xref: /linux/drivers/dma-buf/dma-fence.c (revision 3a2c4d55e32ad65efebdb6de44eef3bfa08bb49d)

// SPDX-License-Identifier: GPL-2.0-only
/*
 * Fence mechanism for dma-buf and to allow for asynchronous dma access
 *
 * Copyright (C) 2012 Canonical Ltd
 * Copyright (C) 2012 Texas Instruments
 *
 * Authors:
 * Rob Clark <robdclark@gmail.com>
 * Maarten Lankhorst <maarten.lankhorst@canonical.com>
 */

#include <linux/slab.h>
#include <linux/export.h>
#include <linux/atomic.h>
#include <linux/dma-fence.h>
#include <linux/sched/signal.h>
#include <linux/seq_file.h>

#define CREATE_TRACE_POINTS
#include <trace/events/dma_fence.h>

EXPORT_TRACEPOINT_SYMBOL(dma_fence_emit);
EXPORT_TRACEPOINT_SYMBOL(dma_fence_enable_signal);
EXPORT_TRACEPOINT_SYMBOL(dma_fence_signaled);

static struct dma_fence dma_fence_stub;

/*
 * fence context counter: each execution context should have its own
 * fence context, this allows checking if fences belong to the same
 * context or not. One device can have multiple separate contexts,
 * and they're used if some engine can run independently of another.
 */
static atomic64_t dma_fence_context_counter = ATOMIC64_INIT(1);

/**
 * DOC: DMA fences overview
 *
 * DMA fences, represented by &struct dma_fence, are the kernel internal
 * synchronization primitive for DMA operations like GPU rendering, video
 * encoding/decoding, or displaying buffers on a screen.
 *
 * A fence is initialized using dma_fence_init() and completed using
 * dma_fence_signal(). Fences are associated with a context, allocated through
 * dma_fence_context_alloc(), and all fences on the same context are
 * fully ordered.
 *
 * Since the purposes of fences is to facilitate cross-device and
 * cross-application synchronization, there's multiple ways to use one:
 *
 * - Individual fences can be exposed as a &sync_file, accessed as a file
 *   descriptor from userspace, created by calling sync_file_create(). This is
 *   called explicit fencing, since userspace passes around explicit
 *   synchronization points.
 *
 * - Some subsystems also have their own explicit fencing primitives, like
 *   &drm_syncobj. Compared to &sync_file, a &drm_syncobj allows the underlying
 *   fence to be updated.
 *
 * - Then there's also implicit fencing, where the synchronization points are
 *   implicitly passed around as part of shared &dma_buf instances. Such
 *   implicit fences are stored in &struct dma_resv through the
 *   &dma_buf.resv pointer.
 */

/**
 * DOC: fence cross-driver contract
 *
 * Since &dma_fence provide a cross driver contract, all drivers must follow the
 * same rules:
 *
 * * Fences must complete in a reasonable time. Fences which represent kernels
 *   and shaders submitted by userspace, which could run forever, must be backed
 *   up by timeout and gpu hang recovery code. Minimally that code must prevent
 *   further command submission and force complete all in-flight fences, e.g.
 *   when the driver or hardware do not support gpu reset, or if the gpu reset
 *   failed for some reason. Ideally the driver supports gpu recovery which only
 *   affects the offending userspace context, and no other userspace
 *   submissions.
 *
 * * Drivers may have different ideas of what completion within a reasonable
 *   time means. Some hang recovery code uses a fixed timeout, others a mix
 *   between observing forward progress and increasingly strict timeouts.
 *   Drivers should not try to second guess timeout handling of fences from
 *   other drivers.
 *
 * * To ensure there's no deadlocks of dma_fence_wait() against other locks
 *   drivers should annotate all code required to reach dma_fence_signal(),
 *   which completes the fences, with dma_fence_begin_signalling() and
 *   dma_fence_end_signalling().
 *
 * * Drivers are allowed to call dma_fence_wait() while holding dma_resv_lock().
 *   This means any code required for fence completion cannot acquire a
 *   &dma_resv lock. Note that this also pulls in the entire established
 *   locking hierarchy around dma_resv_lock() and dma_resv_unlock().
 *
 * * Drivers are allowed to call dma_fence_wait() from their &shrinker
 *   callbacks. This means any code required for fence completion cannot
 *   allocate memory with GFP_KERNEL.
 *
 * * Drivers are allowed to call dma_fence_wait() from their &mmu_notifier
 *   respectively &mmu_interval_notifier callbacks. This means any code required
 *   for fence completion cannot allocate memory with GFP_NOFS or GFP_NOIO.
 *   Only GFP_ATOMIC is permissible, which might fail.
 *
 * Note that only GPU drivers have a reasonable excuse for both requiring
 * &mmu_interval_notifier and &shrinker callbacks at the same time as having to
 * track asynchronous compute work using &dma_fence. No driver outside of
 * drivers/gpu should ever call dma_fence_wait() in such contexts.
 */

static const char *dma_fence_stub_get_name(struct dma_fence *fence)
{
        return "stub";
}

static const struct dma_fence_ops dma_fence_stub_ops = {
	.get_driver_name = dma_fence_stub_get_name,
	.get_timeline_name = dma_fence_stub_get_name,
};

static int __init dma_fence_init_stub(void)
{
	dma_fence_init(&dma_fence_stub, &dma_fence_stub_ops, NULL, 0, 0);
	set_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
		&dma_fence_stub.flags);
	dma_fence_signal(&dma_fence_stub);
	return 0;
}
subsys_initcall(dma_fence_init_stub);

/**
 * dma_fence_get_stub - return a signaled fence
 *
 * Return a stub fence which is already signaled. The fence's timestamp
 * corresponds to the initialisation time of the linux kernel.
 */
struct dma_fence *dma_fence_get_stub(void)
{
	return dma_fence_get(&dma_fence_stub);
}
EXPORT_SYMBOL(dma_fence_get_stub);

/**
 * dma_fence_allocate_private_stub - return a private, signaled fence
 * @timestamp: timestamp when the fence was signaled
 *
 * Return a newly allocated and signaled stub fence.
 */
struct dma_fence *dma_fence_allocate_private_stub(ktime_t timestamp)
{
	struct dma_fence *fence;

	fence = kzalloc_obj(*fence);
	if (fence == NULL)
		return NULL;

	dma_fence_init(fence, &dma_fence_stub_ops, NULL, 0, 0);
	set_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
		&fence->flags);

	dma_fence_signal_timestamp(fence, timestamp);

	return fence;
}
EXPORT_SYMBOL(dma_fence_allocate_private_stub);

/**
 * dma_fence_context_alloc - allocate an array of fence contexts
 * @num: amount of contexts to allocate
 *
 * This function will return the first index of the number of fence contexts
 * allocated.  The fence context is used for setting &dma_fence.context to a
 * unique number by passing the context to dma_fence_init().
 */
u64 dma_fence_context_alloc(unsigned num)
{
	WARN_ON(!num);
	return atomic64_fetch_add(num, &dma_fence_context_counter);
}
EXPORT_SYMBOL(dma_fence_context_alloc);

/**
 * DOC: fence signalling annotation
 *
 * Proving correctness of all the kernel code around &dma_fence through code
 * review and testing is tricky for a few reasons:
 *
 * * It is a cross-driver contract, and therefore all drivers must follow the
 *   same rules for lock nesting order, calling contexts for various functions
 *   and anything else significant for in-kernel interfaces. But it is also
 *   impossible to test all drivers in a single machine, hence brute-force N vs.
 *   N testing of all combinations is impossible. Even just limiting to the
 *   possible combinations is infeasible.
 *
 * * There is an enormous amount of driver code involved. For render drivers
 *   there's the tail of command submission, after fences are published,
 *   scheduler code, interrupt and workers to process job completion,
 *   and timeout, gpu reset and gpu hang recovery code. Plus for integration
 *   with core mm with have &mmu_notifier, respectively &mmu_interval_notifier,
 *   and &shrinker. For modesetting drivers there's the commit tail functions
 *   between when fences for an atomic modeset are published, and when the
 *   corresponding vblank completes, including any interrupt processing and
 *   related workers. Auditing all that code, across all drivers, is not
 *   feasible.
 *
 * * Due to how many other subsystems are involved and the locking hierarchies
 *   this pulls in there is extremely thin wiggle-room for driver-specific
 *   differences. &dma_fence interacts with almost all of the core memory
 *   handling through page fault handlers via &dma_resv, dma_resv_lock() and
 *   dma_resv_unlock(). On the other side it also interacts through all
 *   allocation sites through &mmu_notifier and &shrinker.
 *
 * Furthermore lockdep does not handle cross-release dependencies, which means
 * any deadlocks between dma_fence_wait() and dma_fence_signal() can't be caught
 * at runtime with some quick testing. The simplest example is one thread
 * waiting on a &dma_fence while holding a lock::
 *
 *     lock(A);
 *     dma_fence_wait(B);
 *     unlock(A);
 *
 * while the other thread is stuck trying to acquire the same lock, which
 * prevents it from signalling the fence the previous thread is stuck waiting
 * on::
 *
 *     lock(A);
 *     unlock(A);
 *     dma_fence_signal(B);
 *
 * By manually annotating all code relevant to signalling a &dma_fence we can
 * teach lockdep about these dependencies, which also helps with the validation
 * headache since now lockdep can check all the rules for us::
 *
 *    cookie = dma_fence_begin_signalling();
 *    lock(A);
 *    unlock(A);
 *    dma_fence_signal(B);
 *    dma_fence_end_signalling(cookie);
 *
 * For using dma_fence_begin_signalling() and dma_fence_end_signalling() to
 * annotate critical sections the following rules need to be observed:
 *
 * * All code necessary to complete a &dma_fence must be annotated, from the
 *   point where a fence is accessible to other threads, to the point where
 *   dma_fence_signal() is called. Un-annotated code can contain deadlock issues,
 *   and due to the very strict rules and many corner cases it is infeasible to
 *   catch these just with review or normal stress testing.
 *
 * * &struct dma_resv deserves a special note, since the readers are only
 *   protected by rcu. This means the signalling critical section starts as soon
 *   as the new fences are installed, even before dma_resv_unlock() is called.
 *
 * * The only exception are fast paths and opportunistic signalling code, which
 *   calls dma_fence_signal() purely as an optimization, but is not required to
 *   guarantee completion of a &dma_fence. The usual example is a wait IOCTL
 *   which calls dma_fence_signal(), while the mandatory completion path goes
 *   through a hardware interrupt and possible job completion worker.
 *
 * * To aid composability of code, the annotations can be freely nested, as long
 *   as the overall locking hierarchy is consistent. The annotations also work
 *   both in interrupt and process context. Due to implementation details this
 *   requires that callers pass an opaque cookie from
 *   dma_fence_begin_signalling() to dma_fence_end_signalling().
 *
 * * Validation against the cross driver contract is implemented by priming
 *   lockdep with the relevant hierarchy at boot-up. This means even just
 *   testing with a single device is enough to validate a driver, at least as
 *   far as deadlocks with dma_fence_wait() against dma_fence_signal() are
 *   concerned.
 */
#ifdef CONFIG_LOCKDEP
static struct lockdep_map dma_fence_lockdep_map = {
	.name = "dma_fence_map"
};

/**
 * dma_fence_begin_signalling - begin a critical DMA fence signalling section
 *
 * Drivers should use this to annotate the beginning of any code section
 * required to eventually complete &dma_fence by calling dma_fence_signal().
 *
 * The end of these critical sections are annotated with
 * dma_fence_end_signalling().
 *
 * Returns:
 *
 * Opaque cookie needed by the implementation, which needs to be passed to
 * dma_fence_end_signalling().
 */
bool dma_fence_begin_signalling(void)
{
	/* explicitly nesting ... */
	if (lock_is_held_type(&dma_fence_lockdep_map, 1))
		return true;

	/* rely on might_sleep check for soft/hardirq locks */
	if (in_atomic())
		return true;

	/* ... and non-recursive successful read_trylock */
	lock_acquire(&dma_fence_lockdep_map, 0, 1, 1, 1, NULL, _RET_IP_);

	return false;
}
EXPORT_SYMBOL(dma_fence_begin_signalling);

/**
 * dma_fence_end_signalling - end a critical DMA fence signalling section
 * @cookie: opaque cookie from dma_fence_begin_signalling()
 *
 * Closes a critical section annotation opened by dma_fence_begin_signalling().
 */
void dma_fence_end_signalling(bool cookie)
{
	if (cookie)
		return;

	lock_release(&dma_fence_lockdep_map, _RET_IP_);
}
EXPORT_SYMBOL(dma_fence_end_signalling);

void __dma_fence_might_wait(void)
{
	bool tmp;

	tmp = lock_is_held_type(&dma_fence_lockdep_map, 1);
	if (tmp)
		lock_release(&dma_fence_lockdep_map, _THIS_IP_);
	lock_map_acquire(&dma_fence_lockdep_map);
	lock_map_release(&dma_fence_lockdep_map);
	if (tmp)
		lock_acquire(&dma_fence_lockdep_map, 0, 1, 1, 1, NULL, _THIS_IP_);
}
#endif

/**
 * dma_fence_signal_timestamp_locked - signal completion of a fence
 * @fence: the fence to signal
 * @timestamp: fence signal timestamp in kernel's CLOCK_MONOTONIC time domain
 *
 * Signal completion for software callbacks on a fence, this will unblock
 * dma_fence_wait() calls and run all the callbacks added with
 * dma_fence_add_callback(). Can be called multiple times, but since a fence
 * can only go from the unsignaled to the signaled state and not back, it will
 * only be effective the first time. Set the timestamp provided as the fence
 * signal timestamp.
 *
 * Unlike dma_fence_signal_timestamp(), this function must be called with
 * &dma_fence.lock held.
 */
void dma_fence_signal_timestamp_locked(struct dma_fence *fence,
				      ktime_t timestamp)
{
	const struct dma_fence_ops *ops;
	struct dma_fence_cb *cur, *tmp;
	struct list_head cb_list;

	dma_fence_assert_held(fence);

	if (unlikely(test_and_set_bit(DMA_FENCE_FLAG_SIGNALED_BIT,
				      &fence->flags)))
		return;

	trace_dma_fence_signaled(fence);

	/*
	 * When neither a release nor a wait operation is specified set the ops
	 * pointer to NULL to allow the fence structure to become independent
	 * from who originally issued it.
	 */
	ops = rcu_dereference_protected(fence->ops, true);
	if (!ops->release && !ops->wait)
		RCU_INIT_POINTER(fence->ops, NULL);

	/* Stash the cb_list before replacing it with the timestamp */
	list_replace(&fence->cb_list, &cb_list);

	fence->timestamp = timestamp;
	set_bit(DMA_FENCE_FLAG_TIMESTAMP_BIT, &fence->flags);

	list_for_each_entry_safe(cur, tmp, &cb_list, node) {
		INIT_LIST_HEAD(&cur->node);
		cur->func(fence, cur);
	}
}
EXPORT_SYMBOL(dma_fence_signal_timestamp_locked);

/**
 * dma_fence_signal_timestamp - signal completion of a fence
 * @fence: the fence to signal
 * @timestamp: fence signal timestamp in kernel's CLOCK_MONOTONIC time domain
 *
 * Signal completion for software callbacks on a fence, this will unblock
 * dma_fence_wait() calls and run all the callbacks added with
 * dma_fence_add_callback(). Can be called multiple times, but since a fence
 * can only go from the unsignaled to the signaled state and not back, it will
 * only be effective the first time. Set the timestamp provided as the fence
 * signal timestamp.
 */
void dma_fence_signal_timestamp(struct dma_fence *fence, ktime_t timestamp)
{
	unsigned long flags;

	if (WARN_ON(!fence))
		return;

	dma_fence_lock_irqsave(fence, flags);
	dma_fence_signal_timestamp_locked(fence, timestamp);
	dma_fence_unlock_irqrestore(fence, flags);
}
EXPORT_SYMBOL(dma_fence_signal_timestamp);

/**
 * dma_fence_signal_locked - signal completion of a fence
 * @fence: the fence to signal
 *
 * Signal completion for software callbacks on a fence, this will unblock
 * dma_fence_wait() calls and run all the callbacks added with
 * dma_fence_add_callback(). Can be called multiple times, but since a fence
 * can only go from the unsignaled to the signaled state and not back, it will
 * only be effective the first time.
 *
 * Unlike dma_fence_signal(), this function must be called with &dma_fence.lock
 * held.
 */
void dma_fence_signal_locked(struct dma_fence *fence)
{
	dma_fence_signal_timestamp_locked(fence, ktime_get());
}
EXPORT_SYMBOL(dma_fence_signal_locked);

/**
 * dma_fence_check_and_signal_locked - signal the fence if it's not yet signaled
 * @fence: the fence to check and signal
 *
 * Checks whether a fence was signaled and signals it if it was not yet signaled.
 *
 * Unlike dma_fence_check_and_signal(), this function must be called with
 * &struct dma_fence.lock being held.
 *
 * Return: true if fence has been signaled already, false otherwise.
 */
bool dma_fence_check_and_signal_locked(struct dma_fence *fence)
{
	bool ret;

	ret = dma_fence_test_signaled_flag(fence);
	dma_fence_signal_locked(fence);

	return ret;
}
EXPORT_SYMBOL(dma_fence_check_and_signal_locked);

/**
 * dma_fence_check_and_signal - signal the fence if it's not yet signaled
 * @fence: the fence to check and signal
 *
 * Checks whether a fence was signaled and signals it if it was not yet signaled.
 * All this is done in a race-free manner.
 *
 * Return: true if fence has been signaled already, false otherwise.
 */
bool dma_fence_check_and_signal(struct dma_fence *fence)
{
	unsigned long flags;
	bool ret;

	dma_fence_lock_irqsave(fence, flags);
	ret = dma_fence_check_and_signal_locked(fence);
	dma_fence_unlock_irqrestore(fence, flags);

	return ret;
}
EXPORT_SYMBOL(dma_fence_check_and_signal);

/**
 * dma_fence_signal - signal completion of a fence
 * @fence: the fence to signal
 *
 * Signal completion for software callbacks on a fence, this will unblock
 * dma_fence_wait() calls and run all the callbacks added with
 * dma_fence_add_callback(). Can be called multiple times, but since a fence
 * can only go from the unsignaled to the signaled state and not back, it will
 * only be effective the first time.
 */
void dma_fence_signal(struct dma_fence *fence)
{
	unsigned long flags;
	bool tmp;

	if (WARN_ON(!fence))
		return;

	tmp = dma_fence_begin_signalling();

	dma_fence_lock_irqsave(fence, flags);
	dma_fence_signal_timestamp_locked(fence, ktime_get());
	dma_fence_unlock_irqrestore(fence, flags);

	dma_fence_end_signalling(tmp);
}
EXPORT_SYMBOL(dma_fence_signal);

/**
 * dma_fence_wait_timeout - sleep until the fence gets signaled
 * or until timeout elapses
 * @fence: the fence to wait on
 * @intr: if true, do an interruptible wait
 * @timeout: timeout value in jiffies, or MAX_SCHEDULE_TIMEOUT
 *
 * Returns -ERESTARTSYS if interrupted, 0 if the wait timed out, or the
 * remaining timeout in jiffies on success. Other error values may be
 * returned on custom implementations.
 *
 * Performs a synchronous wait on this fence. It is assumed the caller
 * directly or indirectly (buf-mgr between reservation and committing)
 * holds a reference to the fence, otherwise the fence might be
 * freed before return, resulting in undefined behavior.
 *
 * See also dma_fence_wait() and dma_fence_wait_any_timeout().
 */
signed long
dma_fence_wait_timeout(struct dma_fence *fence, bool intr, signed long timeout)
{
	const struct dma_fence_ops *ops;
	signed long ret;

	if (WARN_ON(timeout < 0))
		return -EINVAL;

	might_sleep();

	__dma_fence_might_wait();

	dma_fence_enable_signaling(fence);

	rcu_read_lock();
	ops = rcu_dereference(fence->ops);
	trace_dma_fence_wait_start(fence);
	if (ops && ops->wait) {
		/*
		 * Implementing the wait ops is deprecated and not supported for
		 * issuers of fences who need their lifetime to be independent
		 * of their module after they signal, so it is ok to use the
		 * ops outside the RCU protected section.
		 */
		rcu_read_unlock();
		ret = ops->wait(fence, intr, timeout);
	} else {
		rcu_read_unlock();
		ret = dma_fence_default_wait(fence, intr, timeout);
	}
	if (trace_dma_fence_wait_end_enabled()) {
		rcu_read_lock();
		trace_dma_fence_wait_end(fence);
		rcu_read_unlock();
	}
	return ret;
}
EXPORT_SYMBOL(dma_fence_wait_timeout);

/**
 * dma_fence_release - default release function for fences
 * @kref: &dma_fence.recfount
 *
 * This is the default release functions for &dma_fence. Drivers shouldn't call
 * this directly, but instead call dma_fence_put().
 */
void dma_fence_release(struct kref *kref)
{
	struct dma_fence *fence =
		container_of(kref, struct dma_fence, refcount);
	const struct dma_fence_ops *ops;

	rcu_read_lock();
	trace_dma_fence_destroy(fence);

	if (!list_empty(&fence->cb_list) &&
	    !dma_fence_test_signaled_flag(fence)) {
		const char __rcu *timeline;
		const char __rcu *driver;
		unsigned long flags;

		driver = dma_fence_driver_name(fence);
		timeline = dma_fence_timeline_name(fence);

		WARN(1,
		     "Fence %s:%s:%llx:%llx released with pending signals!\n",
		     rcu_dereference(driver), rcu_dereference(timeline),
		     fence->context, fence->seqno);

		/*
		 * Failed to signal before release, likely a refcounting issue.
		 *
		 * This should never happen, but if it does make sure that we
		 * don't leave chains dangling. We set the error flag first
		 * so that the callbacks know this signal is due to an error.
		 */
		dma_fence_lock_irqsave(fence, flags);
		fence->error = -EDEADLK;
		dma_fence_signal_locked(fence);
		dma_fence_unlock_irqrestore(fence, flags);
	}

	ops = rcu_dereference(fence->ops);
	if (ops && ops->release)
		ops->release(fence);
	else
		dma_fence_free(fence);
	rcu_read_unlock();
}
EXPORT_SYMBOL(dma_fence_release);

/**
 * dma_fence_free - default release function for &dma_fence.
 * @fence: fence to release
 *
 * This is the default implementation for &dma_fence_ops.release. It calls
 * kfree_rcu() on @fence.
 */
void dma_fence_free(struct dma_fence *fence)
{
	kfree_rcu(fence, rcu);
}
EXPORT_SYMBOL(dma_fence_free);

static bool __dma_fence_enable_signaling(struct dma_fence *fence)
{
	const struct dma_fence_ops *ops;
	bool was_set;

	dma_fence_assert_held(fence);

	was_set = test_and_set_bit(DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,
				   &fence->flags);

	if (dma_fence_test_signaled_flag(fence))
		return false;

	rcu_read_lock();
	ops = rcu_dereference(fence->ops);
	if (!was_set && ops && ops->enable_signaling) {
		trace_dma_fence_enable_signal(fence);

		if (!ops->enable_signaling(fence)) {
			rcu_read_unlock();
			dma_fence_signal_locked(fence);
			return false;
		}
	}
	rcu_read_unlock();

	return true;
}

/**
 * dma_fence_enable_signaling - enable signaling on fence
 * @fence: the fence to enable
 *
 * This will request for sw signaling to be enabled, to make the fence
 * complete as soon as possible. This calls &dma_fence_ops.enable_signaling
 * internally.
 */
void dma_fence_enable_signaling(struct dma_fence *fence)
{
	unsigned long flags;

	dma_fence_lock_irqsave(fence, flags);
	__dma_fence_enable_signaling(fence);
	dma_fence_unlock_irqrestore(fence, flags);
}
EXPORT_SYMBOL(dma_fence_enable_signaling);

/**
 * dma_fence_add_callback - add a callback to be called when the fence
 * is signaled
 * @fence: the fence to wait on
 * @cb: the callback to register
 * @func: the function to call
 *
 * Add a software callback to the fence. The caller should keep a reference to
 * the fence.
 *
 * @cb will be initialized by dma_fence_add_callback(), no initialization
 * by the caller is required. Any number of callbacks can be registered
 * to a fence, but a callback can only be registered to one fence at a time.
 *
 * If fence is already signaled, this function will return -ENOENT (and
 * *not* call the callback).
 *
 * Note that the callback can be called from an atomic context or irq context.
 *
 * Returns 0 in case of success, -ENOENT if the fence is already signaled
 * and -EINVAL in case of error.
 */
int dma_fence_add_callback(struct dma_fence *fence, struct dma_fence_cb *cb,
			   dma_fence_func_t func)
{
	unsigned long flags;
	int ret = 0;

	if (WARN_ON(!fence || !func))
		return -EINVAL;

	if (dma_fence_test_signaled_flag(fence)) {
		INIT_LIST_HEAD(&cb->node);
		return -ENOENT;
	}

	dma_fence_lock_irqsave(fence, flags);
	if (__dma_fence_enable_signaling(fence)) {
		cb->func = func;
		list_add_tail(&cb->node, &fence->cb_list);
	} else {
		INIT_LIST_HEAD(&cb->node);
		ret = -ENOENT;
	}
	dma_fence_unlock_irqrestore(fence, flags);

	return ret;
}
EXPORT_SYMBOL(dma_fence_add_callback);

/**
 * dma_fence_get_status - returns the status upon completion
 * @fence: the dma_fence to query
 *
 * This wraps dma_fence_get_status_locked() to return the error status
 * condition on a signaled fence. See dma_fence_get_status_locked() for more
 * details.
 *
 * Returns 0 if the fence has not yet been signaled, 1 if the fence has
 * been signaled without an error condition, or a negative error code
 * if the fence has been completed in err.
 */
int dma_fence_get_status(struct dma_fence *fence)
{
	unsigned long flags;
	int status;

	dma_fence_lock_irqsave(fence, flags);
	status = dma_fence_get_status_locked(fence);
	dma_fence_unlock_irqrestore(fence, flags);

	return status;
}
EXPORT_SYMBOL(dma_fence_get_status);

/**
 * dma_fence_remove_callback - remove a callback from the signaling list
 * @fence: the fence to wait on
 * @cb: the callback to remove
 *
 * Remove a previously queued callback from the fence. This function returns
 * true if the callback is successfully removed, or false if the fence has
 * already been signaled.
 *
 * *WARNING*:
 * Cancelling a callback should only be done if you really know what you're
 * doing, since deadlocks and race conditions could occur all too easily. For
 * this reason, it should only ever be done on hardware lockup recovery,
 * with a reference held to the fence.
 *
 * Behaviour is undefined if @cb has not been added to @fence using
 * dma_fence_add_callback() beforehand.
 */
bool
dma_fence_remove_callback(struct dma_fence *fence, struct dma_fence_cb *cb)
{
	unsigned long flags;
	bool ret;

	dma_fence_lock_irqsave(fence, flags);
	ret = !list_empty(&cb->node);
	if (ret)
		list_del_init(&cb->node);
	dma_fence_unlock_irqrestore(fence, flags);

	return ret;
}
EXPORT_SYMBOL(dma_fence_remove_callback);

struct default_wait_cb {
	struct dma_fence_cb base;
	struct task_struct *task;
};

static void
dma_fence_default_wait_cb(struct dma_fence *fence, struct dma_fence_cb *cb)
{
	struct default_wait_cb *wait =
		container_of(cb, struct default_wait_cb, base);

	wake_up_state(wait->task, TASK_NORMAL);
}

/**
 * dma_fence_default_wait - default sleep until the fence gets signaled
 * or until timeout elapses
 * @fence: the fence to wait on
 * @intr: if true, do an interruptible wait
 * @timeout: timeout value in jiffies, or MAX_SCHEDULE_TIMEOUT
 *
 * Returns -ERESTARTSYS if interrupted, 0 if the wait timed out, or the
 * remaining timeout in jiffies on success. If timeout is zero the value one is
 * returned if the fence is already signaled for consistency with other
 * functions taking a jiffies timeout.
 */
signed long
dma_fence_default_wait(struct dma_fence *fence, bool intr, signed long timeout)
{
	struct default_wait_cb cb;
	unsigned long flags;
	signed long ret = timeout ? timeout : 1;

	dma_fence_lock_irqsave(fence, flags);

	if (dma_fence_test_signaled_flag(fence))
		goto out;

	if (intr && signal_pending(current)) {
		ret = -ERESTARTSYS;
		goto out;
	}

	if (!timeout) {
		ret = 0;
		goto out;
	}

	cb.base.func = dma_fence_default_wait_cb;
	cb.task = current;
	list_add(&cb.base.node, &fence->cb_list);

	while (!dma_fence_test_signaled_flag(fence) && ret > 0) {
		if (intr)
			__set_current_state(TASK_INTERRUPTIBLE);
		else
			__set_current_state(TASK_UNINTERRUPTIBLE);
		dma_fence_unlock_irqrestore(fence, flags);

		ret = schedule_timeout(ret);

		dma_fence_lock_irqsave(fence, flags);
		if (ret > 0 && intr && signal_pending(current))
			ret = -ERESTARTSYS;
	}

	if (!list_empty(&cb.base.node))
		list_del(&cb.base.node);
	__set_current_state(TASK_RUNNING);

out:
	dma_fence_unlock_irqrestore(fence, flags);
	return ret;
}
EXPORT_SYMBOL(dma_fence_default_wait);

static bool
dma_fence_test_signaled_any(struct dma_fence **fences, uint32_t count,
			    uint32_t *idx)
{
	int i;

	for (i = 0; i < count; ++i) {
		struct dma_fence *fence = fences[i];
		if (dma_fence_test_signaled_flag(fence)) {
			if (idx)
				*idx = i;
			return true;
		}
	}
	return false;
}

/**
 * dma_fence_wait_any_timeout - sleep until any fence gets signaled
 * or until timeout elapses
 * @fences: array of fences to wait on
 * @count: number of fences to wait on
 * @intr: if true, do an interruptible wait
 * @timeout: timeout value in jiffies, or MAX_SCHEDULE_TIMEOUT
 * @idx: used to store the first signaled fence index, meaningful only on
 *	positive return
 *
 * Returns -EINVAL on custom fence wait implementation, -ERESTARTSYS if
 * interrupted, 0 if the wait timed out, or the remaining timeout in jiffies
 * on success.
 *
 * Synchronous waits for the first fence in the array to be signaled. The
 * caller needs to hold a reference to all fences in the array, otherwise a
 * fence might be freed before return, resulting in undefined behavior.
 *
 * See also dma_fence_wait() and dma_fence_wait_timeout().
 */
signed long
dma_fence_wait_any_timeout(struct dma_fence **fences, uint32_t count,
			   bool intr, signed long timeout, uint32_t *idx)
{
	struct default_wait_cb *cb;
	signed long ret = timeout;
	unsigned i;

	if (WARN_ON(!fences || !count || timeout < 0))
		return -EINVAL;

	if (timeout == 0) {
		for (i = 0; i < count; ++i)
			if (dma_fence_is_signaled(fences[i])) {
				if (idx)
					*idx = i;
				return 1;
			}

		return 0;
	}

	cb = kzalloc_objs(struct default_wait_cb, count);
	if (cb == NULL) {
		ret = -ENOMEM;
		goto err_free_cb;
	}

	for (i = 0; i < count; ++i) {
		struct dma_fence *fence = fences[i];

		cb[i].task = current;
		if (dma_fence_add_callback(fence, &cb[i].base,
					   dma_fence_default_wait_cb)) {
			/* This fence is already signaled */
			if (idx)
				*idx = i;
			goto fence_rm_cb;
		}
	}

	while (ret > 0) {
		if (intr)
			set_current_state(TASK_INTERRUPTIBLE);
		else
			set_current_state(TASK_UNINTERRUPTIBLE);

		if (dma_fence_test_signaled_any(fences, count, idx))
			break;

		ret = schedule_timeout(ret);

		if (ret > 0 && intr && signal_pending(current))
			ret = -ERESTARTSYS;
	}

	__set_current_state(TASK_RUNNING);

fence_rm_cb:
	while (i-- > 0)
		dma_fence_remove_callback(fences[i], &cb[i].base);

err_free_cb:
	kfree(cb);

	return ret;
}
EXPORT_SYMBOL(dma_fence_wait_any_timeout);

/**
 * DOC: deadline hints
 *
 * In an ideal world, it would be possible to pipeline a workload sufficiently
 * that a utilization based device frequency governor could arrive at a minimum
 * frequency that meets the requirements of the use-case, in order to minimize
 * power consumption.  But in the real world there are many workloads which
 * defy this ideal.  For example, but not limited to:
 *
 * * Workloads that ping-pong between device and CPU, with alternating periods
 *   of CPU waiting for device, and device waiting on CPU.  This can result in
 *   devfreq and cpufreq seeing idle time in their respective domains and in
 *   result reduce frequency.
 *
 * * Workloads that interact with a periodic time based deadline, such as double
 *   buffered GPU rendering vs vblank sync'd page flipping.  In this scenario,
 *   missing a vblank deadline results in an *increase* in idle time on the GPU
 *   (since it has to wait an additional vblank period), sending a signal to
 *   the GPU's devfreq to reduce frequency, when in fact the opposite is what is
 *   needed.
 *
 * To this end, deadline hint(s) can be set on a &dma_fence via &dma_fence_set_deadline
 * (or indirectly via userspace facing ioctls like &sync_set_deadline).
 * The deadline hint provides a way for the waiting driver, or userspace, to
 * convey an appropriate sense of urgency to the signaling driver.
 *
 * A deadline hint is given in absolute ktime (CLOCK_MONOTONIC for userspace
 * facing APIs).  The time could either be some point in the future (such as
 * the vblank based deadline for page-flipping, or the start of a compositor's
 * composition cycle), or the current time to indicate an immediate deadline
 * hint (Ie. forward progress cannot be made until this fence is signaled).
 *
 * Multiple deadlines may be set on a given fence, even in parallel.  See the
 * documentation for &dma_fence_ops.set_deadline.
 *
 * The deadline hint is just that, a hint.  The driver that created the fence
 * may react by increasing frequency, making different scheduling choices, etc.
 * Or doing nothing at all.
 */

/**
 * dma_fence_set_deadline - set desired fence-wait deadline hint
 * @fence:    the fence that is to be waited on
 * @deadline: the time by which the waiter hopes for the fence to be
 *            signaled
 *
 * Give the fence signaler a hint about an upcoming deadline, such as
 * vblank, by which point the waiter would prefer the fence to be
 * signaled by.  This is intended to give feedback to the fence signaler
 * to aid in power management decisions, such as boosting GPU frequency
 * if a periodic vblank deadline is approaching but the fence is not
 * yet signaled..
 */
void dma_fence_set_deadline(struct dma_fence *fence, ktime_t deadline)
{
	const struct dma_fence_ops *ops;

	rcu_read_lock();
	ops = rcu_dereference(fence->ops);
	if (ops && ops->set_deadline && !dma_fence_is_signaled(fence))
		ops->set_deadline(fence, deadline);
	rcu_read_unlock();
}
EXPORT_SYMBOL(dma_fence_set_deadline);

/**
 * dma_fence_describe - Dump fence description into seq_file
 * @fence: the fence to describe
 * @seq: the seq_file to put the textual description into
 *
 * Dump a textual description of the fence and it's state into the seq_file.
 */
void dma_fence_describe(struct dma_fence *fence, struct seq_file *seq)
{
	const char __rcu *timeline = (const char __rcu *)"";
	const char __rcu *driver = (const char __rcu *)"";
	const char *signaled = "";

	rcu_read_lock();

	if (!dma_fence_is_signaled(fence)) {
		timeline = dma_fence_timeline_name(fence);
		driver = dma_fence_driver_name(fence);
		signaled = "un";
	}

	seq_printf(seq, "%llu:%llu %s %s %ssignalled\n",
		   fence->context, fence->seqno, timeline, driver,
		   signaled);

	rcu_read_unlock();
}
EXPORT_SYMBOL(dma_fence_describe);

static void
__dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops,
	         spinlock_t *lock, u64 context, u64 seqno, unsigned long flags)
{
	BUG_ON(!ops || !ops->get_driver_name || !ops->get_timeline_name);

	kref_init(&fence->refcount);
	/*
	 * While it is counter intuitive to protect a constant function pointer
	 * table by RCU it allows modules to wait for an RCU grace period
	 * before they unload, to make sure that nobody is executing their
	 * functions any more.
	 */
	RCU_INIT_POINTER(fence->ops, ops);
	INIT_LIST_HEAD(&fence->cb_list);
	fence->context = context;
	fence->seqno = seqno;
	fence->flags = flags | BIT(DMA_FENCE_FLAG_INITIALIZED_BIT);
	if (lock) {
		fence->extern_lock = lock;
	} else {
		spin_lock_init(&fence->inline_lock);
		fence->flags |= BIT(DMA_FENCE_FLAG_INLINE_LOCK_BIT);
	}
	fence->error = 0;

	trace_dma_fence_init(fence);
}

/**
 * dma_fence_init - Initialize a custom fence.
 * @fence: the fence to initialize
 * @ops: the dma_fence_ops for operations on this fence
 * @lock: optional irqsafe spinlock to use for locking this fence
 * @context: the execution context this fence is run on
 * @seqno: a linear increasing sequence number for this context
 *
 * Initializes an allocated fence, the caller doesn't have to keep its
 * refcount after committing with this fence, but it will need to hold a
 * refcount again if &dma_fence_ops.enable_signaling gets called.
 *
 * context and seqno are used for easy comparison between fences, allowing
 * to check which fence is later by simply using dma_fence_later().
 *
 * External locks are a relic of legacy use cases that needed a shared lock
 * to serialize signaling when no out-of-order signaling was possible through
 * &dma_fence_ops.signaled. Drivers have abandoned this concept since the
 * introduction of the callback, but the external lock is still around. New
 * users MUST NOT use external locks, as they force the issuer to outlive all
 * fences that reference the lock.
 */
void
dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops,
	       spinlock_t *lock, u64 context, u64 seqno)
{
	__dma_fence_init(fence, ops, lock, context, seqno, 0UL);
}
EXPORT_SYMBOL(dma_fence_init);

/**
 * dma_fence_init64 - Initialize a custom fence with 64-bit seqno support.
 * @fence: the fence to initialize
 * @ops: the dma_fence_ops for operations on this fence
 * @lock: optional irqsafe spinlock to use for locking this fence
 * @context: the execution context this fence is run on
 * @seqno: a linear increasing sequence number for this context
 *
 * Initializes an allocated fence, the caller doesn't have to keep its
 * refcount after committing with this fence, but it will need to hold a
 * refcount again if &dma_fence_ops.enable_signaling gets called.
 *
 * Context and seqno are used for easy comparison between fences, allowing
 * to check which fence is later by simply using dma_fence_later().
 *
 * New users MUST NOT use external locks. Check the documentation in
 * dma_fence_init() to understand the motives behind the legacy use cases.
 */
void
dma_fence_init64(struct dma_fence *fence, const struct dma_fence_ops *ops,
		 spinlock_t *lock, u64 context, u64 seqno)
{
	__dma_fence_init(fence, ops, lock, context, seqno,
			 BIT(DMA_FENCE_FLAG_SEQNO64_BIT));
}
EXPORT_SYMBOL(dma_fence_init64);

/**
 * dma_fence_driver_name - Access the driver name
 * @fence: the fence to query
 *
 * Returns a driver name backing the dma-fence implementation.
 *
 * IMPORTANT CONSIDERATION:
 * Dma-fence contract stipulates that access to driver provided data (data not
 * directly embedded into the object itself), such as the &dma_fence.lock and
 * memory potentially accessed by the &dma_fence.ops functions, is forbidden
 * after the fence has been signalled. Drivers are allowed to free that data,
 * and some do.
 *
 * To allow safe access drivers are mandated to guarantee a RCU grace period
 * between signalling the fence and freeing said data.
 *
 * As such access to the driver name is only valid inside a RCU locked section.
 * The pointer MUST be both queried and USED ONLY WITHIN a SINGLE block guarded
 * by the &rcu_read_lock and &rcu_read_unlock pair.
 */
const char __rcu *dma_fence_driver_name(struct dma_fence *fence)
{
	const struct dma_fence_ops *ops;

	/* RCU protection is required for safe access to returned string */
	ops = rcu_dereference(fence->ops);
	if (ops)
		return (const char __rcu *)ops->get_driver_name(fence);
	else
		return (const char __rcu *)"detached-driver";
}
EXPORT_SYMBOL(dma_fence_driver_name);

/**
 * dma_fence_timeline_name - Access the timeline name
 * @fence: the fence to query
 *
 * Returns a timeline name provided by the dma-fence implementation.
 *
 * IMPORTANT CONSIDERATION:
 * Dma-fence contract stipulates that access to driver provided data (data not
 * directly embedded into the object itself), such as the &dma_fence.lock and
 * memory potentially accessed by the &dma_fence.ops functions, is forbidden
 * after the fence has been signalled. Drivers are allowed to free that data,
 * and some do.
 *
 * To allow safe access drivers are mandated to guarantee a RCU grace period
 * between signalling the fence and freeing said data.
 *
 * As such access to the driver name is only valid inside a RCU locked section.
 * The pointer MUST be both queried and USED ONLY WITHIN a SINGLE block guarded
 * by the &rcu_read_lock and &rcu_read_unlock pair.
 */
const char __rcu *dma_fence_timeline_name(struct dma_fence *fence)
{
	const struct dma_fence_ops *ops;

	/* RCU protection is required for safe access to returned string */
	ops = rcu_dereference(fence->ops);
	if (ops)
		return (const char __rcu *)ops->get_timeline_name(fence);
	else
		return (const char __rcu *)"signaled-timeline";
}
EXPORT_SYMBOL(dma_fence_timeline_name);