request_buffer;
        cmd->bufflen = cmd->request_bufflen;

        return 1;
}

/*
 * Function:   scsi_setup_cmd_retry()
 *
 * Purpose:    Restore the command state for a retry
 *
 * Arguments:  cmd      - command to be restored
 *
 * Returns:    Nothing
 *
 * Notes:      Immediately prior to retrying a command, we need
 *             to restore certain fields that we saved above.
 */
void scsi_setup_cmd_retry(struct scsi_cmnd *cmd)
{
        memcpy(cmd->cmnd, cmd->data_cmnd, sizeof(cmd->data_cmnd));
        cmd->request_buffer = cmd->buffer;
        cmd->request_bufflen = cmd->bufflen;
        cmd->use_sg = cmd->old_use_sg;
        cmd->cmd_len = cmd->old_cmd_len;
        cmd->sc_data_direction = cmd->sc_old_data_direction;
        cmd->underflow = cmd->old_underflow;
}

void scsi_device_unbusy(struct scsi_device *sdev)
{
        struct Scsi_Host *shost = sdev->host;
        unsigned long flags;

        spin_lock_irqsave(shost->host_lock, flags);
        shost->host_busy--;
        if (unlikely(scsi_host_in_recovery(shost) &&
                     shost->host_failed))
                scsi_eh_wakeup(shost);
        spin_unlock(shost->host_lock);
        spin_lock(sdev->request_queue->queue_lock);
        sdev->device_busy--;
        spin_unlock_irqrestore(sdev->request_queue->queue_lock, flags);
}

/*
 * Called for single_lun devices on IO completion. Clear starget_sdev_user,
 * and call blk_run_queue for all the scsi_devices on the target -
 * including current_sdev first.
 *
 * Called with *no* scsi locks held.
 */
static void scsi_single_lun_run(struct scsi_device *current_sdev)
{
        struct Scsi_Host *shost = current_sdev->host;
        struct scsi_device *sdev, *tmp;
        struct scsi_target *starget = scsi_target(current_sdev);
        unsigned long flags;

        spin_lock_irqsave(shost->host_lock, flags);
        starget->starget_sdev_user = NULL;
        spin_unlock_irqrestore(shost->host_lock, flags);

        /*
         * Call blk_run_queue for all LUNs on the target, starting with
         * current_sdev. We race with others (to set starget_sdev_user),
         * but in most cases, we will be first. Ideally, each LU on the
         * target would get some limited time or requests on the target.
         */
        blk_run_queue(current_sdev->request_queue);

        spin_lock_irqsave(shost->host_lock, flags);
        if (starget->starget_sdev_user)
                goto out;
        list_for_each_entry_safe(sdev, tmp, &starget->devices,
                        same_target_siblings) {
                if (sdev == current_sdev)
                        continue;
                if (scsi_device_get(sdev))
                        continue;

                spin_unlock_irqrestore(shost->host_lock, flags);
                blk_run_queue(sdev->request_queue);
                spin_lock_irqsave(shost->host_lock, flags);
        
                scsi_device_put(sdev);
        }
 out:
        spin_unlock_irqrestore(shost->host_lock, flags);
}

/*
 * Function:    scsi_run_queue()
 *
 * Purpose:     Select a proper request queue to serve next
 *
 * Arguments:   q       - last request's queue
 *
 * Returns:     Nothing
 *
 * Notes:       The previous command was completely finished, start
 *              a new one if possible.
 */
static void scsi_run_queue(struct request_queue *q)
{
        struct scsi_device *sdev = q->queuedata;
        struct Scsi_Host *shost = sdev->host;
        unsigned long flags;

        if (sdev->single_lun)
                scsi_single_lun_run(sdev);

        spin_lock_irqsave(shost->host_lock, flags);
        while (!list_empty(&shost->starved_list) &&
               !shost->host_blocked && !shost->host_self_blocked &&
                !((shost->can_queue > 0) &&
                  (shost->host_busy >= shost->can_queue))) {
                /*
                 * As long as shost is accepting commands and we have
                 * starved queues, call blk_run_queue. scsi_request_fn
                 * drops the queue_lock and can add us back to the
                 * starved_list.
                 *
                 * host_lock protects the starved_list and starved_entry.
                 * scsi_request_fn must get the host_lock before checking
                 * or modifying starved_list or starved_entry.
                 */
                sdev = list_entry(shost->starved_list.next,
                                          struct scsi_device, starved_entry);
                list_del_init(&sdev->starved_entry);
                spin_unlock_irqrestore(shost->host_lock, flags);

                blk_run_queue(sdev->request_queue);

                spin_lock_irqsave(shost->host_lock, flags);
                if (unlikely(!list_empty(&sdev->starved_entry)))
                        /*
                         * sdev lost a race, and was put back on the
                         * starved list. This is unlikely but without this
                         * in theory we could loop forever.
                         */
                        break;
        }
        spin_unlock_irqrestore(shost->host_lock, flags);

        blk_run_queue(q);
}

/*
 * Function:    scsi_requeue_command()
 *
 * Purpose:     Handle post-processing of completed commands.
 *
 * Arguments:   q       - queue to operate on
 *              cmd     - command that may need to be requeued.
 *
 * Returns:     Nothing
 *
 * Notes:       After command completion, there may be blocks left
 *              over which weren't finished by the previous command
 *              this can be for a number of reasons - the main one is
 *              I/O errors in the middle of the request, in which case
 *              we need to request the blocks that come after the bad
 *              sector.
 * Notes:       Upon return, cmd is a stale pointer.
 */
static void scsi_requeue_command(struct request_queue *q, struct scsi_cmnd *cmd)
{
        struct request *req = cmd->request;
        unsigned long flags;

        scsi_unprep_request(req);
        spin_lock_irqsave(q->queue_lock, flags);
        blk_requeue_request(q, req);
        spin_unlock_irqrestore(q->queue_lock, flags);

        scsi_run_queue(q);
}

void scsi_next_command(struct scsi_cmnd *cmd)
{
        struct scsi_device *sdev = cmd->device;
        struct request_queue *q = sdev->request_queue;

        /* need to hold a reference on the device before we let go of the cmd */
        get_device(&sdev->sdev_gendev);

        scsi_put_command(cmd);
        scsi_run_queue(q);

        /* ok to remove device now */
        put_device(&sdev->sdev_gendev);
}

void scsi_run_host_queues(struct Scsi_Host *shost)
{
        struct scsi_device *sdev;

        shost_for_each_device(sdev, shost)
                scsi_run_queue(sdev->request_queue);
}

/*
 * Function:    scsi_end_request()
 *
 * Purpose:     Post-processing of completed commands (usually invoked at end
 *              of upper level post-processing and scsi_io_completion).
 *
 * Arguments:   cmd      - command that is complete.
 *              uptodate - 1 if I/O indicates success, <= 0 for I/O error.
 *              bytes    - number of bytes of completed I/O
 *              requeue  - indicates whether we should requeue leftovers.
 *
 * Lock status: Assumed that lock is not held upon entry.
 *
 * Returns:     cmd if requeue required, NULL otherwise.
 *
 * Notes:       This is called for block device requests in order to
 *              mark some number of sectors as complete.
 * 
 *              We are guaranteeing that the request queue will be goosed
 *              at some point during this call.
 * Notes:       If cmd was requeued, upon return it will be a stale pointer.
 */
static struct scsi_cmnd *scsi_end_request(struct scsi_cmnd *cmd, int uptodate,
                                          int bytes, int requeue)
{
        request_queue_t *q = cmd->device->request_queue;
        struct request *req = cmd->request;
        unsigned long flags;

        /*
         * If there are blocks left over at the end, set up the command
         * to queue the remainder of them.
         */
        if (end_that_request_chunk(req, uptodate, bytes)) {
                int leftover = (req->hard_nr_sectors << 9);

                if (blk_pc_request(req))
                        leftover = req->data_len;

                /* kill remainder if no retrys */
                if (!uptodate && blk_noretry_request(req))
                        end_that_request_chunk(req, 0, leftover);
                else {
                        if (requeue) {
                                /*
                                 * Bleah.  Leftovers again.  Stick the
                                 * leftovers in the front of the
                                 * queue, and goose the queue again.
                                 */
                                scsi_requeue_command(q, cmd);
                                cmd = NULL;
                        }
                        return cmd;
                }
        }

        add_disk_randomness(req->rq_disk);

        spin_lock_irqsave(q->queue_lock, flags);
        if (blk_rq_tagged(req))
                blk_queue_end_tag(q, req);
        end_that_request_last(req, uptodate);
        spin_unlock_irqrestore(q->queue_lock, flags);

        /*
         * This will goose the queue request function at the end, so we don't
         * need to worry about launching another command.
         */
        scsi_next_command(cmd);
        return NULL;
}

static struct scatterlist *scsi_alloc_sgtable(struct scsi_cmnd *cmd, gfp_t gfp_mask)
{
        struct scsi_host_sg_pool *sgp;
        struct scatterlist *sgl;

        BUG_ON(!cmd->use_sg);

        switch (cmd->use_sg) {
        case 1 ... 8:
                cmd->sglist_len = 0;
                break;
        case 9 ... 16:
                cmd->sglist_len = 1;
                break;
        case 17 ... 32:
                cmd->sglist_len = 2;
                break;
#if (SCSI_MAX_PHYS_SEGMENTS > 32)
        case 33 ... 64:
                cmd->sglist_len = 3;
                break;
#if (SCSI_MAX_PHYS_SEGMENTS > 64)
        case 65 ... 128:
                cmd->sglist_len = 4;
                break;
#if (SCSI_MAX_PHYS_SEGMENTS  > 128)
        case 129 ... 256:
                cmd->sglist_len = 5;
                break;
#endif
#endif
#endif
        default:
                return NULL;
        }

        sgp = scsi_sg_pools + cmd->sglist_len;
        sgl = mempool_alloc(sgp->pool, gfp_mask);
        return sgl;
}

static void scsi_free_sgtable(struct scatterlist *sgl, int index)
{
        struct scsi_host_sg_pool *sgp;

        BUG_ON(index >= SG_MEMPOOL_NR);

        sgp = scsi_sg_pools + index;
        mempool_free(sgl, sgp->pool);
}

/*
 * Function:    scsi_release_buffers()
 *
 * Purpose:     Completion processing for block device I/O requests.
 *
 * Arguments:   cmd     - command that we are bailing.
 *
 * Lock status: Assumed that no lock is held upon entry.
 *
 * Returns:     Nothing
 *
 * Notes:       In the event that an upper level driver rejects a
 *              command, we must release resources allocated during
 *              the __init_io() function.  Primarily this would involve
 *              the scatter-gather table, and potentially any bounce
 *              buffers.
 */
static void scsi_release_buffers(struct scsi_cmnd *cmd)
{
        struct request *req = cmd->request;

        /*
         * Free up any indirection buffers we allocated for DMA purposes. 
         */
        if (cmd->use_sg)
                scsi_free_sgtable(cmd->request_buffer, cmd->sglist_len);
        else if (cmd->request_buffer != req->buffer)
                kfree(cmd->request_buffer);

        /*
         * Zero these out.  They now point to freed memory, and it is
         * dangerous to hang onto the pointers.
         */
        cmd->buffer  = NULL;
        cmd->bufflen = 0;
        cmd->request_buffer = NULL;
        cmd->request_bufflen = 0;
}

/*
 * Function:    scsi_io_completion()
 *
 * Purpose:     Completion processing for block device I/O requests.
 *
 * Arguments:   cmd   - command that is finished.
 *
 * Lock status: Assumed that no lock is held upon entry.
 *
 * Returns:     Nothing
 *
 * Notes:       This function is matched in terms of capabilities to
 *              the function that created the scatter-gather list.
 *              In other words, if there are no bounce buffers
 *              (the normal case for most drivers), we don't need
 *              the logic to deal with cleaning up afterwards.
 *
 *              We must do one of several things here:
 *
 *              a) Call scsi_end_request.  This will finish off the
 *                 specified number of sectors.  If we are done, the
 *                 command block will be released, and the queue
 *                 function will be goosed.  If we are not done, then
 *                 scsi_end_request will directly goose the queue.
 *
 *              b) We can just use scsi_requeue_command() here.  This would
 *                 be used if we just wanted to retry, for example.
 */
void scsi_io_completion(struct scsi_cmnd *cmd, unsigned int good_bytes,
                        unsigned int block_bytes)
{
        int result = cmd->result;
        int this_count = cmd->bufflen;
        request_queue_t *q = cmd->device->request_queue;
        struct request *req = cmd->request;
        int clear_errors = 1;
        struct scsi_sense_hdr sshdr;
        int sense_valid = 0;
        int sense_deferred = 0;

        /*
         * Free up any indirection buffers we allocated for DMA purposes. 
         * For the case of a READ, we need to copy the data out of the
         * bounce buffer and into the real buffer.
         */
        if (cmd->use_sg)
                scsi_free_sgtable(cmd->buffer, cmd->sglist_len);
        else if (cmd->buffer != req->buffer) {
                if (rq_data_dir(req) == READ) {
                        unsigned long flags;
                        char *to = bio_kmap_irq(req->bio, &flags);
                        memcpy(to, cmd->buffer, cmd->bufflen);
                        bio_kunmap_irq(to, &flags);
                }
                kfree(cmd->buffer);
        }

        if (result) {
                sense_valid = scsi_command_normalize_sense(cmd, &sshdr);
                if (sense_valid)
                        sense_deferred = scsi_sense_is_deferred(&sshdr);
        }
        if (blk_pc_request(req)) { /* SG_IO ioctl from block level */
                req->errors = result;
                if (result) {
                        clear_errors = 0;
                        if (sense_valid && req->sense) {
                                /*
                                 * SG_IO wants current and deferred errors
                                 */
                                int len = 8 + cmd->sense_buffer[7];

                                if (len > SCSI_SENSE_BUFFERSIZE)
                                        len = SCSI_SENSE_BUFFERSIZE;
                                memcpy(req->sense, cmd->sense_buffer,  len);
                                req->sense_len = len;
                        }
                } else
                        req->data_len = cmd->resid;
        }

        /*
         * Zero these out.  They now point to freed memory, and it is
         * dangerous to hang onto the pointers.
         */
        cmd->buffer  = NULL;
        cmd->bufflen = 0;
        cmd->request_buffer = NULL;
        cmd->request_bufflen = 0;

        /*
         * Next deal with any sectors which we were able to correctly
         * handle.
         */
        if (good_bytes >= 0) {
                SCSI_LOG_HLCOMPLETE(1, printk("%ld sectors total, %d bytes done.\n",
                                              req->nr_sectors, good_bytes));
                SCSI_LOG_HLCOMPLETE(1, printk("use_sg is %d\n", cmd->use_sg));

                if (clear_errors)
                        req->errors = 0;
                /*
                 * If multiple sectors are requested in one buffer, then
                 * they will have been finished off by the first command.
                 * If not, then we have a multi-buffer command.
                 *
                 * If block_bytes != 0, it means we had a medium error
                 * of some sort, and that we want to mark some number of
                 * sectors as not uptodate.  Thus we want to inhibit
                 * requeueing right here - we will requeue down below
                 * when we handle the bad sectors.
                 */

                /*
                 * If the command completed without error, then either
                 * finish off the rest of the command, or start a new one.
                 */
                if (scsi_end_request(cmd, 1, good_bytes, result == 0) == NULL)
                        return;
        }
        /*
         * Now, if we were good little boys and girls, Santa left us a request
         * sense buffer.  We can extract information from this, so we
         * can choose a block to remap, etc.
         */
        if (sense_valid && !sense_deferred) {
                switch (sshdr.sense_key) {
                case UNIT_ATTENTION:
                        if (cmd->device->removable) {
                                /* detected disc change.  set a bit 
                                 * and quietly refuse further access.
                                 */
                                cmd->device->changed = 1;
                                scsi_end_request(cmd, 0,
                                                this_count, 1);
                                return;
                        } else {
                                /*
                                * Must have been a power glitch, or a
                                * bus reset.  Could not have been a
                                * media change, so we just retry the
                                * request and see what happens.  
                                */
                                scsi_requeue_command(q, cmd);
                                return;
                        }
                        break;
                case ILLEGAL_REQUEST:
                        /*
                        * If we had an ILLEGAL REQUEST returned, then we may
                        * have performed an unsupported command.  The only
                        * thing this should be would be a ten byte read where
                        * only a six byte read was supported.  Also, on a
                        * system where READ CAPACITY failed, we may have read
                        * past the end of the disk.
                        */
                        if ((cmd->device->use_10_for_rw &&
                            sshdr.asc == 0x20 && sshdr.ascq == 0x00) &&
                            (cmd->cmnd[0] == READ_10 ||
                             cmd->cmnd[0] == WRITE_10)) {
                                cmd->device->use_10_for_rw = 0;
                                /*
                                 * This will cause a retry with a 6-byte
                                 * command.
                                 */
                                scsi_requeue_command(q, cmd);
                                result = 0;
                        } else {
                                scsi_end_request(cmd, 0, this_count, 1);
                                return;
                        }
                        break;
                case NOT_READY:
                        /*
                         * If the device is in the process of becoming ready,
                         * retry.
                         */
                        if (sshdr.asc == 0x04 && sshdr.ascq == 0x01) {
                                scsi_requeue_command(q, cmd);
                                return;
                        }
                        if (!(req->flags & REQ_QUIET))
                                scmd_printk(KERN_INFO, cmd,
                                           "Device not ready.\n");
                        scsi_end_request(cmd, 0, this_count, 1);
                        return;
                case VOLUME_OVERFLOW:
                        if (!(req->flags & REQ_QUIET)) {
                                scmd_printk(KERN_INFO, cmd,
                                           "Volume overflow, CDB: ");
                                __scsi_print_command(cmd->data_cmnd);
                                scsi_print_sense("", cmd);
                        }
                        scsi_end_request(cmd, 0, block_bytes, 1);
                        return;
                default:
                        break;
                }
        }                       /* driver byte != 0 */
        if (host_byte(result) == DID_RESET) {
                /*
                 * Third party bus reset or reset for error
                 * recovery reasons.  Just retry the request
                 * and see what happens.  
                 */
                scsi_requeue_command(q, cmd);
                return;
        }
        if (result) {
                if (!(req->flags & REQ_QUIET)) {
                        scmd_printk(KERN_INFO, cmd,
                                   "SCSI error: return code = 0x%x\n", result);

                        if (driver_byte(result) & DRIVER_SENSE)
                                scsi_print_sense("", cmd);
                }
                /*
                 * Mark a single buffer as not uptodate.  Queue the remainder.
                 * We sometimes get this cruft in the event that a medium error
                 * isn't properly reported.
                 */
                block_bytes = req->hard_cur_sectors << 9;
                if (!block_bytes)
                        block_bytes = req->data_len;
                scsi_end_request(cmd, 0, block_bytes, 1);
        }
}
EXPORT_SYMBOL(scsi_io_completion);

/*
 * Function:    scsi_init_io()
 *
 * Purpose:     SCSI I/O initialize function.
 *
 * Arguments:   cmd   - Command descriptor we wish to initialize
 *
 * Returns:     0 on success
 *              BLKPREP_DEFER if the failure is retryable
 *              BLKPREP_KILL if the failure is fatal
 */
static int scsi_init_io(struct scsi_cmnd *cmd)
{
        struct request     *req = cmd->request;
        struct scatterlist *sgpnt;
        int                count;

        /*
         * if this is a rq->data based REQ_BLOCK_PC, setup for a non-sg xfer
         */
        if ((req->flags & REQ_BLOCK_PC) && !req->bio) {
                cmd->request_bufflen = req->data_len;
                cmd->request_buffer = req->data;
                req->buffer = req->data;
                cmd->use_sg = 0;
                return 0;
        }

        /*
         * we used to not use scatter-gather for single segment request,
         * but now we do (it makes highmem I/O easier to support without
         * kmapping pages)
         */
        cmd->use_sg = req->nr_phys_segments;

        /*
         * if sg table allocation fails, requeue request later.
         */
        sgpnt = scsi_alloc_sgtable(cmd, GFP_ATOMIC);
        if (unlikely(!sgpnt)) {
                scsi_unprep_request(req);
                return BLKPREP_DEFER;
        }

        cmd->request_buffer = (char *) sgpnt;
        cmd->request_bufflen = req->nr_sectors << 9;
        if (blk_pc_request(req))
                cmd->request_bufflen = req->data_len;
        req->buffer = NULL;

        /* 
         * Next, walk the list, and fill in the addresses and sizes of
         * each segment.
         */
        count = blk_rq_map_sg(req->q, req, cmd->request_buffer);

        /*
         * mapped well, send it off
         */
        if (likely(count <= cmd->use_sg)) {
                cmd->use_sg = count;
                return 0;
        }

        printk(KERN_ERR "Incorrect number of segments after building list\n");
        printk(KERN_ERR "counted %d, received %d\n", count, cmd->use_sg);
        printk(KERN_ERR "req nr_sec %lu, cur_nr_sec %u\n", req->nr_sectors,
                        req->current_nr_sectors);

        /* release the command and kill it */
        scsi_release_buffers(cmd);
        scsi_put_command(cmd);
        return BLKPREP_KILL;
}

static int scsi_issue_flush_fn(request_queue_t *q, struct gendisk *disk,
                               sector_t *error_sector)
{
        struct scsi_device *sdev = q->queuedata;
        struct scsi_driver *drv;

        if (sdev->sdev_state != SDEV_RUNNING)
                return -ENXIO;

        drv = *(struct scsi_driver **) disk->private_data;
        if (drv->issue_flush)
                return drv->issue_flush(&sdev->sdev_gendev, error_sector);

        return -EOPNOTSUPP;
}

static void scsi_blk_pc_done(struct scsi_cmnd *cmd)
{
        BUG_ON(!blk_pc_request(cmd->request));
        /*
         * This will complete the whole command with uptodate=1 so
         * as far as the block layer is concerned the command completed
         * successfully. Since this is a REQ_BLOCK_PC command the
         * caller should check the request's errors value
         */
        scsi_io_completion(cmd, cmd->bufflen, 0);
}

static void scsi_setup_blk_pc_cmnd(struct scsi_cmnd *cmd)
{
        struct request *req = cmd->request;

        BUG_ON(sizeof(req->cmd) > sizeof(cmd->cmnd));
        memcpy(cmd->cmnd, req->cmd, sizeof(cmd->cmnd));
        cmd->cmd_len = req->cmd_len;
        if (!req->data_len)
                cmd->sc_data_direction = DMA_NONE;
        else if (rq_data_dir(req) == WRITE)
                cmd->sc_data_direction = DMA_TO_DEVICE;
        else
                cmd->sc_data_direction = DMA_FROM_DEVICE;
        
        cmd->transfersize = req->data_len;
        cmd->allowed = req->retries;
        cmd->timeout_per_command = req->timeout;
        cmd->done = scsi_blk_pc_done;
}

static int scsi_prep_fn(struct request_queue *q, struct request *req)
{
        struct scsi_device *sdev = q->queuedata;
        struct scsi_cmnd *cmd;
        int specials_only = 0;

        /*
         * Just check to see if the device is online.  If it isn't, we
         * refuse to process any commands.  The device must be brought
         * online before trying any recovery commands
         */
        if (unlikely(!scsi_device_online(sdev))) {
                sdev_printk(KERN_ERR, sdev,
                            "rejecting I/O to offline device\n");
                goto kill;
        }
        if (unlikely(sdev->sdev_state != SDEV_RUNNING)) {
                /* OK, we're not in a running state don't prep
                 * user commands */
                if (sdev->sdev_state == SDEV_DEL) {
                        /* Device is fully deleted, no commands
                         * at all allowed down */
                        sdev_printk(KERN_ERR, sdev,
                                    "rejecting I/O to dead device\n");
                        goto kill;
                }
                /* OK, we only allow special commands (i.e. not
                 * user initiated ones */
                specials_only = sdev->sdev_state;
        }

        /*
         * Find the actual device driver associated with this command.
         * The SPECIAL requests are things like character device or
         * ioctls, which did not originate from ll_rw_blk.  Note that
         * the special field is also used to indicate the cmd for
         * the remainder of a partially fulfilled request that can 
         * come up when there is a medium error.  We have to treat
         * these two cases differently.  We differentiate by looking
         * at request->cmd, as this tells us the real story.
         */
        if (req->flags & REQ_SPECIAL && req->special) {
                struct scsi_request *sreq = req->special;

                if (sreq->sr_magic == SCSI_REQ_MAGIC) {
                        cmd = scsi_get_command(sreq->sr_device, GFP_ATOMIC);
                        if (unlikely(!cmd))
                                goto defer;
                        scsi_init_cmd_from_req(cmd, sreq);
                } else
                        cmd = req->special;
        } else if (req->flags & (REQ_CMD | REQ_BLOCK_PC)) {

                if(unlikely(specials_only) && !(req->flags & REQ_SPECIAL)) {
                        if(specials_only == SDEV_QUIESCE ||
                                        specials_only == SDEV_BLOCK)
                                goto defer;
                        
                        sdev_printk(KERN_ERR, sdev,
                                    "rejecting I/O to device being removed\n");
                        goto kill;
                }
                        
                        
                /*
                 * Now try and find a command block that we can use.
                 */
                if (!req->special) {
                        cmd = scsi_get_command(sdev, GFP_ATOMIC);
                        if (unlikely(!cmd))
                                goto defer;
                } else
                        cmd = req->special;
                
                /* pull a tag out of the request if we have one */
                cmd->tag = req->tag;
        } else {
                blk_dump_rq_flags(req, "SCSI bad req");
                goto kill;
        }
        
        /* note the overloading of req->special.  When the tag
         * is active it always means cmd.  If the tag goes
         * back for re-queueing, it may be reset */
        req->special = cmd;
        cmd->request = req;
        
        /*
         * FIXME: drop the lock here because the functions below
         * expect to be called without the queue lock held.  Also,
         * previously, we dequeued the request before dropping the
         * lock.  We hope REQ_STARTED prevents anything untoward from
         * happening now.
         */
        if (req->flags & (REQ_CMD | REQ_BLOCK_PC)) {
                int ret;

                /*
                 * This will do a couple of things:
                 *  1) Fill in the actual SCSI command.
                 *  2) Fill in any other upper-level specific fields
                 * (timeout).
                 *
                 * If this returns 0, it means that the request failed
                 * (reading past end of disk, reading offline device,
                 * etc).   This won't actually talk to the device, but
                 * some kinds of consistency checking may cause the     
                 * request to be rejected immediately.
                 */

                /* 
                 * This sets up the scatter-gather table (allocating if
                 * required).
                 */
                ret = scsi_init_io(cmd);
                switch(ret) {
                        /* For BLKPREP_KILL/DEFER the cmd was released */
                case BLKPREP_KILL:
                        goto kill;
                case BLKPREP_DEFER:
                        goto defer;
                }
                
                /*
                 * Initialize the actual SCSI command for this request.
                 */
                if (req->flags & REQ_BLOCK_PC) {
                        scsi_setup_blk_pc_cmnd(cmd);
                } else if (req->rq_disk) {
                        struct scsi_driver *drv;

                        drv = *(struct scsi_driver **)req->rq_disk->private_data;
                        if (unlikely(!drv->init_command(cmd))) {
                                scsi_release_buffers(cmd);
                                scsi_put_command(cmd);
                                goto kill;
                        }
                }
        }

        /*
         * The request is now prepped, no need to come back here
         */
        req->flags |= REQ_DONTPREP;
        return BLKPREP_OK;

 defer:
        /* If we defer, the elv_next_request() returns NULL, but the
         * queue must be restarted, so we plug here if no returning
         * command will automatically do that. */
        if (sdev->device_busy == 0)
                blk_plug_device(q);
        return BLKPREP_DEFER;
 kill:
        req->errors = DID_NO_CONNECT << 16;
        return BLKPREP_KILL;
}

/*
 * scsi_dev_queue_ready: if we can send requests to sdev, return 1 else
 * return 0.
 *
 * Called with the queue_lock held.
 */
static inline int scsi_dev_queue_ready(struct request_queue *q,
                                  struct scsi_device *sdev)
{
        if (sdev->device_busy >= sdev->queue_depth)
                return 0;
        if (sdev->device_busy == 0 && sdev->device_blocked) {
                /*
                 * unblock after device_blocked iterates to zero
                 */
                if (--sdev->device_blocked == 0) {
                        SCSI_LOG_MLQUEUE(3,
                                   sdev_printk(KERN_INFO, sdev,
                                   "unblocking device at zero depth\n"));
                } else {
                        blk_plug_device(q);
                        return 0;
                }
        }
        if (sdev->device_blocked)
                return 0;

        return 1;
}

/*
 * scsi_host_queue_ready: if we can send requests to shost, return 1 else
 * return 0. We must end up running the queue again whenever 0 is
 * returned, else IO can hang.
 *
 * Called with host_lock held.
 */
static inline int scsi_host_queue_ready(struct request_queue *q,
                                   struct Scsi_Host *shost,
                                   struct scsi_device *sdev)
{
        if (scsi_host_in_recovery(shost))
                return 0;
        if (shost->host_busy == 0 && shost->host_blocked) {
                /*
                 * unblock after host_blocked iterates to zero
                 */
                if (--shost->host_blocked == 0) {
                        SCSI_LOG_MLQUEUE(3,
                                printk("scsi%d unblocking host at zero depth\n",
                                        shost->host_no));
                } else {
                        blk_plug_device(q);
                        return 0;
                }
        }
        if ((shost->can_queue > 0 && shost->host_busy >= shost->can_queue) ||
            shost->host_blocked || shost->host_self_blocked) {
                if (list_empty(&sdev->starved_entry))
                        list_add_tail(&sdev->starved_entry, &shost->starved_list);
                return 0;
        }

        /* We're OK to process the command, so we can't be starved */
        if (!list_empty(&sdev->starved_entry))
                list_del_init(&sdev->starved_entry);

        return 1;
}

/*
 * Kill a request for a dead device
 */
static void scsi_kill_request(struct request *req, request_queue_t *q)
{
        struct scsi_cmnd *cmd = req->special;

        blkdev_dequeue_request(req);

        if (unlikely(cmd == NULL)) {
                printk(KERN_CRIT "impossible request in %s.\n",
                                 __FUNCTION__);
                BUG();
        }

        scsi_init_cmd_errh(cmd);
        cmd->result = DID_NO_CONNECT << 16;
        atomic_inc(&cmd->device->iorequest_cnt);
        __scsi_done(cmd);
}

static void scsi_softirq_done(struct request *rq)
{
        struct scsi_cmnd *cmd = rq->completion_data;
        unsigned long wait_for = cmd->allowed * cmd->timeout_per_command;
        int disposition;

        INIT_LIST_HEAD(&cmd->eh_entry);

        disposition = scsi_decide_disposition(cmd);
        if (disposition != SUCCESS &&
            time_before(cmd->jiffies_at_alloc + wait_for, jiffies)) {
                sdev_printk(KERN_ERR, cmd->device,
                            "timing out command, waited %lus\n",
                            wait_for/HZ);
                disposition = SUCCESS;
        }
                        
        scsi_log_completion(cmd, disposition);

        switch (disposition) {
                case SUCCESS:
                        scsi_finish_command(cmd);
                        break;
                case NEEDS_RETRY:
                        scsi_retry_command(cmd);
                        break;
                case ADD_TO_MLQUEUE:
                        scsi_queue_insert(cmd, SCSI_MLQUEUE_DEVICE_BUSY);
                        break;
                default:
                        if (!scsi_eh_scmd_add(cmd, 0))
                                scsi_finish_command(cmd);
        }
}

/*
 * Function:    scsi_request_fn()
 *
 * Purpose:     Main strategy routine for SCSI.
 *
 * Arguments:   q       - Pointer to actual queue.
 *
 * Returns:     Nothing
 *
 * Lock status: IO request lock assumed to be held when called.
 */
static void scsi_request_fn(struct request_queue *q)
{
        struct scsi_device *sdev = q->queuedata;
        struct Scsi_Host *shost;
        struct scsi_cmnd *cmd;
        struct request *req;

        if (!sdev) {
                printk("scsi: killing requests for dead queue\n");
                while ((req = elv_next_request(q)) != NULL)
                        scsi_kill_request(req, q);
                return;
        }

        if(!get_device(&sdev->sdev_gendev))
                /* We must be tearing the block queue down already */
                return;

        /*
         * To start with, we keep looping until the queue is empty, or until
         * the host is no longer able to accept any more requests.
         */
        shost = sdev->host;
        while (!blk_queue_plugged(q)) {
                int rtn;
                /*
                 * get next queueable request.  We do this early to make sure
                 * that the request is fully prepared even if we cannot 
                 * accept it.
                 */
                req = elv_next_request(q);
                if (!req || !scsi_dev_queue_ready(q, sdev))
                        break;

                if (unlikely(!scsi_device_online(sdev))) {
                        sdev_printk(KERN_ERR, sdev,
                                    "rejecting I/O to offline device\n");
                        scsi_kill_request(req, q);
                        continue;
                }


                /*
                 * Remove the request from the request list.
                 */
                if (!(blk_queue_tagged(q) && !blk_queue_start_tag(q, req)))
                        blkdev_dequeue_request(req);
                sdev->device_busy++;

                spin_unlock(q->queue_lock);
                cmd = req->special;
                if (unlikely(cmd == NULL)) {
                        printk(KERN_CRIT "impossible request in %s.\n"
                                         "please mail a stack trace to "
                                         "linux-scsi@vger.kernel.org",
                                         __FUNCTION__);
                        BUG();
                }
                spin_lock(shost->host_lock);

                if (!scsi_host_queue_ready(q, shost, sdev))
                        goto not_ready;
                if (sdev->single_lun) {
                        if (scsi_target(sdev)->starget_sdev_user &&
                            scsi_target(sdev)->starget_sdev_user != sdev)
                                goto not_ready;
                        scsi_target(sdev)->starget_sdev_user = sdev;
                }
                shost->host_busy++;

                /*
                 * XXX(hch): This is rather suboptimal, scsi_dispatch_cmd will
                 *              take the lock again.
                 */
                spin_unlock_irq(shost->host_lock);

                /*
                 * Finally, initialize any error handling parameters, and set up
                 * the timers for timeouts.
                 */
                scsi_init_cmd_errh(cmd);

                /*
                 * Dispatch the command to the low-level driver.
                 */
                rtn = scsi_dispatch_cmd(cmd);
                spin_lock_irq(q->queue_lock);
                if(rtn) {
                        /* we're refusing the command; because of
                         * the way locks get dropped, we need to 
                         * check here if plugging is required */
                        if(sdev->device_busy == 0)
                                blk_plug_device(q);

                        break;
                }
        }

        goto out;

 not_ready:
        spin_unlock_irq(shost->host_lock);

        /*
         * lock q, handle tag, requeue req, and decrement device_busy. We
         * must return with queue_lock held.
         *
         * Decrementing device_busy without checking it is OK, as all such
         * cases (host limits or settings) should run the queue at some
         * later time.
         */
        spin_lock_irq(q->queue_lock);
        blk_requeue_request(q, req);
        sdev->device_busy--;
        if(sdev->device_busy == 0)
                blk_plug_device(q);
 out:
        /* must be careful here...if we trigger the ->remove() function
         * we cannot be holding the q lock */
        spin_unlock_irq(q->queue_lock);
        put_device(&sdev->sdev_gendev);
        spin_lock_irq(q->queue_lock);
}

u64 scsi_calculate_bounce_limit(struct Scsi_Host *shost)
{
        struct device *host_dev;
        u64 bounce_limit = 0xffffffff;

        if (shost->unchecked_isa_dma)
                return BLK_BOUNCE_ISA;
        /*
         * Platforms with virtual-DMA translation
         * hardware have no practical limit.
         */
        if (!PCI_DMA_BUS_IS_PHYS)
                return BLK_BOUNCE_ANY;

        host_dev = scsi_get_device(shost);
        if (host_dev && host_dev->dma_mask)
                bounce_limit = *host_dev->dma_mask;

        return bounce_limit;
}
EXPORT_SYMBOL(scsi_calculate_bounce_limit);

struct request_queue *scsi_alloc_queue(struct scsi_device *sdev)
{
        struct Scsi_Host *shost = sdev->host;
        struct request_queue *q;

        q = blk_init_queue(scsi_request_fn, NULL);
        if (!q)
                return NULL;

        blk_queue_prep_rq(q, scsi_prep_fn);

        blk_queue_max_hw_segments(q, shost->sg_tablesize);
        blk_queue_max_phys_segments(q, SCSI_MAX_PHYS_SEGMENTS);
        blk_queue_max_sectors(q, shost->max_sectors);
        blk_queue_bounce_limit(q, scsi_calculate_bounce_limit(shost));
        blk_queue_segment_boundary(q, shost->dma_boundary);
        blk_queue_issue_flush_fn(q, scsi_issue_flush_fn);
        blk_queue_softirq_done(q, scsi_softirq_done);

        if (!shost->use_clustering)
                clear_bit(QUEUE_FLAG_CLUSTER, &q->queue_flags);
        return q;
}

void scsi_free_queue(struct request_queue *q)
{
        blk_cleanup_queue(q);
}

/*
 * Function:    scsi_block_requests()
 *
 * Purpose:     Utility function used by low-level drivers to prevent further
 *              commands from being queued to the device.
 *
 * Arguments:   shost       - Host in question
 *
 * Returns:     Nothing
 *
 * Lock status: No locks are assumed held.
 *
 * Notes:       There is no timer nor any other means by which the requests
 *              get unblocked other than the low-level driver calling
 *              scsi_unblock_requests().
 */
void scsi_block_requests(struct Scsi_Host *shost)
{
        shost->host_self_blocked = 1;
}
EXPORT_SYMBOL(scsi_block_requests);

/*
 * Function:    scsi_unblock_requests()
 *
 * Purpose:     Utility function used by low-level drivers to allow further
 *              commands from being queued to the device.
 *
 * Arguments:   shost       - Host in question
 *
 * Returns:     Nothing
 *
 * Lock status: No locks are assumed held.
 *
 * Notes:       There is no timer nor any other means by which the requests
 *              get unblocked other than the low-level driver calling
 *              scsi_unblock_requests().
 *
 *              This is done as an API function so that changes to the
 *              internals of the scsi mid-layer won't require wholesale
 *              changes to drivers that use this feature.
 */
void scsi_unblock_requests(struct Scsi_Host *shost)
{
        shost->host_self_blocked = 0;
        scsi_run_host_queues(shost);
}
EXPORT_SYMBOL(scsi_unblock_requests);

int __init scsi_init_queue(void)
{
        int i;

        scsi_io_context_cache = kmem_cache_create("scsi_io_context",
                                        sizeof(struct scsi_io_context),
                                        0, 0, NULL, NULL);
        if (!scsi_io_context_cache) {
                printk(KERN_ERR "SCSI: can't init scsi io context cache\n");
                return -ENOMEM;
        }

        for (i = 0; i < SG_MEMPOOL_NR; i++) {
                struct scsi_host_sg_pool *sgp = scsi_sg_pools + i;
                int size = sgp->size * sizeof(struct scatterlist);

                sgp->slab = kmem_cache_create(sgp->name, size, 0,
                                SLAB_HWCACHE_ALIGN, NULL, NULL);
                if (!sgp->slab) {
                        printk(KERN_ERR "SCSI: can't init sg slab %s\n",
                                        sgp->name);
                }

                sgp->pool = mempool_create(SG_MEMPOOL_SIZE,
                                mempool_alloc_slab, mempool_free_slab,
                                sgp->slab);
                if (!sgp->pool) {
                        printk(KERN_ERR "SCSI: can't init sg mempool %s\n",
                                        sgp->name);
                }
        }

        return 0;
}

void scsi_exit_queue(void)
{
        int i;

        kmem_cache_destroy(scsi_io_context_cache);

        for (i = 0; i < SG_MEMPOOL_NR; i++) {
                struct scsi_host_sg_pool *sgp = scsi_sg_pools + i;
                mempool_destroy(sgp->pool);
                kmem_cache_destroy(sgp->slab);
        }
}
/**
 *      scsi_mode_sense - issue a mode sense, falling back from 10 to 
 *              six bytes if necessary.
 *      @sdev:  SCSI device to be queried
 *      @dbd:   set if mode sense will allow block descriptors to be returned
 *      @modepage: mode page being requested
 *      @buffer: request buffer (may not be smaller than eight bytes)
 *      @len:   length of request buffer.
 *      @timeout: command timeout
 *      @retries: number of retries before failing
 *      @data: returns a structure abstracting the mode header data
 *      @sense: place to put sense data (or NULL if no sense to be collected).
 *              must be SCSI_SENSE_BUFFERSIZE big.
 *
 *      Returns zero if unsuccessful, or the header offset (either 4
 *      or 8 depending on whether a six or ten byte command was
 *      issued) if successful.
 **/
int
scsi_mode_sense(struct scsi_device *sdev, int dbd, int modepage,
                  unsigned char *buffer, int len, int timeout, int retries,
                  struct scsi_mode_data *data, struct scsi_sense_hdr *sshdr) {
        unsigned char cmd[12];
        int use_10_for_ms;
        int header_length;
        int result;
        struct scsi_sense_hdr my_sshdr;

        memset(data, 0, sizeof(*data));
        memset(&cmd[0], 0, 12);
        cmd[1] = dbd & 0x18;    /* allows DBD and LLBA bits */
        cmd[2] = modepage;

        /* caller might not be interested in sense, but we need it */
        if (!sshdr)
                sshdr = &my_sshdr;

 retry:
        use_10_for_ms = sdev->use_10_for_ms;

        if (use_10_for_ms) {
                if (len < 8)
                        len = 8;

                cmd[0] = MODE_SENSE_10;
                cmd[8] = len;
                header_length = 8;
        } else {
                if (len < 4)
                        len = 4;

                cmd[0] = MODE_SENSE;
                cmd[4] = len;
                header_length = 4;
        }

        memset(buffer, 0, len);

        result = scsi_execute_req(sdev, cmd, DMA_FROM_DEVICE, buffer, len,
                                  sshdr, timeout, retries);

        /* This code looks awful: what it's doing is making sure an
         * ILLEGAL REQUEST sense return identifies the actual command
         * byte as the problem.  MODE_SENSE commands can return
         * ILLEGAL REQUEST if the code page isn't supported */

        if (use_10_for_ms && !scsi_status_is_good(result) &&
            (driver_byte(result) & DRIVER_SENSE)) {
                if (scsi_sense_valid(sshdr)) {
                        if ((sshdr->sense_key == ILLEGAL_REQUEST) &&
                            (sshdr->asc == 0x20) && (sshdr->ascq == 0)) {
                                /* 
                                 * Invalid command operation code
                                 */
                                sdev->use_10_for_ms = 0;
                                goto retry;
                        }
                }
        }

        if(scsi_status_is_good(result)) {
                data->header_length = header_length;
                if(use_10_for_ms) {
                        data->length = buffer[0]*256 + buffer[1] + 2;
                        data->medium_type = buffer[2];
                        data->device_specific = buffer[3];
                        data->longlba = buffer[4] & 0x01;
                        data->block_descriptor_length = buffer[6]*256
                                + buffer[7];
                } else {
                        data->length = buffer[0] + 1;
                        data->medium_type = buffer[1];
                        data->device_specific = buffer[2];
                        data->block_descriptor_length = buffer[3];
                }
        }

        return result;
}
EXPORT_SYMBOL(scsi_mode_sense);

int
scsi_test_unit_ready(struct scsi_device *sdev, int timeout, int retries)
{
        char cmd[] = {
                TEST_UNIT_READY, 0, 0, 0, 0, 0,
        };
        struct scsi_sense_hdr sshdr;
        int result;
        
        result = scsi_execute_req(sdev, cmd, DMA_NONE, NULL, 0, &sshdr,
                                  timeout, retries);

        if ((driver_byte(result) & DRIVER_SENSE) && sdev->removable) {

                if ((scsi_sense_valid(&sshdr)) &&
                    ((sshdr.sense_key == UNIT_ATTENTION) ||
                     (sshdr.sense_key == NOT_READY))) {
                        sdev->changed = 1;
                        result = 0;
                }
        }
        return result;
}
EXPORT_SYMBOL(scsi_test_unit_ready);

/**
 *      scsi_device_set_state - Take the given device through the device
 *              state model.
 *      @sdev:  scsi device to change the state of.
 *      @state: state to change to.
 *
 *      Returns zero if unsuccessful or an error if the requested 
 *      transition is illegal.
 **/
int
scsi_device_set_state(struct scsi_device *sdev, enum scsi_device_state state)
{
        enum scsi_device_state oldstate = sdev->sdev_state;

        if (state == oldstate)
                return 0;

        switch (state) {
        case SDEV_CREATED:
                /* There are no legal states that come back to
                 * created.  This is the manually initialised start
                 * state */
                goto illegal;
                        
        case SDEV_RUNNING:
                switch (oldstate) {
                case SDEV_CREATED:
                case SDEV_OFFLINE:
                case SDEV_QUIESCE:
                case SDEV_BLOCK:
                        break;
                default:
                        goto illegal;
                }
                break;

        case SDEV_QUIESCE:
                switch (oldstate) {
                case SDEV_RUNNING:
                case SDEV_OFFLINE:
                        break;
                default:
                        goto illegal;
                }
                break;

        case SDEV_OFFLINE:
                switch (oldstate) {
                case SDEV_CREATED:
                case SDEV_RUNNING:
                case SDEV_QUIESCE:
                case SDEV_BLOCK:
                        break;
                default:
                        goto illegal;
                }
                break;

        case SDEV_BLOCK:
                switch (oldstate) {
                case SDEV_CREATED:
                case SDEV_RUNNING:
                        break;
                default:
                        goto illegal;
                }
                break;

        case SDEV_CANCEL:
                switch (oldstate) {
                case SDEV_CREATED:
                case SDEV_RUNNING:
                case SDEV_OFFLINE:
                case SDEV_BLOCK:
                        break;
                default:
                        goto illegal;
                }
                break;

        case SDEV_DEL:
                switch (oldstate) {
                case SDEV_CANCEL:
                        break;
                default:
                        goto illegal;
                }
                break;

        }
        sdev->sdev_state = state;
        return 0;

 illegal:
        SCSI_LOG_ERROR_RECOVERY(1, 
                                sdev_printk(KERN_ERR, sdev,
                                            "Illegal state transition %s->%s\n",
                                            scsi_device_state_name(oldstate),
                                            scsi_device_state_name(state))
                                );
        return -EINVAL;
}
EXPORT_SYMBOL(scsi_device_set_state);

/**
 *      scsi_device_quiesce - Block user issued commands.
 *      @sdev:  scsi device to quiesce.
 *
 *      This works by trying to transition to the SDEV_QUIESCE state
 *      (which must be a legal transition).  When the device is in this
 *      state, only special requests will be accepted, all others will
 *      be deferred.  Since special requests may also be requeued requests,
 *      a successful return doesn't guarantee the device will be 
 *      totally quiescent.
 *
 *      Must be called with user context, may sleep.
 *
 *      Returns zero if unsuccessful or an error if not.
 **/
int
scsi_device_quiesce(struct scsi_device *sdev)
{
        int err = scsi_device_set_state(sdev, SDEV_QUIESCE);
        if (err)
                return err;

        scsi_run_queue(sdev->request_queue);
        while (sdev->device_busy) {
                msleep_interruptible(200);
                scsi_run_queue(sdev->request_queue);
        }
        return 0;
}
EXPORT_SYMBOL(scsi_device_quiesce);

/**
 *      scsi_device_resume - Restart user issued commands to a quiesced device.
 *      @sdev:  scsi device to resume.
 *
 *      Moves the device from quiesced back to running and restarts the
 *      queues.
 *
 *      Must be called with user context, may sleep.
 **/
void
scsi_device_resume(struct scsi_device *sdev)
{
        if(scsi_device_set_state(sdev, SDEV_RUNNING))
                return;
        scsi_run_queue(sdev->request_queue);
}
EXPORT_SYMBOL(scsi_device_resume);

static void
device_quiesce_fn(struct scsi_device *sdev, void *data)
{
        scsi_device_quiesce(sdev);
}

void
scsi_target_quiesce(struct scsi_target *starget)
{
        starget_for_each_device(starget, NULL, device_quiesce_fn);
}
EXPORT_SYMBOL(scsi_target_quiesce);

static void
device_resume_fn(struct scsi_device *sdev, void *data)
{
        scsi_device_resume(sdev);
}

void
scsi_target_resume(struct scsi_target *starget)
{
        starget_for_each_device(starget, NULL, device_resume_fn);
}
EXPORT_SYMBOL(scsi_target_resume);

/**
 * scsi_internal_device_block - internal function to put a device
 *                              temporarily into the SDEV_BLOCK state
 * @sdev:       device to block
 *
 * Block request made by scsi lld's to temporarily stop all
 * scsi commands on the specified device.  Called from interrupt
 * or normal process context.
 *
 * Returns zero if successful or error if not
 *
 * Notes:       
 *      This routine transitions the device to the SDEV_BLOCK state
 *      (which must be a legal transition).  When the device is in this
 *      state, all commands are deferred until the scsi lld reenables
 *      the device with scsi_device_unblock or device_block_tmo fires.
 *      This routine assumes the host_lock is held on entry.
 **/
int
scsi_internal_device_block(struct scsi_device *sdev)
{
        request_queue_t *q = sdev->request_queue;
        unsigned long flags;
        int err = 0;

        err = scsi_device_set_state(sdev, SDEV_BLOCK);
        if (err)
                return err;

        /* 
         * The device has transitioned to SDEV_BLOCK.  Stop the
         * block layer from calling the midlayer with this device's
         * request queue. 
         */
        spin_lock_irqsave(q->queue_lock, flags);
        blk_stop_queue(q);
        spin_unlock_irqrestore(q->queue_lock, flags);

        return 0;
}
EXPORT_SYMBOL_GPL(scsi_internal_device_block);
 
/**
 * scsi_internal_device_unblock - resume a device after a block request
 * @sdev:       device to resume
 *
 * Called by scsi lld's or the midlayer to restart the device queue
 * for the previously suspended scsi device.  Called from interrupt or
 * normal process context.
 *
 * Returns zero if successful or error if not.
 *
 * Notes:       
 *      This routine transitions the device to the SDEV_RUNNING state
 *      (which must be a legal transition) allowing the midlayer to
 *      goose the queue for this device.  This routine assumes the 
 *      host_lock is held upon entry.
 **/
int
scsi_internal_device_unblock(struct scsi_device *sdev)
{
        request_queue_t *q = sdev->request_queue; 
        int err;
        unsigned long flags;
        
        /* 
         * Try to transition the scsi device to SDEV_RUNNING
         * and goose the device queue if successful.  
         */
        err = scsi_device_set_state(sdev, SDEV_RUNNING);
        if (err)
                return err;

        spin_lock_irqsave(q->queue_lock, flags);
        blk_start_queue(q);
        spin_unlock_irqrestore(q->queue_lock, flags);

        return 0;
}
EXPORT_SYMBOL_GPL(scsi_internal_device_unblock);

static void
device_block(struct scsi_device *sdev, void *data)
{
        scsi_internal_device_block(sdev);
}

static int
target_block(struct device *dev, void *data)
{
        if (scsi_is_target_device(dev))
                starget_for_each_device(to_scsi_target(dev), NULL,
                                        device_block);
        return 0;
}

void
scsi_target_block(struct device *dev)
{
        if (scsi_is_target_device(dev))
                starget_for_each_device(to_scsi_target(dev), NULL,
                                        device_block);
        else
                device_for_each_child(dev, NULL, target_block);
}
EXPORT_SYMBOL_GPL(scsi_target_block);

static void
device_unblock(struct scsi_device *sdev, void *data)
{
        scsi_internal_device_unblock(sdev);
}

static int
target_unblock(struct device *dev, void *data)
{
        if (scsi_is_target_device(dev))
                starget_for_each_device(to_scsi_target(dev), NULL,
                                        device_unblock);
        return 0;
}

void
scsi_target_unblock(struct device *dev)
{
        if (scsi_is_target_device(dev))
                starget_for_each_device(to_scsi_target(dev), NULL,
                                        device_unblock);
        else
                device_for_each_child(dev, NULL, target_unblock);
}
EXPORT_SYMBOL_GPL(scsi_target_unblock);