-----Original Message----- From: Imre Deak imre.deak@intel.com Sent: Tuesday, August 5, 2025 6:20 PM To: Dmitry Baryshkov dmitry.baryshkov@oss.qualcomm.com Cc: Huhulea, Nicusor Liviu (FT FDS CES LX PBU 1) nicusor.huhulea@siemens.com; stable@vger.kernel.org; dri- devel@lists.freedesktop.org; intel-gfx@lists.freedesktop.org; cip-dev@lists.cip- project.org; shradhagupta@linux.microsoft.com; jouni.hogander@intel.com; neil.armstrong@linaro.org; jani.nikula@linux.intel.com; maarten.lankhorst@linux.intel.com; mripard@kernel.org; tzimmermann@suse.de; airlied@gmail.com; daniel@ffwll.ch; joonas.lahtinen@linux.intel.com; rodrigo.vivi@intel.com; laurentiu.palcu@oss.nxp.com; Hombourger, Cedric (FT FDS CES LX) cedric.hombourger@siemens.com; Bobade, Shrikant Krishnarao (FT FDS CES LX PBU 1) shrikant.bobade@siemens.com Subject: Re: [PATCH] drm/probe-helper: fix output polling not resuming after HPD IRQ storm
On Tue, Aug 05, 2025 at 01:46:51PM +0300, Dmitry Baryshkov wrote:
On Mon, Aug 04, 2025 at 11:13:59PM +0300, Nicusor Huhulea wrote:
A regression in output polling was introduced by commit 4ad8d57d902fbc7c82507cfc1b031f3a07c3de6e ("drm: Check output polling initialized before disabling") in the 6.1.y stable
tree.
As a result, when the i915 driver detects an HPD IRQ storm and attempts to switch from IRQ-based hotplug detection to polling, output polling
fails to resume.
The root cause is the use of dev->mode_config.poll_running. Once poll_running is set (during the first connector detection) the calls to drm_kms_helper_poll_enable(), such as intel_hpd_irq_storm_switch_to_polling() fails to schedule output_poll_work as
expected.
Therefore, after an IRQ storm disables HPD IRQs, polling does not start,
breaking hotplug detection.
Why doesn't disable path use drm_kms_helper_poll_disable() ?
In general i915 doesn't disable polling as a whole after an IRQ storm and a 2 minute delay (or runtime resuming), since on some other connectors the polling may be still required.
Also, in the 6.1.y stable tree drm_kms_helper_poll_disable() is:
if (drm_WARN_ON(dev, !dev->mode_config.poll_enabled)) return; cancel_delayed_work_sync(&dev->mode_config.output_poll_work);so calling that wouldn't really fix the problem, which is clearly just an incorrect backport of the upstream commit 5abffb66d12bcac8 ("drm: Check output polling initialized before disabling") to the v6.1.y stable tree by commit 4ad8d57d902f ("drm: Check output polling initialized before disabling") in v6.1.y.
The upstream commit did not add the check for dev->mode_config.poll_running in drm_kms_helper_poll_enable(), the condition was only part of the diff context in the commit. Hence adding the condition in the 4ad8d57d902f backport commit was incorrect.
The fix is to remove the dev->mode_config.poll_running in the check condition, ensuring polling is always scheduled as requested.
I'm not a full-time kernel developer, but I spent some time trying to really understand both the rationale and the effects of commit "Check output polling initialized before disabling." Here's how I see the issue: That commit was introduced mainly as a defensive measure, to protect drivers such as hyperv-drm that do not initialize connector polling. In those drivers, calling drm_kms_helper_poll_disable() without proper polling setup could trigger warnings or even stack traces, since there are no outputs to poll and the polling helpers don't apply. From what I understand, the poll_running variable was added to prevent cases where polling was accidentally disabled for drivers that were never set up for it. However, this fix ended up creating a new class of breakage, which I have observed and describe here.
To me, the core logic should be simple: if polling is needed, then we should allow it to proceed (regardless of whether it's been scheduled previously).
Looking at the drm_kms_helper_poll_disable() if (drm_WARN_ON(dev, !dev->mode_config.poll_enabled)) return;
If the driver never enabled polling (that is, drm_kms_helper_poll_enable() was never called), then the disable operation is effectively a no-op-totally safe for drivers like hyperv-drm.
And in the enable function drm_kms_helper_poll_enable(..): if (drm_WARN_ON_ONCE(dev, !dev->mode_config.poll_enabled) || - !drm_kms_helper_poll || dev->mode_config.poll_running) + !drm_kms_helper_poll) return; The main thing being guarded here is whether polling has actually been initialized: 1.For polling drivers like i915, removing poll_running from the enable path is both safe and necessary: it fixes the regression with HPD polling after IRQ storms 2.For non-polling drivers like hyperv-drm, the existing checks on poll_enabled in both enable and disable functions are sufficient. Removing poll_running doesn't affect these drivers-they don't go through the polling code paths, so no polling gets scheduled or canceled by mistake
Therefore based on my understanding and testing removing poll_running guard not only fixes a real bug but also does not introduce new regressions for drivers that don't use output polling.
Nicu
Notes: Initial analysis, assumptions, device testing details, the correct fix and detailed rationale were discussed here https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flo re.kernel.org%2Fstable%2FaI32HUzrT95nS_H9%40ideak-
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-- With best wishes Dmitry