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drm/syncobj: Add better overview documentation for syncobj (v2)

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This patch only brings the syncobj documentation up-to-date for the
original form of syncobj.  It does not contain any information about the
design of timeline syncobjs.

v2: Incorporate feedback from Lionel and Christian:
 - Mention actual ioctl and flag names
 - Better language around reference counting
 - Misc. language cleanups

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190812142211.15885-1-jason@jlekstrand.net
This commit is contained in:
Jason Ekstrand 2019-08-12 09:22:11 -05:00 committed by Lionel Landwerlin
parent 12db36bc3c
commit f246ff5c23
1 changed files with 89 additions and 13 deletions
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102
drivers/gpu/drm/drm_syncobj.c
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@ -29,21 +29,97 @@
/**
* DOC: Overview
*
* DRM synchronisation objects (syncobj, see struct &drm_syncobj) are
* persistent objects that contain an optional fence. The fence can be updated
* with a new fence, or be NULL.
*
* syncobj's can be waited upon, where it will wait for the underlying
* fence.
*
* syncobj's can be export to fd's and back, these fd's are opaque and
* have no other use case, except passing the syncobj between processes.
*
* DRM synchronisation objects (syncobj, see struct &drm_syncobj) provide a
* container for a synchronization primitive which can be used by userspace
* to explicitly synchronize GPU commands, can be shared between userspace
* processes, and can be shared between different DRM drivers.
* Their primary use-case is to implement Vulkan fences and semaphores.
* The syncobj userspace API provides ioctls for several operations:
*
* syncobj have a kref reference count, but also have an optional file.
* The file is only created once the syncobj is exported.
* The file takes a reference on the kref.
* - Creation and destruction of syncobjs
* - Import and export of syncobjs to/from a syncobj file descriptor
* - Import and export a syncobj's underlying fence to/from a sync file
* - Reset a syncobj (set its fence to NULL)
* - Signal a syncobj (set a trivially signaled fence)
* - Wait for a syncobj's fence to appear and be signaled
*
* 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
* setting its pointer to a fence which is already signaled.
*
*
* Host-side wait on syncobjs
* --------------------------
*
* &DRM_IOCTL_SYNCOBJ_WAIT takes an array of syncobj handles and does a
* host-side wait on all of the syncobj fences simultaneously.
* If &DRM_SYNCOBJ_WAIT_FLAGS_WAIT_ALL is set, the wait ioctl will wait on
* all of the syncobj fences to be signaled before it returns.
* Otherwise, it returns once at least one syncobj fence has been signaled
* and the index of a signaled fence is written back to the client.
*
* Unlike the enqueued GPU work dependencies which fail if they see a NULL
* fence in a syncobj, if &DRM_SYNCOBJ_WAIT_FLAGS_WAIT_FOR_SUBMIT is set,
* the host-side wait will first wait for the syncobj to receive a non-NULL
* fence and then wait on that fence.
* If &DRM_SYNCOBJ_WAIT_FLAGS_WAIT_FOR_SUBMIT is not set and any one of the
* syncobjs in the array has a NULL fence, -EINVAL will be returned.
* Assuming the syncobj starts off with a NULL fence, this allows a client
* to do a host wait in one thread (or process) which waits on GPU work
* submitted in another thread (or process) without having to manually
* synchronize between the two.
* This requirement is inherited from the Vulkan fence API.
*
*
* Import/export of syncobjs
* -------------------------
*
* &DRM_IOCTL_SYNCOBJ_FD_TO_HANDLE and &DRM_IOCTL_SYNCOBJ_HANDLE_TO_FD
* provide two mechanisms for import/export of syncobjs.
*
* The first lets the client import or export an entire syncobj to a file
* descriptor.
* These fd's are opaque and have no other use case, except passing the
* syncobj between processes.
* All exported file descriptors and any syncobj handles created as a
* result of importing those file descriptors own a reference to the
* same underlying struct &drm_syncobj and the syncobj can be used
* persistently across all the processes with which it is shared.
* The syncobj is freed only once the last reference is dropped.
* Unlike dma-buf, importing a syncobj creates a new handle (with its own
* reference) for every import instead of de-duplicating.
* The primary use-case of this persistent import/export is for shared
* Vulkan fences and semaphores.
*
* The second import/export mechanism, which is indicated by
* &DRM_SYNCOBJ_FD_TO_HANDLE_FLAGS_IMPORT_SYNC_FILE or
* &DRM_SYNCOBJ_HANDLE_TO_FD_FLAGS_EXPORT_SYNC_FILE lets the client
* import/export the syncobj's current fence from/to a &sync_file.
* When a syncobj is exported to a sync file, that sync file wraps the
* sycnobj's fence at the time of export and any later signal or reset
* operations on the syncobj will not affect the exported sync file.
* When a sync file is imported into a syncobj, the syncobj's fence is set
* to the fence wrapped by that sync file.
* Because sync files are immutable, resetting or signaling the syncobj
* will not affect any sync files whose fences have been imported into the
* syncobj.
*/
#include <linux/anon_inodes.h>