/*
* 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_ */
