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

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Message-ID: <afefc748-c466-4697-b8e6-7791b9ebbfed@suse.de>
Date: Thu, 23 Oct 2025 09:51:06 +0200
From: Thomas Zimmermann <tzimmermann@...e.de>
To: Chintan Patel <chintanlike@...il.com>, maarten.lankhorst@...ux.intel.com,
 maxime.ripard@...nel.org, airlied@...il.com, simona@...ll.ch
Cc: dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 syzbot+147ba789658184f0ce04@...kaller.appspotmail.com
Subject: Re: [PATCH v3] drm/vblank: downgrade vblank wait timeout from WARN to
 error

Hi

Am 03.10.25 um 05:23 schrieb Chintan Patel:
> 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.
>
> 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>

 From my side

Acked-by: Thomas Zimmermann <tzimmermann@...e.de>

See [1] for another such report

[1] 
https://lore.kernel.org/virtualization/202510221555.8c60c069-lkp@intel.com/T/#u

If there's also a way to raise priority of the vblank timer, we should 
explore that as well.

Best regards
Thomas

>
> 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);
>   }

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)