linux-kernel - Re: [PATCH v3] drm/vblank: downgrade vblank wait timeout from WARN to error

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Message-ID: <5871956eef6c89f8361faae84c515c0e8356fc71@intel.com>
Date: Fri, 03 Oct 2025 14:11:15 +0300
From: Jani Nikula <jani.nikula@...ux.intel.com>
To: Chintan Patel <chintanlike@...il.com>,
 maarten.lankhorst@...ux.intel.com, maxime.ripard@...nel.org,
 tzimmermann@...e.de, airlied@...il.com, simona@...ll.ch
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 syzbot+147ba789658184f0ce04@...kaller.appspotmail.com, Chintan Patel
 <chintanlike@...il.com>
Subject: Re: [PATCH v3] drm/vblank: downgrade vblank wait timeout from WARN
 to error

On Thu, 02 Oct 2025, Chintan Patel <chintanlike@...il.com> 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 heavy scheduling pressure or in rare cases of delayed vblank
> handling, and are not always a kernel bug.
>
> Replace the WARN with drm_err() messages that report the timeout
> without crashing the system. Developers can still enable drm.debug
> to diagnose genuine problems.

No need to resend for this, can be fixed while applying, but the last
sentence is no longer meaningful with drm_err().

BR,
Jani.

>
> 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
>
> v3:
>  - Use drm_err() instead of drm_dbg_kms() (suggested by Ville Syrjälä)
>  - Remove unnecessary curr = drm_vblank_count() (suggested by Thomas Zimmermann)
>  - Fix commit message wording (“invalid userspace calls” → “delayed vblank handling”)
> ---
>  drivers/gpu/drm/drm_vblank.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/drm_vblank.c b/drivers/gpu/drm/drm_vblank.c
> index 46f59883183d..0664aea1b924 100644
> --- a/drivers/gpu/drm/drm_vblank.c
> +++ b/drivers/gpu/drm/drm_vblank.c
> @@ -1305,7 +1305,8 @@ 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);
> +	if (!ret)
> +		drm_err(dev, "vblank wait timed out on crtc %i\n", pipe);
>  
>  	drm_vblank_put(dev, pipe);
>  }

-- 
Jani Nikula, Intel