[Intel-gfx] [PATCH 17/17] drm/i915: Support explicit fencing for execbuf

[Chris Wilson]

On Sun, Aug 28, 2016 at 09:46:24PM +0100, Chris Wilson wrote:
> +/* Setting I915_EXEC_FENCE_OUT causes the ioctl to return a sync_file fd
> + * in the upper_32_bits(rsvd2) upon success. Ownership of the fd is given
> + * to the caller, and it should be close() after use. (The fd is a regular
> + * file descriptor and will be cleaned up on process termination. It holds
> + * a reference to the request, but nothing else.)
> + *
> + * The sync_file fd can be combined with other sync_file and passed either
> + * to execbuf using I915_EXEC_FENCE_IN, to atomic KMS ioctls (so that a flip
> + * will only occur after this request completes), or to other devices.

Addendum:

Using I915_EXEC_FENCE_OUT requires use of
DRM_IOCTL_I915_GEM_EXECBUFFER2_WR ioctl so that the result is written
back to userspace. Failure to do so will cause the out-fence to always
be reported as zero, and the real fence fd to be leaked.

-- 
Chris Wilson, Intel Open Source Technology Centre