linux-kernel - Re: [PATCH v2] drm/vblank: downgrade vblank wait timeout from WARN to debug

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Message-ID: <aN5klZdl2MZdPSpU@intel.com>
Date: Thu, 2 Oct 2025 14:40:05 +0300
From: Ville Syrjälä <ville.syrjala@...ux.intel.com>
To: Chintan Patel <chintanlike@...il.com>
Cc: maarten.lankhorst@...ux.intel.com, maxime.ripard@...nel.org,
	tzimmermann@...e.de, airlied@...il.com, simona@...ll.ch,
	dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	syzbot+147ba789658184f0ce04@...kaller.appspotmail.com
Subject: Re: [PATCH v2] drm/vblank: downgrade vblank wait timeout from WARN
 to debug

On Wed, Oct 01, 2025 at 07:57:23PM -0700, Chintan Patel wrote:
> When wait_event_timeout() in drm_wait_one_vblank() times out, the
> current WARN can cause unnecessary kernel panics in environments
> with panic_on_warn set (e.g. CI, fuzzing). These timeouts can happen
> under scheduler pressure or from invalid userspace calls, so they are
> not always a kernel bug.

"invalid userspace calls" should never reach this far.
That would be a kernel bug.

> 
> Replace the WARN with drm_dbg_kms() messages that provide useful
> context (last and current vblank counters) without crashing the
> system. Developers can still enable drm.debug to diagnose genuine
> problems.
> 
> Reported-by: syzbot+147ba789658184f0ce04@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=147ba789658184f0ce04
> Tested-by: syzbot+147ba789658184f0ce04@...kaller.appspotmail.com
> 
> Signed-off-by: Chintan Patel <chintanlike@...il.com>
> 
> v2:
>  - Drop unnecessary in-code comment (suggested by Thomas Zimmermann)
>  - Remove else branch, only log timeout case
> ---
>  drivers/gpu/drm/drm_vblank.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
> index 46f59883183d..a94570668cba 100644
> --- a/drivers/gpu/drm/drm_vblank.c
> +++ b/drivers/gpu/drm/drm_vblank.c
> @@ -1289,7 +1289,7 @@ void drm_wait_one_vblank(struct drm_device *dev, unsigned int pipe)
>  {
>  	struct drm_vblank_crtc *vblank = drm_vblank_crtc(dev, pipe);
>  	int ret;
> -	u64 last;
> +	u64 last, curr;
>  
>  	if (drm_WARN_ON(dev, pipe >= dev->num_crtcs))
>  		return;
> @@ -1305,7 +1305,12 @@ void drm_wait_one_vblank(struct drm_device *dev, unsigned int pipe)
>  				 last != drm_vblank_count(dev, pipe),
>  				 msecs_to_jiffies(100));
>  
> -	drm_WARN(dev, ret == 0, "vblank wait timed out on crtc %i\n", pipe);
> +	curr = drm_vblank_count(dev, pipe);
> +
> +	if (ret == 0) {
> +		drm_dbg_kms(dev, "WAIT_VBLANK: timeout crtc=%d, last=%llu, curr=%llu\n",
> +			pipe, last, curr);

It should at the very least be a drm_err(). Though the backtrace can
be useful in figuring out where the problem is coming from, so not
too happy about this change.

> +	}
>  
>  	drm_vblank_put(dev, pipe);
>  }
> -- 
> 2.43.0

-- 
Ville Syrjälä
Intel