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Diffstat (limited to 'drivers/gpu/drm/i915/i915_active.h')
| -rw-r--r-- | drivers/gpu/drm/i915/i915_active.h | 425 |
1 files changed, 425 insertions, 0 deletions
diff --git a/drivers/gpu/drm/i915/i915_active.h b/drivers/gpu/drm/i915/i915_active.h new file mode 100644 index 000000000000..12b5c1d287d1 --- /dev/null +++ b/drivers/gpu/drm/i915/i915_active.h @@ -0,0 +1,425 @@ +/* + * SPDX-License-Identifier: MIT + * + * Copyright © 2019 Intel Corporation + */ + +#ifndef _I915_ACTIVE_H_ +#define _I915_ACTIVE_H_ + +#include <linux/lockdep.h> + +#include "i915_active_types.h" +#include "i915_request.h" + +/* + * We treat requests as fences. This is not be to confused with our + * "fence registers" but pipeline synchronisation objects ala GL_ARB_sync. + * We use the fences to synchronize access from the CPU with activity on the + * GPU, for example, we should not rewrite an object's PTE whilst the GPU + * is reading them. We also track fences at a higher level to provide + * implicit synchronisation around GEM objects, e.g. set-domain will wait + * for outstanding GPU rendering before marking the object ready for CPU + * access, or a pageflip will wait until the GPU is complete before showing + * the frame on the scanout. + * + * In order to use a fence, the object must track the fence it needs to + * serialise with. For example, GEM objects want to track both read and + * write access so that we can perform concurrent read operations between + * the CPU and GPU engines, as well as waiting for all rendering to + * complete, or waiting for the last GPU user of a "fence register". The + * object then embeds a #i915_active_request to track the most recent (in + * retirement order) request relevant for the desired mode of access. + * The #i915_active_request is updated with i915_active_request_set() to + * track the most recent fence request, typically this is done as part of + * i915_vma_move_to_active(). + * + * When the #i915_active_request completes (is retired), it will + * signal its completion to the owner through a callback as well as mark + * itself as idle (i915_active_request.request == NULL). The owner + * can then perform any action, such as delayed freeing of an active + * resource including itself. + */ + +void i915_active_retire_noop(struct i915_active_request *active, + struct i915_request *request); + +/** + * i915_active_request_init - prepares the activity tracker for use + * @active - the active tracker + * @rq - initial request to track, can be NULL + * @func - a callback when then the tracker is retired (becomes idle), + * can be NULL + * + * i915_active_request_init() prepares the embedded @active struct for use as + * an activity tracker, that is for tracking the last known active request + * associated with it. When the last request becomes idle, when it is retired + * after completion, the optional callback @func is invoked. + */ +static inline void +i915_active_request_init(struct i915_active_request *active, + struct i915_request *rq, + i915_active_retire_fn retire) +{ + RCU_INIT_POINTER(active->request, rq); + INIT_LIST_HEAD(&active->link); + active->retire = retire ?: i915_active_retire_noop; +} + +#define INIT_ACTIVE_REQUEST(name) i915_active_request_init((name), NULL, NULL) + +/** + * i915_active_request_set - updates the tracker to watch the current request + * @active - the active tracker + * @request - the request to watch + * + * __i915_active_request_set() watches the given @request for completion. Whilst + * that @request is busy, the @active reports busy. When that @request is + * retired, the @active tracker is updated to report idle. + */ +static inline void +__i915_active_request_set(struct i915_active_request *active, + struct i915_request *request) +{ + list_move(&active->link, &request->active_list); + rcu_assign_pointer(active->request, request); +} + +int __must_check +i915_active_request_set(struct i915_active_request *active, + struct i915_request *rq); + +/** + * i915_active_request_set_retire_fn - updates the retirement callback + * @active - the active tracker + * @fn - the routine called when the request is retired + * @mutex - struct_mutex used to guard retirements + * + * i915_active_request_set_retire_fn() updates the function pointer that + * is called when the final request associated with the @active tracker + * is retired. + */ +static inline void +i915_active_request_set_retire_fn(struct i915_active_request *active, + i915_active_retire_fn fn, + struct mutex *mutex) +{ + lockdep_assert_held(mutex); + active->retire = fn ?: i915_active_retire_noop; +} + +static inline struct i915_request * +__i915_active_request_peek(const struct i915_active_request *active) +{ + /* + * Inside the error capture (running with the driver in an unknown + * state), we want to bend the rules slightly (a lot). + * + * Work is in progress to make it safer, in the meantime this keeps + * the known issue from spamming the logs. + */ + return rcu_dereference_protected(active->request, 1); +} + +/** + * i915_active_request_raw - return the active request + * @active - the active tracker + * + * i915_active_request_raw() returns the current request being tracked, or NULL. + * It does not obtain a reference on the request for the caller, so the caller + * must hold struct_mutex. + */ +static inline struct i915_request * +i915_active_request_raw(const struct i915_active_request *active, + struct mutex *mutex) +{ + return rcu_dereference_protected(active->request, + lockdep_is_held(mutex)); +} + +/** + * i915_active_request_peek - report the active request being monitored + * @active - the active tracker + * + * i915_active_request_peek() returns the current request being tracked if + * still active, or NULL. It does not obtain a reference on the request + * for the caller, so the caller must hold struct_mutex. + */ +static inline struct i915_request * +i915_active_request_peek(const struct i915_active_request *active, + struct mutex *mutex) +{ + struct i915_request *request; + + request = i915_active_request_raw(active, mutex); + if (!request || i915_request_completed(request)) + return NULL; + + return request; +} + +/** + * i915_active_request_get - return a reference to the active request + * @active - the active tracker + * + * i915_active_request_get() returns a reference to the active request, or NULL + * if the active tracker is idle. The caller must hold struct_mutex. + */ +static inline struct i915_request * +i915_active_request_get(const struct i915_active_request *active, + struct mutex *mutex) +{ + return i915_request_get(i915_active_request_peek(active, mutex)); +} + +/** + * __i915_active_request_get_rcu - return a reference to the active request + * @active - the active tracker + * + * __i915_active_request_get() returns a reference to the active request, + * or NULL if the active tracker is idle. The caller must hold the RCU read + * lock, but the returned pointer is safe to use outside of RCU. + */ +static inline struct i915_request * +__i915_active_request_get_rcu(const struct i915_active_request *active) +{ + /* + * Performing a lockless retrieval of the active request is super + * tricky. SLAB_TYPESAFE_BY_RCU merely guarantees that the backing + * slab of request objects will not be freed whilst we hold the + * RCU read lock. It does not guarantee that the request itself + * will not be freed and then *reused*. Viz, + * + * Thread A Thread B + * + * rq = active.request + * retire(rq) -> free(rq); + * (rq is now first on the slab freelist) + * active.request = NULL + * + * rq = new submission on a new object + * ref(rq) + * + * To prevent the request from being reused whilst the caller + * uses it, we take a reference like normal. Whilst acquiring + * the reference we check that it is not in a destroyed state + * (refcnt == 0). That prevents the request being reallocated + * whilst the caller holds on to it. To check that the request + * was not reallocated as we acquired the reference we have to + * check that our request remains the active request across + * the lookup, in the same manner as a seqlock. The visibility + * of the pointer versus the reference counting is controlled + * by using RCU barriers (rcu_dereference and rcu_assign_pointer). + * + * In the middle of all that, we inspect whether the request is + * complete. Retiring is lazy so the request may be completed long + * before the active tracker is updated. Querying whether the + * request is complete is far cheaper (as it involves no locked + * instructions setting cachelines to exclusive) than acquiring + * the reference, so we do it first. The RCU read lock ensures the + * pointer dereference is valid, but does not ensure that the + * seqno nor HWS is the right one! However, if the request was + * reallocated, that means the active tracker's request was complete. + * If the new request is also complete, then both are and we can + * just report the active tracker is idle. If the new request is + * incomplete, then we acquire a reference on it and check that + * it remained the active request. + * + * It is then imperative that we do not zero the request on + * reallocation, so that we can chase the dangling pointers! + * See i915_request_alloc(). + */ + do { + struct i915_request *request; + + request = rcu_dereference(active->request); + if (!request || i915_request_completed(request)) + return NULL; + + /* + * An especially silly compiler could decide to recompute the + * result of i915_request_completed, more specifically + * re-emit the load for request->fence.seqno. A race would catch + * a later seqno value, which could flip the result from true to + * false. Which means part of the instructions below might not + * be executed, while later on instructions are executed. Due to + * barriers within the refcounting the inconsistency can't reach + * past the call to i915_request_get_rcu, but not executing + * that while still executing i915_request_put() creates + * havoc enough. Prevent this with a compiler barrier. + */ + barrier(); + + request = i915_request_get_rcu(request); + + /* + * What stops the following rcu_access_pointer() from occurring + * before the above i915_request_get_rcu()? If we were + * to read the value before pausing to get the reference to + * the request, we may not notice a change in the active + * tracker. + * + * The rcu_access_pointer() is a mere compiler barrier, which + * means both the CPU and compiler are free to perform the + * memory read without constraint. The compiler only has to + * ensure that any operations after the rcu_access_pointer() + * occur afterwards in program order. This means the read may + * be performed earlier by an out-of-order CPU, or adventurous + * compiler. + * + * The atomic operation at the heart of + * i915_request_get_rcu(), see dma_fence_get_rcu(), is + * atomic_inc_not_zero() which is only a full memory barrier + * when successful. That is, if i915_request_get_rcu() + * returns the request (and so with the reference counted + * incremented) then the following read for rcu_access_pointer() + * must occur after the atomic operation and so confirm + * that this request is the one currently being tracked. + * + * The corresponding write barrier is part of + * rcu_assign_pointer(). + */ + if (!request || request == rcu_access_pointer(active->request)) + return rcu_pointer_handoff(request); + + i915_request_put(request); + } while (1); +} + +/** + * i915_active_request_get_unlocked - return a reference to the active request + * @active - the active tracker + * + * i915_active_request_get_unlocked() returns a reference to the active request, + * or NULL if the active tracker is idle. The reference is obtained under RCU, + * so no locking is required by the caller. + * + * The reference should be freed with i915_request_put(). + */ +static inline struct i915_request * +i915_active_request_get_unlocked(const struct i915_active_request *active) +{ + struct i915_request *request; + + rcu_read_lock(); + request = __i915_active_request_get_rcu(active); + rcu_read_unlock(); + + return request; +} + +/** + * i915_active_request_isset - report whether the active tracker is assigned + * @active - the active tracker + * + * i915_active_request_isset() returns true if the active tracker is currently + * assigned to a request. Due to the lazy retiring, that request may be idle + * and this may report stale information. + */ +static inline bool +i915_active_request_isset(const struct i915_active_request *active) +{ + return rcu_access_pointer(active->request); +} + +/** + * i915_active_request_retire - waits until the request is retired + * @active - the active request on which to wait + * + * i915_active_request_retire() waits until the request is completed, + * and then ensures that at least the retirement handler for this + * @active tracker is called before returning. If the @active + * tracker is idle, the function returns immediately. + */ +static inline int __must_check +i915_active_request_retire(struct i915_active_request *active, + struct mutex *mutex) +{ + struct i915_request *request; + long ret; + + request = i915_active_request_raw(active, mutex); + if (!request) + return 0; + + ret = i915_request_wait(request, + I915_WAIT_INTERRUPTIBLE | I915_WAIT_LOCKED, + MAX_SCHEDULE_TIMEOUT); + if (ret < 0) + return ret; + + list_del_init(&active->link); + RCU_INIT_POINTER(active->request, NULL); + + active->retire(active, request); + + return 0; +} + +/* + * GPU activity tracking + * + * Each set of commands submitted to the GPU compromises a single request that + * signals a fence upon completion. struct i915_request combines the + * command submission, scheduling and fence signaling roles. If we want to see + * if a particular task is complete, we need to grab the fence (struct + * i915_request) for that task and check or wait for it to be signaled. More + * often though we want to track the status of a bunch of tasks, for example + * to wait for the GPU to finish accessing some memory across a variety of + * different command pipelines from different clients. We could choose to + * track every single request associated with the task, but knowing that + * each request belongs to an ordered timeline (later requests within a + * timeline must wait for earlier requests), we need only track the + * latest request in each timeline to determine the overall status of the + * task. + * + * struct i915_active provides this tracking across timelines. It builds a + * composite shared-fence, and is updated as new work is submitted to the task, + * forming a snapshot of the current status. It should be embedded into the + * different resources that need to track their associated GPU activity to + * provide a callback when that GPU activity has ceased, or otherwise to + * provide a serialisation point either for request submission or for CPU + * synchronisation. + */ + +void i915_active_init(struct drm_i915_private *i915, + struct i915_active *ref, + void (*retire)(struct i915_active *ref)); + +int i915_active_ref(struct i915_active *ref, + u64 timeline, + struct i915_request *rq); + +int i915_active_wait(struct i915_active *ref); + +int i915_request_await_active(struct i915_request *rq, + struct i915_active *ref); +int i915_request_await_active_request(struct i915_request *rq, + struct i915_active_request *active); + +bool i915_active_acquire(struct i915_active *ref); + +static inline void i915_active_cancel(struct i915_active *ref) +{ + GEM_BUG_ON(ref->count != 1); + ref->count = 0; +} + +void i915_active_release(struct i915_active *ref); + +static inline bool +i915_active_is_idle(const struct i915_active *ref) +{ + return !ref->count; +} + +#if IS_ENABLED(CONFIG_DRM_I915_DEBUG_GEM) +void i915_active_fini(struct i915_active *ref); +#else +static inline void i915_active_fini(struct i915_active *ref) { } +#endif + +int i915_global_active_init(void); +void i915_global_active_exit(void); + +#endif /* _I915_ACTIVE_H_ */ |