[Intel-gfx] [PATCH] drm/i915: Do a quick check on whether the fence is already signaled first

[Tvrtko Ursulin]

On 26/04/2017 11:15, Chris Wilson wrote:
> Now that we try to signal the fence from inside the interrupt handler,
> when we reach the signaler thread, the fence is most likely already
> signaled. Skip manipulating the bottom-half locks if this is so.
>
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> ---
>  drivers/gpu/drm/i915/intel_breadcrumbs.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/intel_breadcrumbs.c b/drivers/gpu/drm/i915/intel_breadcrumbs.c
> index 8f52fd5f6102..5f79c8135b3f 100644
> --- a/drivers/gpu/drm/i915/intel_breadcrumbs.c
> +++ b/drivers/gpu/drm/i915/intel_breadcrumbs.c
> @@ -616,9 +616,12 @@ static int intel_breadcrumbs_signaler(void *arg)
>  			request = i915_gem_request_get_rcu(request);
>  		rcu_read_unlock();
>  		if (signal_complete(request)) {
> -			local_bh_disable();
> -			dma_fence_signal(&request->fence);
> -			local_bh_enable(); /* kick start the tasklets */
> +			if (!test_bit(DMA_FENCE_FLAG_SIGNALED_BIT,
> +				      &request->fence.flags)) {
> +				local_bh_disable();
> +				dma_fence_signal(&request->fence);
> +				local_bh_enable(); /* kick start the tasklet */
> +			}
>
>  			spin_lock_irq(&b->rb_lock);
>
>

What are you referring to by "bottom-half locks" in the commit msg? 
AFAICS it would only skip kicking the tasklets with this change. That 
may be worth it, I haven't measured, but I don't see a difference wrt 
any locks.

In fact we could change this to:

	if (!dma_fence_signal(...)) {
		local_bh_disable();
		local_bh_enable();
	}

If we wanted to avoid touching the flags directly, but then would have a 
function call..

Regards,

Tvrtko