2 * SPDX-License-Identifier: MIT
4 * Copyright © 2019 Intel Corporation
7 #ifndef _I915_ACTIVE_H_
8 #define _I915_ACTIVE_H_
10 #include <linux/lockdep.h>
12 #include "i915_active_types.h"
13 #include "i915_request.h"
16 * We treat requests as fences. This is not be to confused with our
17 * "fence registers" but pipeline synchronisation objects ala GL_ARB_sync.
18 * We use the fences to synchronize access from the CPU with activity on the
19 * GPU, for example, we should not rewrite an object's PTE whilst the GPU
20 * is reading them. We also track fences at a higher level to provide
21 * implicit synchronisation around GEM objects, e.g. set-domain will wait
22 * for outstanding GPU rendering before marking the object ready for CPU
23 * access, or a pageflip will wait until the GPU is complete before showing
24 * the frame on the scanout.
26 * In order to use a fence, the object must track the fence it needs to
27 * serialise with. For example, GEM objects want to track both read and
28 * write access so that we can perform concurrent read operations between
29 * the CPU and GPU engines, as well as waiting for all rendering to
30 * complete, or waiting for the last GPU user of a "fence register". The
31 * object then embeds a #i915_active_request to track the most recent (in
32 * retirement order) request relevant for the desired mode of access.
33 * The #i915_active_request is updated with i915_active_request_set() to
34 * track the most recent fence request, typically this is done as part of
35 * i915_vma_move_to_active().
37 * When the #i915_active_request completes (is retired), it will
38 * signal its completion to the owner through a callback as well as mark
39 * itself as idle (i915_active_request.request == NULL). The owner
40 * can then perform any action, such as delayed freeing of an active
41 * resource including itself.
44 void i915_active_retire_noop(struct i915_active_request *active,
45 struct i915_request *request);
48 * i915_active_request_init - prepares the activity tracker for use
49 * @active - the active tracker
50 * @rq - initial request to track, can be NULL
51 * @func - a callback when then the tracker is retired (becomes idle),
54 * i915_active_request_init() prepares the embedded @active struct for use as
55 * an activity tracker, that is for tracking the last known active request
56 * associated with it. When the last request becomes idle, when it is retired
57 * after completion, the optional callback @func is invoked.
60 i915_active_request_init(struct i915_active_request *active,
61 struct i915_request *rq,
62 i915_active_retire_fn retire)
64 RCU_INIT_POINTER(active->request, rq);
65 INIT_LIST_HEAD(&active->link);
66 active->retire = retire ?: i915_active_retire_noop;
69 #define INIT_ACTIVE_REQUEST(name) i915_active_request_init((name), NULL, NULL)
72 * i915_active_request_set - updates the tracker to watch the current request
73 * @active - the active tracker
74 * @request - the request to watch
76 * __i915_active_request_set() watches the given @request for completion. Whilst
77 * that @request is busy, the @active reports busy. When that @request is
78 * retired, the @active tracker is updated to report idle.
81 __i915_active_request_set(struct i915_active_request *active,
82 struct i915_request *request)
84 list_move(&active->link, &request->active_list);
85 rcu_assign_pointer(active->request, request);
89 i915_active_request_set(struct i915_active_request *active,
90 struct i915_request *rq);
93 * i915_active_request_set_retire_fn - updates the retirement callback
94 * @active - the active tracker
95 * @fn - the routine called when the request is retired
96 * @mutex - struct_mutex used to guard retirements
98 * i915_active_request_set_retire_fn() updates the function pointer that
99 * is called when the final request associated with the @active tracker
103 i915_active_request_set_retire_fn(struct i915_active_request *active,
104 i915_active_retire_fn fn,
107 lockdep_assert_held(mutex);
108 active->retire = fn ?: i915_active_retire_noop;
112 * i915_active_request_raw - return the active request
113 * @active - the active tracker
115 * i915_active_request_raw() returns the current request being tracked, or NULL.
116 * It does not obtain a reference on the request for the caller, so the caller
117 * must hold struct_mutex.
119 static inline struct i915_request *
120 i915_active_request_raw(const struct i915_active_request *active,
123 return rcu_dereference_protected(active->request,
124 lockdep_is_held(mutex));
128 * i915_active_request_peek - report the active request being monitored
129 * @active - the active tracker
131 * i915_active_request_peek() returns the current request being tracked if
132 * still active, or NULL. It does not obtain a reference on the request
133 * for the caller, so the caller must hold struct_mutex.
135 static inline struct i915_request *
136 i915_active_request_peek(const struct i915_active_request *active,
139 struct i915_request *request;
141 request = i915_active_request_raw(active, mutex);
142 if (!request || i915_request_completed(request))
149 * i915_active_request_get - return a reference to the active request
150 * @active - the active tracker
152 * i915_active_request_get() returns a reference to the active request, or NULL
153 * if the active tracker is idle. The caller must hold struct_mutex.
155 static inline struct i915_request *
156 i915_active_request_get(const struct i915_active_request *active,
159 return i915_request_get(i915_active_request_peek(active, mutex));
163 * __i915_active_request_get_rcu - return a reference to the active request
164 * @active - the active tracker
166 * __i915_active_request_get() returns a reference to the active request,
167 * or NULL if the active tracker is idle. The caller must hold the RCU read
168 * lock, but the returned pointer is safe to use outside of RCU.
170 static inline struct i915_request *
171 __i915_active_request_get_rcu(const struct i915_active_request *active)
174 * Performing a lockless retrieval of the active request is super
175 * tricky. SLAB_TYPESAFE_BY_RCU merely guarantees that the backing
176 * slab of request objects will not be freed whilst we hold the
177 * RCU read lock. It does not guarantee that the request itself
178 * will not be freed and then *reused*. Viz,
182 * rq = active.request
183 * retire(rq) -> free(rq);
184 * (rq is now first on the slab freelist)
185 * active.request = NULL
187 * rq = new submission on a new object
190 * To prevent the request from being reused whilst the caller
191 * uses it, we take a reference like normal. Whilst acquiring
192 * the reference we check that it is not in a destroyed state
193 * (refcnt == 0). That prevents the request being reallocated
194 * whilst the caller holds on to it. To check that the request
195 * was not reallocated as we acquired the reference we have to
196 * check that our request remains the active request across
197 * the lookup, in the same manner as a seqlock. The visibility
198 * of the pointer versus the reference counting is controlled
199 * by using RCU barriers (rcu_dereference and rcu_assign_pointer).
201 * In the middle of all that, we inspect whether the request is
202 * complete. Retiring is lazy so the request may be completed long
203 * before the active tracker is updated. Querying whether the
204 * request is complete is far cheaper (as it involves no locked
205 * instructions setting cachelines to exclusive) than acquiring
206 * the reference, so we do it first. The RCU read lock ensures the
207 * pointer dereference is valid, but does not ensure that the
208 * seqno nor HWS is the right one! However, if the request was
209 * reallocated, that means the active tracker's request was complete.
210 * If the new request is also complete, then both are and we can
211 * just report the active tracker is idle. If the new request is
212 * incomplete, then we acquire a reference on it and check that
213 * it remained the active request.
215 * It is then imperative that we do not zero the request on
216 * reallocation, so that we can chase the dangling pointers!
217 * See i915_request_alloc().
220 struct i915_request *request;
222 request = rcu_dereference(active->request);
223 if (!request || i915_request_completed(request))
227 * An especially silly compiler could decide to recompute the
228 * result of i915_request_completed, more specifically
229 * re-emit the load for request->fence.seqno. A race would catch
230 * a later seqno value, which could flip the result from true to
231 * false. Which means part of the instructions below might not
232 * be executed, while later on instructions are executed. Due to
233 * barriers within the refcounting the inconsistency can't reach
234 * past the call to i915_request_get_rcu, but not executing
235 * that while still executing i915_request_put() creates
236 * havoc enough. Prevent this with a compiler barrier.
240 request = i915_request_get_rcu(request);
243 * What stops the following rcu_access_pointer() from occurring
244 * before the above i915_request_get_rcu()? If we were
245 * to read the value before pausing to get the reference to
246 * the request, we may not notice a change in the active
249 * The rcu_access_pointer() is a mere compiler barrier, which
250 * means both the CPU and compiler are free to perform the
251 * memory read without constraint. The compiler only has to
252 * ensure that any operations after the rcu_access_pointer()
253 * occur afterwards in program order. This means the read may
254 * be performed earlier by an out-of-order CPU, or adventurous
257 * The atomic operation at the heart of
258 * i915_request_get_rcu(), see dma_fence_get_rcu(), is
259 * atomic_inc_not_zero() which is only a full memory barrier
260 * when successful. That is, if i915_request_get_rcu()
261 * returns the request (and so with the reference counted
262 * incremented) then the following read for rcu_access_pointer()
263 * must occur after the atomic operation and so confirm
264 * that this request is the one currently being tracked.
266 * The corresponding write barrier is part of
267 * rcu_assign_pointer().
269 if (!request || request == rcu_access_pointer(active->request))
270 return rcu_pointer_handoff(request);
272 i915_request_put(request);
277 * i915_active_request_get_unlocked - return a reference to the active request
278 * @active - the active tracker
280 * i915_active_request_get_unlocked() returns a reference to the active request,
281 * or NULL if the active tracker is idle. The reference is obtained under RCU,
282 * so no locking is required by the caller.
284 * The reference should be freed with i915_request_put().
286 static inline struct i915_request *
287 i915_active_request_get_unlocked(const struct i915_active_request *active)
289 struct i915_request *request;
292 request = __i915_active_request_get_rcu(active);
299 * i915_active_request_isset - report whether the active tracker is assigned
300 * @active - the active tracker
302 * i915_active_request_isset() returns true if the active tracker is currently
303 * assigned to a request. Due to the lazy retiring, that request may be idle
304 * and this may report stale information.
307 i915_active_request_isset(const struct i915_active_request *active)
309 return rcu_access_pointer(active->request);
313 * i915_active_request_retire - waits until the request is retired
314 * @active - the active request on which to wait
316 * i915_active_request_retire() waits until the request is completed,
317 * and then ensures that at least the retirement handler for this
318 * @active tracker is called before returning. If the @active
319 * tracker is idle, the function returns immediately.
321 static inline int __must_check
322 i915_active_request_retire(struct i915_active_request *active,
325 struct i915_request *request;
328 request = i915_active_request_raw(active, mutex);
332 ret = i915_request_wait(request,
333 I915_WAIT_INTERRUPTIBLE | I915_WAIT_LOCKED,
334 MAX_SCHEDULE_TIMEOUT);
338 list_del_init(&active->link);
339 RCU_INIT_POINTER(active->request, NULL);
341 active->retire(active, request);
347 * GPU activity tracking
349 * Each set of commands submitted to the GPU compromises a single request that
350 * signals a fence upon completion. struct i915_request combines the
351 * command submission, scheduling and fence signaling roles. If we want to see
352 * if a particular task is complete, we need to grab the fence (struct
353 * i915_request) for that task and check or wait for it to be signaled. More
354 * often though we want to track the status of a bunch of tasks, for example
355 * to wait for the GPU to finish accessing some memory across a variety of
356 * different command pipelines from different clients. We could choose to
357 * track every single request associated with the task, but knowing that
358 * each request belongs to an ordered timeline (later requests within a
359 * timeline must wait for earlier requests), we need only track the
360 * latest request in each timeline to determine the overall status of the
363 * struct i915_active provides this tracking across timelines. It builds a
364 * composite shared-fence, and is updated as new work is submitted to the task,
365 * forming a snapshot of the current status. It should be embedded into the
366 * different resources that need to track their associated GPU activity to
367 * provide a callback when that GPU activity has ceased, or otherwise to
368 * provide a serialisation point either for request submission or for CPU
372 void i915_active_init(struct drm_i915_private *i915,
373 struct i915_active *ref,
374 void (*retire)(struct i915_active *ref));
376 int i915_active_ref(struct i915_active *ref,
378 struct i915_request *rq);
380 int i915_active_wait(struct i915_active *ref);
382 int i915_request_await_active(struct i915_request *rq,
383 struct i915_active *ref);
384 int i915_request_await_active_request(struct i915_request *rq,
385 struct i915_active_request *active);
387 bool i915_active_acquire(struct i915_active *ref);
389 static inline void i915_active_cancel(struct i915_active *ref)
391 GEM_BUG_ON(ref->count != 1);
395 void i915_active_release(struct i915_active *ref);
398 i915_active_is_idle(const struct i915_active *ref)
403 #if IS_ENABLED(CONFIG_DRM_I915_DEBUG_GEM)
404 void i915_active_fini(struct i915_active *ref);
406 static inline void i915_active_fini(struct i915_active *ref) { }
409 #endif /* _I915_ACTIVE_H_ */