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

Thomas Zimmermann
2025-10-02T06:34:57Z

Hi

Am 02.10.25 um 04:57 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 scheduler pressure or from invalid userspace calls, so they are
> not always 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@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=147ba789658184f0ce04
> Tested-by: syzbot+147ba789658184f0ce04@syzkaller.appspotmail.com
>
> Signed-off-by: Chintan Patel <chintanlike@gmail.com>

There should be no empty lines among those tags

>
> 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);

Please don't call drm_vblank_count() here. It's not necessary for
regular operation. Simply keep the debug message as-is.

> +
> + if (ret == 0) {

"if (!ret)" is the preferred style.

> + drm_dbg_kms(dev, "WAIT_VBLANK: timeout crtc=%d, last=%llu, curr=%llu\n",
> + pipe, last, curr);

Aligning the pipe argument with dev from the previous line is the
preferred style.

Best regards
Thomas

> + }
>
> 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)