linux-kernel - Re: [PATCH] drm/msm: Fix fence rollover issue

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Message-ID: <991331d8-ddda-a816-d279-fdaed90b43c1@linaro.org>
Date:   Thu, 16 Jun 2022 11:27:39 +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@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/msm: Fix fence rollover issue

On 15/06/2022 19:24, Rob Clark wrote:
> From: Rob Clark <robdclark@...omium.org>
> 
> And while we are at it, let's start the fence counter close to the
> rollover point so that if issues slip in, they are more obvious.
> 
> Signed-off-by: Rob Clark <robdclark@...omium.org>

Should it also have

Fixes: fde5de6cb461 ("drm/msm: move fence code to it's own file")

Or maybe

Fixes: 5f3aee4ceb5b ("drm/msm: Handle fence rollover")

Otherwise:

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


> ---
>   drivers/gpu/drm/msm/msm_fence.c | 13 +++++++++++--
>   1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/msm/msm_fence.c b/drivers/gpu/drm/msm/msm_fence.c
> index 3df255402a33..a35a6746c7cd 100644
> --- a/drivers/gpu/drm/msm/msm_fence.c
> +++ b/drivers/gpu/drm/msm/msm_fence.c
> @@ -28,6 +28,14 @@ msm_fence_context_alloc(struct drm_device *dev, volatile uint32_t *fenceptr,
>   	fctx->fenceptr = fenceptr;
>   	spin_lock_init(&fctx->spinlock);
>   
> +	/*
> +	 * Start out close to the 32b fence rollover point, so we can
> +	 * catch bugs with fence comparisons.
> +	 */
> +	fctx->last_fence = 0xffffff00;
> +	fctx->completed_fence = fctx->last_fence;
> +	*fctx->fenceptr = fctx->last_fence;

This looks like a debugging hack. But probably it's fine to have it, as 
it wouldn't cause any side effects.

> +
>   	return fctx;
>   }
>   
> @@ -46,11 +54,12 @@ bool msm_fence_completed(struct msm_fence_context *fctx, uint32_t fence)
>   		(int32_t)(*fctx->fenceptr - fence) >= 0;
>   }
>   
> -/* called from workqueue */
> +/* called from irq handler */
>   void msm_update_fence(struct msm_fence_context *fctx, uint32_t fence)
>   {
>   	spin_lock(&fctx->spinlock);
> -	fctx->completed_fence = max(fence, fctx->completed_fence);
> +	if (fence_after(fence, fctx->completed_fence))
> +		fctx->completed_fence = fence;
>   	spin_unlock(&fctx->spinlock);
>   }
>   


-- 
With best wishes
Dmitry