linux-kernel - Re: [PATCH] drm/scheduler: Simplify spsc_queue_count check in drm_sched_entity_select

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Date:   Tue, 2 Oct 2018 14:56:49 +0000
From:   "Koenig, Christian" <Christian.Koenig@....com>
To:     Nathan Chancellor <natechancellor@...il.com>,
        David Airlie <airlied@...ux.ie>
CC:     "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "Deucher, Alexander" <Alexander.Deucher@....com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/scheduler: Simplify spsc_queue_count check in
 drm_sched_entity_select_rq

Am 02.10.2018 um 08:41 schrieb Nathan Chancellor:
> Clang generates a warning when it sees a logical not followed by a
> conditional operator like ==, >, or <.
>
> drivers/gpu/drm/scheduler/sched_entity.c:470:6: warning: logical not is
> only applied to the left hand side of this comparison
> [-Wlogical-not-parentheses]
>          if (!spsc_queue_count(&entity->job_queue) == 0 ||
>              ^                                     ~~
> drivers/gpu/drm/scheduler/sched_entity.c:470:6: note: add parentheses
> after the '!' to evaluate the comparison first
>          if (!spsc_queue_count(&entity->job_queue) == 0 ||
>              ^
>               (                                        )
> drivers/gpu/drm/scheduler/sched_entity.c:470:6: note: add parentheses
> around left hand side expression to silence this warning
>          if (!spsc_queue_count(&entity->job_queue) == 0 ||
>              ^
>              (                                    )
> 1 warning generated.
>
> It assumes the author might have made a mistake in their logic:
>
> if (!a == b) -> if (!(a == b))
>
> Sometimes that is the case; other times, it's just a super convoluted
> way of saying 'if (a)' when b = 0:
>
> if (!1 == 0) -> if (0 == 0) -> if (true)
>
> Alternatively:
>
> if (!1 == 0) -> if (!!1) -> if (1)
>
> Simplify this comparison so that Clang doesn't complain.
>
> Fixes: 35e160e781a0 ("drm/scheduler: change entities rq even earlier")
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>

Reviewed-by: Christian König <christian.koenig@....com>

Going to pick that up for pushing upstream as well.

Thanks,
Christian.

> ---
>   drivers/gpu/drm/scheduler/sched_entity.c | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/scheduler/sched_entity.c b/drivers/gpu/drm/scheduler/sched_entity.c
> index 4e5e95c0cab5..3e22a54a99c2 100644
> --- a/drivers/gpu/drm/scheduler/sched_entity.c
> +++ b/drivers/gpu/drm/scheduler/sched_entity.c
> @@ -467,8 +467,7 @@ void drm_sched_entity_select_rq(struct drm_sched_entity *entity)
>   	struct dma_fence *fence;
>   	struct drm_sched_rq *rq;
>   
> -	if (!spsc_queue_count(&entity->job_queue) == 0 ||
> -	    entity->num_rq_list <= 1)
> +	if (spsc_queue_count(&entity->job_queue) || entity->num_rq_list <= 1)
>   		return;
>   
>   	fence = READ_ONCE(entity->last_scheduled);