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

[Lionel Landwerlin]

Thanks a lot for writing this :)
I now have a canvas to add stuff on!

Just a couple of questions below.

-Lionel

On 02/08/2019 20:40, 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.
> ---
>   drivers/gpu/drm/drm_syncobj.c | 81 +++++++++++++++++++++++++++++++----
>   1 file changed, 73 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
> index 1438dcb3ebb1..03cc888744fb 100644
> --- a/drivers/gpu/drm/drm_syncobj.c
> +++ b/drivers/gpu/drm/drm_syncobj.c
> @@ -29,17 +29,82 @@
>   /**
>    * 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
> + * 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.


I've been wondering whether we should call it a container for a 
synchronization primitive rather than a primitive itself.

Sync files are also a container, just we fewer features (no host 
operations).

It's not really important :)


> + * 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 a syncobj's fence to 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 simply a wrapper around a pointer to a struct
> + * &dma_fence which may be NULL.
> + * 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.
>    *
> - * Their primary use-case is to implement Vulkan fences and semaphores.


Maybe add a line about creation (and its signaled flag)?


> + *
> + * Host-side wait on syncobjs
> + * --------------------------
> + *
> + * The userspace syncobj API provides an ioctl for waiting on a set of
> + * syncobjs.
> + * The wait ioctl takes an array of syncobj handles and a flag indicating
> + * whether it should return immediately once one syncobj has been signaled
> + * or if it should wait for all the syncobjs to be signaled.
> + * Unlike the enqueued GPU work dependencies, the host-side wait ioctl
> + * will also optionally wait for the syncobj to first receive a fence and
> + * then wait on that fence.
> + * 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.


Should we list the flags & ioctl names?


> + *
> + *
> + * Import/export of syncobjs
> + * -------------------------
> + *
> + * The userspace syncobj API provides 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 syncobj handles which are created as the result of such an import
> + * operation refer to the same underlying syncobj as was used for the
> + * export and the syncobj can be used persistently across all the processes
> + * with which it is shared.
> + * Unlike dma-buf, importing a syncobj creates a new handle 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 lets the client export the syncobj's
> + * current fence to/from 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.
>    *


This 3 lines below seem to be redundant now?

Maybe rephrase with something like :


"Sharing a syncobj increases its refcount. The syncobj is destroyed when 
a client release the last reference."


>    * syncobj have a kref reference count, but also have an optional file.
>    * The file is only created once the syncobj is exported.