[Intel-gfx] [RFC] i915: Add fence fds to execbuffer2 uapi

[Chris Wilson]

On Mon, Jun 27, 2016 at 01:18:59PM -0700, Chad Versace wrote:
> On Mon 27 Jun 2016, Chad Versace wrote:
> > Let the hight 32 bits of drm_i915_gem_execbuffer2::rsvd1 contain an
> > input and/or output fence fd, whose presence is controlled by flags.
> > Also add I915_PARAM_HAS_FENCE_FD.
> > 
> > Signed-off-by: Chad Versace <chad.versace at intel.com>
> > ---
> >  include/uapi/drm/i915_drm.h | 24 ++++++++++++++++++++++--
> >  1 file changed, 22 insertions(+), 2 deletions(-)
> 
> Oops. git-send-email stripped the notes to the patch. Here's the notes:
> 
> This RFC proposes a uapi that integrates execbuf with Android sync fds.  Of
> course, this is *only* an RFC because other devs are working on the i915
> internals, and this patch depends on that work.

Why not just use the earlier patches for the uAPI as well?

> Why am I sending an RFC this early? I will soon begin prototyping Intel's
> Mesa implementation of EGL_ANDROID_native_fence_sync, and that prototype will
> be easier to write if I have a rough expectation of i915's eventual fence fd
> uapi.
> 
> Please provide feedback: Does this roughly look like the uapi that the i915
> devs expect?

Not quite. You have to use separate in/out dwords (i.e. rsvd2) in order
to ensure that we don't overwite the in-fence when dealing with error
paths (i.e. so that userspace can feed in the same execbuf parameters
following EINTR, and you don't have confusion between in/out parameters).
You have to also mark the ioctl as writing the new structures which is an
ABI break and so requires a new identifier (otherwise you break userspace
passing in the args from read-only memory).

Playind devil's advocate, an alternative to every driver implementing
their own fence extension for execbuf/cmdsubmit would be to add support
for explicit sync_fences to be added via dmabuf. (Instead of setting the
fence on the execbuf, you would set the fence on the batch buffer obj,
or surface of interest - though for CreateSync, it would have to be the
batch. Extracting the fence is then supplied by querying the batch buffer
dmabuf. It's not as explicit, but I suspect such uABI will be added to
dmabuf and will be required to be supported in the driver to handle
implicit fencing between PRIME anyway.) 
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre