[Intel-gfx] [CI] drm/i915: Skip execlists_dequeue() early if the list is empty

[Michal Wajdeczko]

On Fri, Mar 17, 2017 at 11:27:22AM +0000, Chris Wilson wrote:
> Do an early read of the execlists' queue before we take the spinlock and
> start checking. This is safe as the first writer to the execlists queue
> will cause the tasklet to be run again after a memory barrier.
> 
> v2: Keep guc in sync with execlists queue changes
> 
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
> Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> Cc: Michał Winiarski <michal.winiarski at intel.com>
> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin at linux.intel.com>
> Reviewed-by: Michał Winiarski <michal.winiarski at intel.com>
> ---
>  drivers/gpu/drm/i915/i915_guc_submission.c | 4 ++++
>  drivers/gpu/drm/i915/intel_lrc.c           | 4 ++++
>  2 files changed, 8 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/i915_guc_submission.c b/drivers/gpu/drm/i915/i915_guc_submission.c
> index a3636b31ebc3..78718a8e70c3 100644
> --- a/drivers/gpu/drm/i915/i915_guc_submission.c
> +++ b/drivers/gpu/drm/i915/i915_guc_submission.c
> @@ -577,6 +577,10 @@ static bool i915_guc_dequeue(struct intel_engine_cs *engine)
>  	struct rb_node *rb;
>  	bool submit = false;
>  
> +	/* After execlist_first is updated, a new tasklet must be scheduled */

Hmm, I think this comment is litlle misleading when placed right to the "if" that do early return.
Either extend it with the info from the commit message, or move down.

-Michal

> +	if (!READ_ONCE(engine->execlist_first))
> +		return false;
> +
>  	spin_lock_irqsave(&engine->timeline->lock, flags);
>  	rb = engine->execlist_first;
>  	while (rb) {
> diff --git a/drivers/gpu/drm/i915/intel_lrc.c b/drivers/gpu/drm/i915/intel_lrc.c
> index becde55b02a3..7adc588d1a19 100644
> --- a/drivers/gpu/drm/i915/intel_lrc.c
> +++ b/drivers/gpu/drm/i915/intel_lrc.c
> @@ -403,6 +403,10 @@ static void execlists_dequeue(struct intel_engine_cs *engine)
>  	struct rb_node *rb;
>  	bool submit = false;
>  
> +	/* After execlist_first is updated, a new tasklet must be scheduled */
> +	if (!READ_ONCE(engine->execlist_first))
> +		return;
> +
>  	last = port->request;
>  	if (last)
>  		/* WaIdleLiteRestore:bdw,skl
> -- 
> 2.11.0
> 
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