---
Documentation/driver-api/dma-buf.rst | 6 ++
drivers/dma-buf/dma-fence.c | 147 +++++++++++++++++++--------
2 files changed, 109 insertions(+), 44 deletions(-)
diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst
index dc384f2f7f34..b541e97c7ab1 100644
--- a/Documentation/driver-api/dma-buf.rst
+++ b/Documentation/driver-api/dma-buf.rst
@@ -130,6 +130,12 @@ Reservation Objects
DMA Fences
----------
+.. kernel-doc:: drivers/dma-buf/dma-fence.c
+ :doc: DMA fences overview
+
+DMA Fences Functions Reference
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
.. kernel-doc:: drivers/dma-buf/dma-fence.c
:export:
diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
index 7a92f85a4cec..1551ca7df394 100644
--- a/drivers/dma-buf/dma-fence.c
+++ b/drivers/dma-buf/dma-fence.c
@@ -38,12 +38,43 @@ EXPORT_TRACEPOINT_SYMBOL(dma_fence_enable_signal);
*/
static atomic64_t dma_fence_context_counter = ATOMIC64_INIT(0);
+/**
+ * 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 reservation_object through the
+ * &dma_buf.resv pointer.
+ */
+
/**
* dma_fence_context_alloc - allocate an array of fence contexts
- * @num: [in] amount of contexts to allocate
+ * @num: amount of contexts to allocate
*
- * This function will return the first index of the number of fences allocated.
- * The fence context is used for setting fence->context to a unique number.
+ * 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)
{
@@ -59,10 +90,14 @@ EXPORT_SYMBOL(dma_fence_context_alloc);
* 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 unsignaled to signaled state, it will only be effective
- * the first time.
+ * 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.
*
- * Unlike dma_fence_signal, this function must be called with fence->lock held.
+ * Returns 0 on success and a negative error value when @fence has been
+ * signalled already.
*/
int dma_fence_signal_locked(struct dma_fence *fence)
{
@@ -102,8 +137,11 @@ EXPORT_SYMBOL(dma_fence_signal_locked);
* 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 unsignaled to signaled state, it will only be effective
- * the first time.
+ * can only go from the unsignaled to the signaled state and not back, it will
+ * only be effective the first time.
+ *
+ * Returns 0 on success and a negative error value when @fence has been
+ * signalled already.
*/
int dma_fence_signal(struct dma_fence *fence)
{
@@ -136,9 +174,9 @@ EXPORT_SYMBOL(dma_fence_signal);
/**
* dma_fence_wait_timeout - sleep until the fence gets signaled
* or until timeout elapses
- * @fence: [in] the fence to wait on
- * @intr: [in] if true, do an interruptible wait
- * @timeout: [in] timeout value in jiffies, or MAX_SCHEDULE_TIMEOUT
+ * @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
@@ -148,6 +186,8 @@ EXPORT_SYMBOL(dma_fence_signal);
* 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)
@@ -167,6 +207,13 @@ dma_fence_wait_timeout(struct dma_fence *fence, bool intr, signed long timeout)
}
EXPORT_SYMBOL(dma_fence_wait_timeout);
+/**
+ * dma_fence_release - default relese 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 =
@@ -184,6 +231,13 @@ void dma_fence_release(struct kref *kref)
}
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);
@@ -192,10 +246,11 @@ EXPORT_SYMBOL(dma_fence_free);
/**
* dma_fence_enable_sw_signaling - enable signaling on fence
- * @fence: [in] the fence to enable
+ * @fence: the fence to enable
*
- * this will request for sw signaling to be enabled, to make the fence
- * complete as soon as possible
+ * 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_sw_signaling(struct dma_fence *fence)
{
@@ -220,24 +275,24 @@ EXPORT_SYMBOL(dma_fence_enable_sw_signaling);
/**
* dma_fence_add_callback - add a callback to be called when the fence
* is signaled
- * @fence: [in] the fence to wait on
- * @cb: [in] the callback to register
- * @func: [in] the function to call
+ * @fence: the fence to wait on
+ * @cb: the callback to register
+ * @func: the function to call
*
- * cb will be initialized by dma_fence_add_callback, no initialization
+ * @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.
*
* Note that the callback can be called from an atomic context. If
* fence is already signaled, this function will return -ENOENT (and
- * *not* call the callback)
+ * *not* call the callback).
*
* Add a software callback to the fence. Same restrictions apply to
- * refcount as it does to dma_fence_wait, however the caller doesn't need to
- * keep a refcount to fence afterwards: when software access is enabled,
- * the creator of the fence is required to keep the fence alive until
- * after it signals with dma_fence_signal. The callback itself can be called
- * from irq context.
+ * refcount as it does to dma_fence_wait(), however the caller doesn't need to
+ * keep a refcount to fence afterward dma_fence_add_callback() has returned:
+ * when software access is enabled, the creator of the fence is required to keep
+ * the fence alive until after it signals with dma_fence_signal(). The callback
+ * itself can be called from irq context.
*
* Returns 0 in case of success, -ENOENT if the fence is already signaled
* and -EINVAL in case of error.
@@ -286,7 +341,7 @@ EXPORT_SYMBOL(dma_fence_add_callback);
/**
* dma_fence_get_status - returns the status upon completion
- * @fence: [in] the dma_fence to query
+ * @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
@@ -311,8 +366,8 @@ EXPORT_SYMBOL(dma_fence_get_status);
/**
* dma_fence_remove_callback - remove a callback from the signaling list
- * @fence: [in] the fence to wait on
- * @cb: [in] the callback to remove
+ * @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
@@ -323,6 +378,9 @@ EXPORT_SYMBOL(dma_fence_get_status);
* 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)
@@ -359,9 +417,9 @@ dma_fence_default_wait_cb(struct dma_fence *fence, struct dma_fence_cb *cb)
/**
* dma_fence_default_wait - default sleep until the fence gets signaled
* or until timeout elapses
- * @fence: [in] the fence to wait on
- * @intr: [in] if true, do an interruptible wait
- * @timeout: [in] timeout value in jiffies, or MAX_SCHEDULE_TIMEOUT
+ * @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
@@ -454,12 +512,12 @@ dma_fence_test_signaled_any(struct dma_fence **fences, uint32_t count,
/**
* dma_fence_wait_any_timeout - sleep until any fence gets signaled
* or until timeout elapses
- * @fences: [in] array of fences to wait on
- * @count: [in] number of fences to wait on
- * @intr: [in] if true, do an interruptible wait
- * @timeout: [in] timeout value in jiffies, or MAX_SCHEDULE_TIMEOUT
- * @idx: [out] the first signaled fence index, meaningful only on
- * positive return
+ * @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
@@ -468,6 +526,8 @@ dma_fence_test_signaled_any(struct dma_fence **fences, uint32_t count,
* 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,
@@ -540,19 +600,18 @@ EXPORT_SYMBOL(dma_fence_wait_any_timeout);
/**
* dma_fence_init - Initialize a custom fence.
- * @fence: [in] the fence to initialize
- * @ops: [in] the dma_fence_ops for operations on this fence
- * @lock: [in] the irqsafe spinlock to use for locking this fence
- * @context: [in] the execution context this fence is run on
- * @seqno: [in] a linear increasing sequence number for this context
+ * @fence: the fence to initialize
+ * @ops: the dma_fence_ops for operations on this fence
+ * @lock: the 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. This can
- * be used for other implementing other types of fence.
+ * 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.
+ * to check which fence is later by simply using dma_fence_later().
*/
void
dma_fence_init(struct dma_fence *fence, const struct dma_fence_ops *ops,
