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([2601:1c0:5780:9200:326a:8939:a05c:b9d5]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-339c4a324ccsm516382a91.18.2025.10.02.19.54.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 02 Oct 2025 19:54:55 -0700 (PDT) Message-ID: <318392fd-9700-4570-bb39-31a4d08bd4a9@gmail.com> Date: Thu, 2 Oct 2025 19:54:55 -0700 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] drm/vblank: downgrade vblank wait timeout from WARN to debug To: =?UTF-8?B?VmlsbGUgU3lyasOkbMOk?= Cc: maarten.lankhorst@linux.intel.com, maxime.ripard@kernel.org, tzimmermann@suse.de, airlied@gmail.com, simona@ffwll.ch, dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org, syzbot+147ba789658184f0ce04@syzkaller.appspotmail.com References: <20251002025723.9430-1-chintanlike@gmail.com> Content-Language: en-US From: Chintan Patel In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit 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@syzkaller.appspotmail.com >> Closes: https://syzkaller.appspot.com/bug?extid=147ba789658184f0ce04 >> Tested-by: syzbot+147ba789658184f0ce04@syzkaller.appspotmail.com >> >> Signed-off-by: Chintan Patel >> >> 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