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drm/syncobj: Add documentation for timeline syncobj 

We've added a set of new APIs to manipulate syncobjs holding timelines
of dma_fence. This adds a bit of documentation about how this works.

v2: Small language nits (Lionel)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/348578/
Cc: Christian Koenig <Christian.Koenig@amd.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: David(ChunMing) Zhou <David1.Zhou@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
This commit is contained in:
Lionel Landwerlin 2020-01-14 14:19:28 +02:00 committed by Christian König
parent d1d511d516
commit 77d1a6dbe8
1 changed files with 74 additions and 13 deletions
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87
drivers/gpu/drm/drm_syncobj.c
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@ -43,27 +43,66 @@
* - Signal a syncobj (set a trivially signaled fence)
* - Wait for a syncobj's fence to appear and be signaled
*
* The syncobj userspace API also provides operations to manipulate a syncobj
* in terms of a timeline of struct &dma_fence_chain rather than a single
* struct &dma_fence, through the following operations:
*
* - Signal a given point on the timeline
* - Wait for a given point to appear and/or be signaled
* - Import and export from/to a given point of a timeline
*
* At it's core, a syncobj is simply a wrapper around a pointer to a struct
* &dma_fence which may be NULL.
* When a syncobj is first created, its pointer is either NULL or a pointer
* to an already signaled fence depending on whether the
* &DRM_SYNCOBJ_CREATE_SIGNALED flag is passed to
* &DRM_IOCTL_SYNCOBJ_CREATE.
* When GPU work which signals a syncobj is enqueued in a DRM driver,
* the syncobj fence is replaced with a fence which will be signaled by the
* completion of that work.
* When GPU work which waits on a syncobj is enqueued in a DRM driver, the
* driver retrieves syncobj's current fence at the time the work is enqueued
* waits on that fence before submitting the work to hardware.
* If the syncobj's fence is NULL, the enqueue operation is expected to fail.
* All manipulation of the syncobjs's fence happens in terms of the current
* fence at the time the ioctl is called by userspace regardless of whether
* that operation is an immediate host-side operation (signal or reset) or
* or an operation which is enqueued in some driver queue.
* &DRM_IOCTL_SYNCOBJ_RESET and &DRM_IOCTL_SYNCOBJ_SIGNAL can be used to
* manipulate a syncobj from the host by resetting its pointer to NULL or
*
* If the syncobj is considered as a binary (its state is either signaled or
* unsignaled) primitive, when GPU work is enqueued in a DRM driver to signal
* the syncobj, the syncobj's fence is replaced with a fence which will be
* signaled by the completion of that work.
* If the syncobj is considered as a timeline primitive, when GPU work is
* enqueued in a DRM driver to signal the a given point of the syncobj, a new
* struct &dma_fence_chain pointing to the DRM driver's fence and also
* pointing to the previous fence that was in the syncobj. The new struct
* &dma_fence_chain fence replace the syncobj's fence and will be signaled by
* completion of the DRM driver's work and also any work associated with the
* fence previously in the syncobj.
*
* When GPU work which waits on a syncobj is enqueued in a DRM driver, at the
* time the work is enqueued, it waits on the syncobj's fence before
* submitting the work to hardware. That fence is either :
*
* - The syncobj's current fence if the syncobj is considered as a binary
* primitive.
* - The struct &dma_fence associated with a given point if the syncobj is
* considered as a timeline primitive.
*
* If the syncobj's fence is NULL or not present in the syncobj's timeline,
* the enqueue operation is expected to fail.
*
* With binary syncobj, all manipulation of the syncobjs's fence happens in
* terms of the current fence at the time the ioctl is called by userspace
* regardless of whether that operation is an immediate host-side operation
* (signal or reset) or or an operation which is enqueued in some driver
* queue. &DRM_IOCTL_SYNCOBJ_RESET and &DRM_IOCTL_SYNCOBJ_SIGNAL can be used
* to manipulate a syncobj from the host by resetting its pointer to NULL or
* setting its pointer to a fence which is already signaled.
*
* With a timeline syncobj, all manipulation of the synobj's fence happens in
* terms of a u64 value referring to point in the timeline. See
* dma_fence_chain_find_seqno() to see how a given point is found in the
* timeline.
*
* Note that applications should be careful to always use timeline set of
* ioctl() when dealing with syncobj considered as timeline. Using a binary
* set of ioctl() with a syncobj considered as timeline could result incorrect
* synchronization. The use of binary syncobj is supported through the
* timeline set of ioctl() by using a point value of 0, this will reproduce
* the behavior of the binary set of ioctl() (for example replace the
* syncobj's fence when signaling).
*
*
* Host-side wait on syncobjs
* --------------------------
@ -87,6 +126,16 @@
* synchronize between the two.
* This requirement is inherited from the Vulkan fence API.
*
* Similarly, &DRM_IOCTL_SYNCOBJ_TIMELINE_WAIT takes an array of syncobj
* handles as well as an array of u64 points and does a host-side wait on all
* of syncobj fences at the given points simultaneously.
*
* &DRM_IOCTL_SYNCOBJ_TIMELINE_WAIT also adds the ability to wait for a given
* fence to materialize on the timeline without waiting for the fence to be
* signaled by using the &DRM_SYNCOBJ_WAIT_FLAGS_WAIT_AVAILABLE flag. This
* requirement is inherited from the wait-before-signal behavior required by
* the Vulkan timeline semaphore API.
*
*
* Import/export of syncobjs
* -------------------------
@ -120,6 +169,18 @@
* Because sync files are immutable, resetting or signaling the syncobj
* will not affect any sync files whose fences have been imported into the
* syncobj.
*
*
* Import/export of timeline points in timeline syncobjs
* -----------------------------------------------------
*
* &DRM_IOCTL_SYNCOBJ_TRANSFER provides a mechanism to transfer a struct
* &dma_fence_chain of a syncobj at a given u64 point to another u64 point
* into another syncobj.
*
* Note that if you want to transfer a struct &dma_fence_chain from a given
* point on a timeline syncobj from/into a binary syncobj, you can use the
* point 0 to mean take/replace the fence in the syncobj.
*/
#include <linux/anon_inodes.h>