drm/i915: Fix false-positive assert_rpm_wakelock_held in i915_pmic_bus_access_notifier v2

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.

Changes in v2:
-Reword comment explaining why disabling the wakeref asserts is
 ok and necessary

Cc: stable@vger.kernel.org
Reported-by: FKr <bugs-freedesktop@ubermail.me>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171110150301.9601-2-hdegoede@redhat.com
(cherry picked from commit ce30560c)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_uncore.c

@@ -1240,8 +1240,15 @@ 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.
+		 *
+		 * The notifier is unregistered during intel_runtime_suspend(),
+		 * so it's ok to access the HW here without holding a RPM
+		 * wake reference -> disable wakeref asserts for the time of
+		 * the access.
 		 */
+		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);