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

[Lionel Landwerlin]

On 06/08/2019 19:19, Jason Ekstrand wrote:
> 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 at jlekstrand.net>

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin at intel.com>

> ---
>   drivers/gpu/drm/drm_syncobj.c | 98 +++++++++++++++++++++++++++++++----
>   1 file changed, 87 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
> index 1438dcb3ebb1..4b5c7b0ed714 100644
> --- a/drivers/gpu/drm/drm_syncobj.c
> +++ b/drivers/gpu/drm/drm_syncobj.c
> @@ -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.
> + * 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's can be waited upon, where it will wait for the underlying
> - * fence.
> + *  - 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
>    *
> - * 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.
> + * 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.
>    *
> - * Their primary use-case is to implement Vulkan fences and semaphores.
>    *
> - * 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.
> + * 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>