linux-kernel - Re: [PATCH] drm/msm: Avoid rounding down to zero jiffies

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Date:   Tue, 28 Mar 2023 18:28:21 +0300
From:   Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
To:     Rob Clark <robdclark@...il.com>, dri-devel@...ts.freedesktop.org
Cc:     freedreno@...ts.freedesktop.org, linux-arm-msm@...r.kernel.org,
        Rob Clark <robdclark@...omium.org>,
        Abhinav Kumar <quic_abhinavk@...cinc.com>,
        Sean Paul <sean@...rly.run>, David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/msm: Avoid rounding down to zero jiffies

On 25/03/2023 00:00, Rob Clark wrote:
> From: Rob Clark <robdclark@...omium.org>
> 
> If userspace asked for a timeout greater than zero, but less than a
> jiffy, they clearly weren't planning on spinning.  So it is better
> to round up to one.
> 
> This fixes an issue with supertuxkart that was (for some reason)
> spinning on a gl sync with 1ms timeout.  CPU time for a demo lap
> drops from:
> 
>    15.83user 20.98system 0:47.46elapsed 77%CPU
> 
> to:
> 
>    8.84user 2.30system 0:46.67elapsed 23%CPU

Interesting. We potentially increased the timeout, but the overall 
(elapsed) time has decreased. Nevertheless:

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>

> 
> Signed-off-by: Rob Clark <robdclark@...omium.org>
> ---
>   drivers/gpu/drm/msm/msm_drv.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/msm/msm_drv.h b/drivers/gpu/drm/msm/msm_drv.h
> index 9f0c184b02a0..7936aa6cad03 100644
> --- a/drivers/gpu/drm/msm/msm_drv.h
> +++ b/drivers/gpu/drm/msm/msm_drv.h
> @@ -548,7 +548,7 @@ static inline unsigned long timeout_to_jiffies(const ktime_t *timeout)
>   		remaining_jiffies = ktime_divns(rem, NSEC_PER_SEC / HZ);
>   	}
>   
> -	return clamp(remaining_jiffies, 0LL, (s64)INT_MAX);
> +	return clamp(remaining_jiffies, 1LL, (s64)INT_MAX);
>   }
>   
>   /* Driver helpers */

-- 
With best wishes
Dmitry