Re: [PATCH v4 38/38] drm/i915: Allow scheduler to manage inter-ring object synchronisation

Chris Wilson
2016-01-11T22:07:04Z

On Mon, Jan 11, 2016 at 06:43:07PM +0000, John.C.Harrison@Intel.com wrote:
> From: John Harrison <John.C.Harrison@Intel.com>
>
> The scheduler has always tracked batch buffer dependencies based on
> DRM object usage. This means that it will not submit a batch on one
> ring that has outstanding dependencies still executing on other rings.
> This is exactly the same synchronisation performed by
> i915_gem_object_sync() using hardware semaphores where available and
> CPU stalls where not (e.g. in execlist mode and/or on Gen8 hardware).
>
> Unfortunately, when a batch buffer is submitted to the driver the
> _object_sync() call happens first. Thus in case where hardware
> semaphores are disabled, the driver has already stalled until the
> dependency has been resolved.

But this should just add the dependency to the request in the scheduler
callback for i915_gem_object_sync_to, or better renamed as
i915_gem_request_submit_after. Without a scheduler we can do the
optimisation of doing that work inline, with a scheduler we can just
track the dependency.
-Chris

--
Chris Wilson, Intel Open Source Technology Centre