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

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Message-ID: <318392fd-9700-4570-bb39-31a4d08bd4a9@gmail.com>
Date: Thu, 2 Oct 2025 19:54:55 -0700
From: Chintan Patel <chintanlike@...il.com>
To: Ville Syrjälä <ville.syrjala@...ux.intel.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 10/2/25 04:40, Ville Syrjälä wrote:
> 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.


Thanks Ville for the feedback.I am still learning as I am new here!

You’re right, “invalid userspace calls” was a poor choice of wording —
I’ll drop that from the commit message. The main goal is to avoid
unnecessary panics in fuzzing/CI with panic_on_warn, while still
reporting the error clearly.

I’ll update the patch to use drm_err() instead of drm_dbg_kms(), and
drop the extra drm_vblank_count() call per Thomas’ earlier comment.

Best regards,
Chintan