[Intel-gfx] [PATCH] drm/i915: Propagate fence->error across semaphores

[Chris Wilson]

Quoting Chris Wilson (2020-05-05 17:13:02)
> Replacing an inter-engine fence with a semaphore reduced the HW
> execution latency, but that comes at a cost. For normal fences, we are
> able to propagate the metadata such as errors along with the signaling.
> For semaphores, we are missing this error propagation so add it in the
> back channel we use to monitor the semaphore overload.
> 
> This raises a valid point on whether error propagation is sufficient in
> the semaphore case if it is coupled to a fatal error, such as EFAULT. It
> is not, and we should teach ourselves not to use a semaphore if we would
> chain up to an external fence whose error we must not ignore.
> 
> Fixes: ef4688497512 ("drm/i915: Propagate fence errors")
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Cc: Matthew Auld <matthew.auld at intel.com>
> ---
>  drivers/gpu/drm/i915/i915_request.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_request.c b/drivers/gpu/drm/i915/i915_request.c
> index 9c5de07db47d..96a8c7a1be73 100644
> --- a/drivers/gpu/drm/i915/i915_request.c
> +++ b/drivers/gpu/drm/i915/i915_request.c
> @@ -614,6 +614,9 @@ semaphore_notify(struct i915_sw_fence *fence, enum i915_sw_fence_notify state)
>  
>         switch (state) {
>         case FENCE_COMPLETE:
> +               if (unlikely(fence->error))
> +                       i915_request_set_error_once(rq, fence->error);

This is just horrible. I don't like it even as a hack.
-Chris