+++ /dev/null
-Usually, i2c devices are controlled by a kernel driver. But it is also
-possible to access all devices on an adapter from userspace, through
-the /dev interface. You need to load module i2c-dev for this.
-
-Each registered i2c adapter gets a number, counting from 0. You can
-examine /sys/class/i2c-dev/ to see what number corresponds to which adapter.
-Alternatively, you can run "i2cdetect -l" to obtain a formatted list of all
-i2c adapters present on your system at a given time. i2cdetect is part of
-the i2c-tools package.
-
-I2C device files are character device files with major device number 89
-and a minor device number corresponding to the number assigned as
-explained above. They should be called "i2c-%d" (i2c-0, i2c-1, ...,
-i2c-10, ...). All 256 minor device numbers are reserved for i2c.
-
-
-C example
-=========
-
-So let's say you want to access an i2c adapter from a C program.
-First, you need to include these two headers:
-
- #include <linux/i2c-dev.h>
- #include <i2c/smbus.h>
-
-Now, you have to decide which adapter you want to access. You should
-inspect /sys/class/i2c-dev/ or run "i2cdetect -l" to decide this.
-Adapter numbers are assigned somewhat dynamically, so you can not
-assume much about them. They can even change from one boot to the next.
-
-Next thing, open the device file, as follows:
-
- int file;
- int adapter_nr = 2; /* probably dynamically determined */
- char filename[20];
-
- snprintf(filename, 19, "/dev/i2c-%d", adapter_nr);
- file = open(filename, O_RDWR);
- if (file < 0) {
- /* ERROR HANDLING; you can check errno to see what went wrong */
- exit(1);
- }
-
-When you have opened the device, you must specify with what device
-address you want to communicate:
-
- int addr = 0x40; /* The I2C address */
-
- if (ioctl(file, I2C_SLAVE, addr) < 0) {
- /* ERROR HANDLING; you can check errno to see what went wrong */
- exit(1);
- }
-
-Well, you are all set up now. You can now use SMBus commands or plain
-I2C to communicate with your device. SMBus commands are preferred if
-the device supports them. Both are illustrated below.
-
- __u8 reg = 0x10; /* Device register to access */
- __s32 res;
- char buf[10];
-
- /* Using SMBus commands */
- res = i2c_smbus_read_word_data(file, reg);
- if (res < 0) {
- /* ERROR HANDLING: i2c transaction failed */
- } else {
- /* res contains the read word */
- }
-
- /*
- * Using I2C Write, equivalent of
- * i2c_smbus_write_word_data(file, reg, 0x6543)
- */
- buf[0] = reg;
- buf[1] = 0x43;
- buf[2] = 0x65;
- if (write(file, buf, 3) != 3) {
- /* ERROR HANDLING: i2c transaction failed */
- }
-
- /* Using I2C Read, equivalent of i2c_smbus_read_byte(file) */
- if (read(file, buf, 1) != 1) {
- /* ERROR HANDLING: i2c transaction failed */
- } else {
- /* buf[0] contains the read byte */
- }
-
-Note that only a subset of the I2C and SMBus protocols can be achieved by
-the means of read() and write() calls. In particular, so-called combined
-transactions (mixing read and write messages in the same transaction)
-aren't supported. For this reason, this interface is almost never used by
-user-space programs.
-
-IMPORTANT: because of the use of inline functions, you *have* to use
-'-O' or some variation when you compile your program!
-
-
-Full interface description
-==========================
-
-The following IOCTLs are defined:
-
-ioctl(file, I2C_SLAVE, long addr)
- Change slave address. The address is passed in the 7 lower bits of the
- argument (except for 10 bit addresses, passed in the 10 lower bits in this
- case).
-
-ioctl(file, I2C_TENBIT, long select)
- Selects ten bit addresses if select not equals 0, selects normal 7 bit
- addresses if select equals 0. Default 0. This request is only valid
- if the adapter has I2C_FUNC_10BIT_ADDR.
-
-ioctl(file, I2C_PEC, long select)
- Selects SMBus PEC (packet error checking) generation and verification
- if select not equals 0, disables if select equals 0. Default 0.
- Used only for SMBus transactions. This request only has an effect if the
- the adapter has I2C_FUNC_SMBUS_PEC; it is still safe if not, it just
- doesn't have any effect.
-
-ioctl(file, I2C_FUNCS, unsigned long *funcs)
- Gets the adapter functionality and puts it in *funcs.
-
-ioctl(file, I2C_RDWR, struct i2c_rdwr_ioctl_data *msgset)
- Do combined read/write transaction without stop in between.
- Only valid if the adapter has I2C_FUNC_I2C. The argument is
- a pointer to a
-
- struct i2c_rdwr_ioctl_data {
- struct i2c_msg *msgs; /* ptr to array of simple messages */
- int nmsgs; /* number of messages to exchange */
- }
-
- The msgs[] themselves contain further pointers into data buffers.
- The function will write or read data to or from that buffers depending
- on whether the I2C_M_RD flag is set in a particular message or not.
- The slave address and whether to use ten bit address mode has to be
- set in each message, overriding the values set with the above ioctl's.
-
-ioctl(file, I2C_SMBUS, struct i2c_smbus_ioctl_data *args)
- If possible, use the provided i2c_smbus_* methods described below instead
- of issuing direct ioctls.
-
-You can do plain i2c transactions by using read(2) and write(2) calls.
-You do not need to pass the address byte; instead, set it through
-ioctl I2C_SLAVE before you try to access the device.
-
-You can do SMBus level transactions (see documentation file smbus-protocol
-for details) through the following functions:
- __s32 i2c_smbus_write_quick(int file, __u8 value);
- __s32 i2c_smbus_read_byte(int file);
- __s32 i2c_smbus_write_byte(int file, __u8 value);
- __s32 i2c_smbus_read_byte_data(int file, __u8 command);
- __s32 i2c_smbus_write_byte_data(int file, __u8 command, __u8 value);
- __s32 i2c_smbus_read_word_data(int file, __u8 command);
- __s32 i2c_smbus_write_word_data(int file, __u8 command, __u16 value);
- __s32 i2c_smbus_process_call(int file, __u8 command, __u16 value);
- __s32 i2c_smbus_read_block_data(int file, __u8 command, __u8 *values);
- __s32 i2c_smbus_write_block_data(int file, __u8 command, __u8 length,
- __u8 *values);
-All these transactions return -1 on failure; you can read errno to see
-what happened. The 'write' transactions return 0 on success; the
-'read' transactions return the read value, except for read_block, which
-returns the number of values read. The block buffers need not be longer
-than 32 bytes.
-
-The above functions are made available by linking against the libi2c library,
-which is provided by the i2c-tools project. See:
-https://git.kernel.org/pub/scm/utils/i2c-tools/i2c-tools.git/.
-
-
-Implementation details
-======================
-
-For the interested, here's the code flow which happens inside the kernel
-when you use the /dev interface to I2C:
-
-1* Your program opens /dev/i2c-N and calls ioctl() on it, as described in
-section "C example" above.
-
-2* These open() and ioctl() calls are handled by the i2c-dev kernel
-driver: see i2c-dev.c:i2cdev_open() and i2c-dev.c:i2cdev_ioctl(),
-respectively. You can think of i2c-dev as a generic I2C chip driver
-that can be programmed from user-space.
-
-3* Some ioctl() calls are for administrative tasks and are handled by
-i2c-dev directly. Examples include I2C_SLAVE (set the address of the
-device you want to access) and I2C_PEC (enable or disable SMBus error
-checking on future transactions.)
-
-4* Other ioctl() calls are converted to in-kernel function calls by
-i2c-dev. Examples include I2C_FUNCS, which queries the I2C adapter
-functionality using i2c.h:i2c_get_functionality(), and I2C_SMBUS, which
-performs an SMBus transaction using i2c-core-smbus.c:i2c_smbus_xfer().
-
-The i2c-dev driver is responsible for checking all the parameters that
-come from user-space for validity. After this point, there is no
-difference between these calls that came from user-space through i2c-dev
-and calls that would have been performed by kernel I2C chip drivers
-directly. This means that I2C bus drivers don't need to implement
-anything special to support access from user-space.
-
-5* These i2c.h functions are wrappers to the actual implementation of
-your I2C bus driver. Each adapter must declare callback functions
-implementing these standard calls. i2c.h:i2c_get_functionality() calls
-i2c_adapter.algo->functionality(), while
-i2c-core-smbus.c:i2c_smbus_xfer() calls either
-adapter.algo->smbus_xfer() if it is implemented, or if not,
-i2c-core-smbus.c:i2c_smbus_xfer_emulated() which in turn calls
-i2c_adapter.algo->master_xfer().
-
-After your I2C bus driver has processed these requests, execution runs
-up the call chain, with almost no processing done, except by i2c-dev to
-package the returned data, if any, in suitable format for the ioctl.
git.samba.org / sfrench / cifs-2.6.git / blobdiff