[Intel-gfx] [PATCH 20/47] drm/i915/guc: Disable semaphores when using GuC scheduling

[John Harrison]

On 6/24/2021 00:04, Matthew Brost wrote:
> Semaphores are an optimization and not required for basic GuC submission
> to work properly. Disable until we have time to do the implementation to
> enable semaphores and tune them for performance. Also long direction is
> just to delete semaphores from the i915 so another reason to not enable
> these for GuC submission.
>
> v2: Reword commit message
>
> Cc: John Harrison <john.c.harrison at intel.com>
> Signed-off-by: Matthew Brost <matthew.brost at intel.com>
I think the commit description does not really match the patch content. 
The description is valid but the 'disable' is done by simply not setting 
the enable flag (done in the execlist back end and presumably not done 
in the GuC back end). However, what the patch is actually doing seems to 
be fixing bugs with the 'are semaphores enabled' mechanism. I.e. 
correcting pieces of code that used semaphores without checking if they 
are enabled. And presumably this would be broken if someone tried to 
disable semaphores in execlist mode for any reason?

So I think keeping the existing comment text is fine but something 
should be added to explain the actual changes.

John.


> ---
>   drivers/gpu/drm/i915/gem/i915_gem_context.c | 6 ++++--
>   1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gem/i915_gem_context.c b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> index 7720b8c22c81..5c07e6abf16a 100644
> --- a/drivers/gpu/drm/i915/gem/i915_gem_context.c
> +++ b/drivers/gpu/drm/i915/gem/i915_gem_context.c
> @@ -230,7 +230,8 @@ static void intel_context_set_gem(struct intel_context *ce,
>   		ce->timeline = intel_timeline_get(ctx->timeline);
>   
>   	if (ctx->sched.priority >= I915_PRIORITY_NORMAL &&
> -	    intel_engine_has_timeslices(ce->engine))
> +	    intel_engine_has_timeslices(ce->engine) &&
> +	    intel_engine_has_semaphores(ce->engine))
>   		__set_bit(CONTEXT_USE_SEMAPHORES, &ce->flags);
>   
>   	intel_context_set_watchdog_us(ce, ctx->watchdog.timeout_us);
> @@ -1938,7 +1939,8 @@ static int __apply_priority(struct intel_context *ce, void *arg)
>   	if (!intel_engine_has_timeslices(ce->engine))
>   		return 0;
>   
> -	if (ctx->sched.priority >= I915_PRIORITY_NORMAL)
> +	if (ctx->sched.priority >= I915_PRIORITY_NORMAL &&
> +	    intel_engine_has_semaphores(ce->engine))
>   		intel_context_set_use_semaphores(ce);
>   	else
>   		intel_context_clear_use_semaphores(ce);