[PATCH v2 1/4] drm/i915: Fix false-positive assert_rpm_wakelock_held in i915_pmic_bus_access_notifier

[Imre Deak]

Hi,

On Thu, Jul 06, 2017 at 09:24:47PM +0200, Hans de Goede wrote:
> assert_rpm_wakelock_held is triggered from i915_pmic_bus_access_notifier
> even though it gets unregistered on (runtime) suspend, this is caused
> by a race happening under the following circumstances:
> 
> intel_runtime_pm_put does:
> 
>    atomic_dec(&dev_priv->pm.wakeref_count);
> 
>    pm_runtime_mark_last_busy(kdev);
>    pm_runtime_put_autosuspend(kdev);
> 
> And pm_runtime_put_autosuspend calls intel_runtime_suspend from
> a workqueue, so there is ample of time between the atomic_dec() and
> intel_runtime_suspend() unregistering the notifier. If the notifier
> gets called in this windowd assert_rpm_wakelock_held falsely triggers
> (at this point we're not runtime-suspended yet).
> 
> This commit adds disable_rpm_wakeref_asserts and
> enable_rpm_wakeref_asserts calls around the
> intel_uncore_forcewake_get(FORCEWAKE_ALL) call in
> i915_pmic_bus_access_notifier fixing the false-positive WARN_ON.
> 
> Reported-by: FKr <bugs-freedesktop at ubermail.me>
> Signed-off-by: Hans de Goede <hdegoede at redhat.com>
> ---
> Changes in v2:
> -Rebase on current (July 6th 2017) drm-next
> ---
>  drivers/gpu/drm/i915/intel_uncore.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/intel_uncore.c b/drivers/gpu/drm/i915/intel_uncore.c
> index 9882724bc2b6..168b28a87f76 100644
> --- a/drivers/gpu/drm/i915/intel_uncore.c
> +++ b/drivers/gpu/drm/i915/intel_uncore.c
> @@ -1171,8 +1171,15 @@ static int i915_pmic_bus_access_notifier(struct notifier_block *nb,
>  		 * bus, which will be busy after this notification, leading to:
>  		 * "render: timed out waiting for forcewake ack request."
>  		 * errors.
> +		 *
> +		 * This notifier may get called between intel_runtime_pm_put()
> +		 * doing atomic_dec(wakeref_count) and intel_runtime_resume()
> +		 * unregistering this notifier, which leads to false-positive
> +		 * assert_rpm_wakelock_held() triggering.

the following would describe better the reason for disabling wakeref asserts.
That is we access the HW without holding a runtime PM reference, but it's ok
here since it's handled as a special case during runtime suspend:

		* The notifier is unregistered during intel_runtime_suspend(),
		* so it's ok to access the HW here without holding an RPM
		* wake reference -> disable wakeref asserts for the time of
		* the access.

With that this looks ok:
Reviewed-by: Imre Deak <imre.deak at intel.com>


>  		 */
> +		disable_rpm_wakeref_asserts(dev_priv);
>  		intel_uncore_forcewake_get(dev_priv, FORCEWAKE_ALL);
> +		enable_rpm_wakeref_asserts(dev_priv);
>  		break;
>  	case MBI_PMIC_BUS_ACCESS_END:
>  		intel_uncore_forcewake_put(dev_priv, FORCEWAKE_ALL);
> -- 
> 2.13.0
>