Re: [PATCH v3] drm/atomic-helper: tear down HPD/polling in drm_atomic_helper_shutdown()

[Dmitry Baryshkov]

On Mon, Sep 21, 2026 at 04:49:52PM +0530, Mahadevan P wrote:
> drm_atomic_helper_shutdown() disables all CRTCs but leaves output
> polling and IRQ-driven hot-plug detection running. On reboot, a late
> DP hot-plug-detect (HPD) IRQ can fire after apps_smmu has already
> disabled translation for the display subsystem, causing the HPD
> thread to kick off a new modeset that drives DPU/DP hardware and DMA
> through a stale IOMMU mapping.
> 
> drm_atomic_helper_shutdown() disables all CRTCs first, but a pending
> HPD IRQ thread wakes up afterwards, reads the DPCD, and fires an
> unsolicited hotplug event that triggers a second atomic commit
> turning the display back on -- right as the IOMMU is disabling
> translation:
> 
>   systemd-shutdown[1]: Rebooting.
>   msm_dpu: drm_atomic_commit: committing (shutdown disabling CRTCs)
>   arm-smmu 3da0000.iommu: disabling translation
>   msm_dpu: drm_dp_read_dpcd_caps (late HPD IRQ thread wakes up)
>   msm_dpu: drm_sysfs_connector_hotplug_event: DP-1 hotplug event
>   msm_dpu: drm_client_modeset_probe: DP-1 found preferred mode
>   msm_dpu: drm_atomic_commit: committing (unsolicited, re-enables display)
>   dpu_crtc_commit_kickoff: crtc94 first commit
>   arm-smmu 15200000.iommu: disabling translation
> 
> Call drm_kms_helper_poll_disable() to tear this down: it stops the
> output poll worker and calls each connector's
> &drm_connector_helper_funcs.disable_hpd, which for HPD-capable bridges
> masks the interrupt in hardware and then waits for an in-flight HPD
> handler under bridge->hpd_mutex. Suspend the in-kernel clients as
> well, so that a hotplug event which still gets through is recorded in
> client->hotplug_pending instead of being probed and committed.
> 
> Reported on Qualcomm platforms such as lemans-evk and monaco-evk
> during reboot stress testing.
> 
> Assisted-by: LLM
> Signed-off-by: Mahadevan P <mahadevan.p@xxxxxxxxxxxxxxxx>


Acked-by: Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxxxxxxxx>

Though I'd really wait for somebody with deeper understanding of this
part to review it.



-- 
With best wishes
Dmitry