1/*
  2 * SPDX-License-Identifier: MIT
  3 *
  4 * Copyright © 2019 Intel Corporation
  5 */
  6
  7#ifndef _I915_ACTIVE_H_
  8#define _I915_ACTIVE_H_
  9
 10#include <linux/lockdep.h>
 11
 12#include "i915_active_types.h"
 13#include "i915_request.h"
 14
 15/*
 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.
 25 *
 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().
 36 *
 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.
 42 */
 43
 44void i915_active_retire_noop(struct i915_active_request *active,
 45			     struct i915_request *request);
 46
 47/**
 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),
 52 *         can be NULL
 53 *
 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.
 58 */
 59static inline void
 60i915_active_request_init(struct i915_active_request *active,
 61			 struct mutex *lock,
 62			 struct i915_request *rq,
 63			 i915_active_retire_fn retire)
 64{
 65	RCU_INIT_POINTER(active->request, rq);
 66	INIT_LIST_HEAD(&active->link);
 67	active->retire = retire ?: i915_active_retire_noop;
 68#if IS_ENABLED(CONFIG_DRM_I915_DEBUG_GEM)
 69	active->lock = lock;
 70#endif
 71}
 72
 73#define INIT_ACTIVE_REQUEST(name, lock) \
 74	i915_active_request_init((name), (lock), NULL, NULL)
 75
 76/**
 77 * i915_active_request_set - updates the tracker to watch the current request
 78 * @active - the active tracker
 79 * @request - the request to watch
 80 *
 81 * __i915_active_request_set() watches the given @request for completion. Whilst
 82 * that @request is busy, the @active reports busy. When that @request is
 83 * retired, the @active tracker is updated to report idle.
 84 */
 85static inline void
 86__i915_active_request_set(struct i915_active_request *active,
 87			  struct i915_request *request)
 88{
 89#if IS_ENABLED(CONFIG_DRM_I915_DEBUG_GEM)
 90	lockdep_assert_held(active->lock);
 91#endif
 92	list_move(&active->link, &request->active_list);
 93	rcu_assign_pointer(active->request, request);
 94}
 95
 96int __must_check
 97i915_active_request_set(struct i915_active_request *active,
 98			struct i915_request *rq);
 99
100/**
101 * i915_active_request_raw - return the active request
102 * @active - the active tracker
103 *
104 * i915_active_request_raw() returns the current request being tracked, or NULL.
105 * It does not obtain a reference on the request for the caller, so the caller
106 * must hold struct_mutex.
107 */
108static inline struct i915_request *
109i915_active_request_raw(const struct i915_active_request *active,
110			struct mutex *mutex)
111{
112	return rcu_dereference_protected(active->request,
113					 lockdep_is_held(mutex));
114}
115
116/**
117 * i915_active_request_peek - report the active request being monitored
118 * @active - the active tracker
119 *
120 * i915_active_request_peek() returns the current request being tracked if
121 * still active, or NULL. It does not obtain a reference on the request
122 * for the caller, so the caller must hold struct_mutex.
123 */
124static inline struct i915_request *
125i915_active_request_peek(const struct i915_active_request *active,
126			 struct mutex *mutex)
127{
128	struct i915_request *request;
129
130	request = i915_active_request_raw(active, mutex);
131	if (!request || i915_request_completed(request))
132		return NULL;
133
134	return request;
135}
136
137/**
138 * i915_active_request_get - return a reference to the active request
139 * @active - the active tracker
140 *
141 * i915_active_request_get() returns a reference to the active request, or NULL
142 * if the active tracker is idle. The caller must hold struct_mutex.
143 */
144static inline struct i915_request *
145i915_active_request_get(const struct i915_active_request *active,
146			struct mutex *mutex)
147{
148	return i915_request_get(i915_active_request_peek(active, mutex));
149}
150
151/**
152 * __i915_active_request_get_rcu - return a reference to the active request
153 * @active - the active tracker
154 *
155 * __i915_active_request_get() returns a reference to the active request,
156 * or NULL if the active tracker is idle. The caller must hold the RCU read
157 * lock, but the returned pointer is safe to use outside of RCU.
158 */
159static inline struct i915_request *
160__i915_active_request_get_rcu(const struct i915_active_request *active)
161{
162	/*
163	 * Performing a lockless retrieval of the active request is super
164	 * tricky. SLAB_TYPESAFE_BY_RCU merely guarantees that the backing
165	 * slab of request objects will not be freed whilst we hold the
166	 * RCU read lock. It does not guarantee that the request itself
167	 * will not be freed and then *reused*. Viz,
168	 *
169	 * Thread A			Thread B
170	 *
171	 * rq = active.request
172	 *				retire(rq) -> free(rq);
173	 *				(rq is now first on the slab freelist)
174	 *				active.request = NULL
175	 *
176	 *				rq = new submission on a new object
177	 * ref(rq)
178	 *
179	 * To prevent the request from being reused whilst the caller
180	 * uses it, we take a reference like normal. Whilst acquiring
181	 * the reference we check that it is not in a destroyed state
182	 * (refcnt == 0). That prevents the request being reallocated
183	 * whilst the caller holds on to it. To check that the request
184	 * was not reallocated as we acquired the reference we have to
185	 * check that our request remains the active request across
186	 * the lookup, in the same manner as a seqlock. The visibility
187	 * of the pointer versus the reference counting is controlled
188	 * by using RCU barriers (rcu_dereference and rcu_assign_pointer).
189	 *
190	 * In the middle of all that, we inspect whether the request is
191	 * complete. Retiring is lazy so the request may be completed long
192	 * before the active tracker is updated. Querying whether the
193	 * request is complete is far cheaper (as it involves no locked
194	 * instructions setting cachelines to exclusive) than acquiring
195	 * the reference, so we do it first. The RCU read lock ensures the
196	 * pointer dereference is valid, but does not ensure that the
197	 * seqno nor HWS is the right one! However, if the request was
198	 * reallocated, that means the active tracker's request was complete.
199	 * If the new request is also complete, then both are and we can
200	 * just report the active tracker is idle. If the new request is
201	 * incomplete, then we acquire a reference on it and check that
202	 * it remained the active request.
203	 *
204	 * It is then imperative that we do not zero the request on
205	 * reallocation, so that we can chase the dangling pointers!
206	 * See i915_request_alloc().
207	 */
208	do {
209		struct i915_request *request;
210
211		request = rcu_dereference(active->request);
212		if (!request || i915_request_completed(request))
213			return NULL;
214
215		/*
216		 * An especially silly compiler could decide to recompute the
217		 * result of i915_request_completed, more specifically
218		 * re-emit the load for request->fence.seqno. A race would catch
219		 * a later seqno value, which could flip the result from true to
220		 * false. Which means part of the instructions below might not
221		 * be executed, while later on instructions are executed. Due to
222		 * barriers within the refcounting the inconsistency can't reach
223		 * past the call to i915_request_get_rcu, but not executing
224		 * that while still executing i915_request_put() creates
225		 * havoc enough.  Prevent this with a compiler barrier.
226		 */
227		barrier();
228
229		request = i915_request_get_rcu(request);
230
231		/*
232		 * What stops the following rcu_access_pointer() from occurring
233		 * before the above i915_request_get_rcu()? If we were
234		 * to read the value before pausing to get the reference to
235		 * the request, we may not notice a change in the active
236		 * tracker.
237		 *
238		 * The rcu_access_pointer() is a mere compiler barrier, which
239		 * means both the CPU and compiler are free to perform the
240		 * memory read without constraint. The compiler only has to
241		 * ensure that any operations after the rcu_access_pointer()
242		 * occur afterwards in program order. This means the read may
243		 * be performed earlier by an out-of-order CPU, or adventurous
244		 * compiler.
245		 *
246		 * The atomic operation at the heart of
247		 * i915_request_get_rcu(), see dma_fence_get_rcu(), is
248		 * atomic_inc_not_zero() which is only a full memory barrier
249		 * when successful. That is, if i915_request_get_rcu()
250		 * returns the request (and so with the reference counted
251		 * incremented) then the following read for rcu_access_pointer()
252		 * must occur after the atomic operation and so confirm
253		 * that this request is the one currently being tracked.
254		 *
255		 * The corresponding write barrier is part of
256		 * rcu_assign_pointer().
257		 */
258		if (!request || request == rcu_access_pointer(active->request))
259			return rcu_pointer_handoff(request);
260
261		i915_request_put(request);
262	} while (1);
263}
264
265/**
266 * i915_active_request_get_unlocked - return a reference to the active request
267 * @active - the active tracker
268 *
269 * i915_active_request_get_unlocked() returns a reference to the active request,
270 * or NULL if the active tracker is idle. The reference is obtained under RCU,
271 * so no locking is required by the caller.
272 *
273 * The reference should be freed with i915_request_put().
274 */
275static inline struct i915_request *
276i915_active_request_get_unlocked(const struct i915_active_request *active)
277{
278	struct i915_request *request;
279
280	rcu_read_lock();
281	request = __i915_active_request_get_rcu(active);
282	rcu_read_unlock();
283
284	return request;
285}
286
287/**
288 * i915_active_request_isset - report whether the active tracker is assigned
289 * @active - the active tracker
290 *
291 * i915_active_request_isset() returns true if the active tracker is currently
292 * assigned to a request. Due to the lazy retiring, that request may be idle
293 * and this may report stale information.
294 */
295static inline bool
296i915_active_request_isset(const struct i915_active_request *active)
297{
298	return rcu_access_pointer(active->request);
299}
300
301/**
302 * i915_active_request_retire - waits until the request is retired
303 * @active - the active request on which to wait
304 *
305 * i915_active_request_retire() waits until the request is completed,
306 * and then ensures that at least the retirement handler for this
307 * @active tracker is called before returning. If the @active
308 * tracker is idle, the function returns immediately.
309 */
310static inline int __must_check
311i915_active_request_retire(struct i915_active_request *active,
312			   struct mutex *mutex)
313{
314	struct i915_request *request;
315	long ret;
316
317	request = i915_active_request_raw(active, mutex);
318	if (!request)
319		return 0;
320
321	ret = i915_request_wait(request,
322				I915_WAIT_INTERRUPTIBLE,
323				MAX_SCHEDULE_TIMEOUT);
324	if (ret < 0)
325		return ret;
326
327	list_del_init(&active->link);
328	RCU_INIT_POINTER(active->request, NULL);
329
330	active->retire(active, request);
331
332	return 0;
333}
334
335/*
336 * GPU activity tracking
337 *
338 * Each set of commands submitted to the GPU compromises a single request that
339 * signals a fence upon completion. struct i915_request combines the
340 * command submission, scheduling and fence signaling roles. If we want to see
341 * if a particular task is complete, we need to grab the fence (struct
342 * i915_request) for that task and check or wait for it to be signaled. More
343 * often though we want to track the status of a bunch of tasks, for example
344 * to wait for the GPU to finish accessing some memory across a variety of
345 * different command pipelines from different clients. We could choose to
346 * track every single request associated with the task, but knowing that
347 * each request belongs to an ordered timeline (later requests within a
348 * timeline must wait for earlier requests), we need only track the
349 * latest request in each timeline to determine the overall status of the
350 * task.
351 *
352 * struct i915_active provides this tracking across timelines. It builds a
353 * composite shared-fence, and is updated as new work is submitted to the task,
354 * forming a snapshot of the current status. It should be embedded into the
355 * different resources that need to track their associated GPU activity to
356 * provide a callback when that GPU activity has ceased, or otherwise to
357 * provide a serialisation point either for request submission or for CPU
358 * synchronisation.
359 */
360
361void __i915_active_init(struct drm_i915_private *i915,
362			struct i915_active *ref,
363			int (*active)(struct i915_active *ref),
364			void (*retire)(struct i915_active *ref),
365			struct lock_class_key *key);
366#define i915_active_init(i915, ref, active, retire) do {		\
367	static struct lock_class_key __key;				\
368									\
369	__i915_active_init(i915, ref, active, retire, &__key);		\
370} while (0)
371
372int i915_active_ref(struct i915_active *ref,
373		    struct intel_timeline *tl,
374		    struct i915_request *rq);
375
376int i915_active_wait(struct i915_active *ref);
377
378int i915_request_await_active(struct i915_request *rq,
379			      struct i915_active *ref);
380int i915_request_await_active_request(struct i915_request *rq,
381				      struct i915_active_request *active);
382
383int i915_active_acquire(struct i915_active *ref);
384void i915_active_release(struct i915_active *ref);
385void __i915_active_release_nested(struct i915_active *ref, int subclass);
386
387bool i915_active_trygrab(struct i915_active *ref);
388void i915_active_ungrab(struct i915_active *ref);
389
390static inline bool
391i915_active_is_idle(const struct i915_active *ref)
392{
393	return !atomic_read(&ref->count);
394}
395
396#if IS_ENABLED(CONFIG_DRM_I915_DEBUG_GEM)
397void i915_active_fini(struct i915_active *ref);
398#else
399static inline void i915_active_fini(struct i915_active *ref) { }
400#endif
401
402int i915_active_acquire_preallocate_barrier(struct i915_active *ref,
403					    struct intel_engine_cs *engine);
404void i915_active_acquire_barrier(struct i915_active *ref);
405void i915_request_add_active_barriers(struct i915_request *rq);
406
407#endif /* _I915_ACTIVE_H_ */