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

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.