drm/i915: fix the SDE irq dmesg warnings properly

We had the "The master control interrupt lied (SDE)!" check and error
message in place for a long time without any problems, until

commit aaf5ec2e
Author: Sonika Jindal <sonika.jindal@intel.com>
Date:   Wed Jul 8 17:07:47 2015 +0530

    drm/i915: Handle HPD when it has actually occurred

caused the errors to start happening. This was bisected and reported,
but the error message was silenced in

commit 97e5ed11
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Oct 23 10:56:12 2015 +0200

    drm/i915: shut up gen8+ SDE irq dmesg noise

shooting the messenger while the debugging for why Sonika's commit
triggered the errors was still in progress.

It looks like we need to read and acknowledge the PCH_PORT_HOTPLUG
register even though the hotplug trigger indicates there isn't a hotplug
irq to handle. The PCH doesn't seem to really ack the the interrupt to
the CPU unless we touch the hotplug register.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Sonika Jindal <sonika.jindal@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=92084
Fixes: aaf5ec2e ("drm/i915: Handle HPD when it has actually occurred")
[Jani: added a comment and amended the commit message while applying]
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448462843-32739-1-git-send-email-jani.nikula@intel.com

drivers/gpu/drm/i915/i915_irq.c

@@ -1824,8 +1824,24 @@ static void ibx_hpd_irq_handler(struct drm_device *dev, u32 hotplug_trigger,
 	struct drm_i915_private *dev_priv = to_i915(dev);
 	u32 dig_hotplug_reg, pin_mask = 0, long_mask = 0;
 
+	/*
+	 * Somehow the PCH doesn't seem to really ack the interrupt to the CPU
+	 * unless we touch the hotplug register, even if hotplug_trigger is
+	 * zero. Not acking leads to "The master control interrupt lied (SDE)!"
+	 * errors.
+	 */
 	dig_hotplug_reg = I915_READ(PCH_PORT_HOTPLUG);
+	if (!hotplug_trigger) {
+		u32 mask = PORTA_HOTPLUG_STATUS_MASK |
+			PORTD_HOTPLUG_STATUS_MASK |
+			PORTC_HOTPLUG_STATUS_MASK |
+			PORTB_HOTPLUG_STATUS_MASK;
+		dig_hotplug_reg &= ~mask;
+	}
+
 	I915_WRITE(PCH_PORT_HOTPLUG, dig_hotplug_reg);
+	if (!hotplug_trigger)
+		return;
 
 	intel_get_hpd_pins(&pin_mask, &long_mask, hotplug_trigger,
 			   dig_hotplug_reg, hpd,
@@ -1840,7 +1856,6 @@ static void ibx_irq_handler(struct drm_device *dev, u32 pch_iir)
 	int pipe;
 	u32 hotplug_trigger = pch_iir & SDE_HOTPLUG_MASK;
 
-	if (hotplug_trigger)
 	ibx_hpd_irq_handler(dev, hotplug_trigger, hpd_ibx);
 
 	if (pch_iir & SDE_AUDIO_POWER_MASK) {
@@ -1934,7 +1949,6 @@ static void cpt_irq_handler(struct drm_device *dev, u32 pch_iir)
 	int pipe;
 	u32 hotplug_trigger = pch_iir & SDE_HOTPLUG_MASK_CPT;
 
-	if (hotplug_trigger)
 	ibx_hpd_irq_handler(dev, hotplug_trigger, hpd_cpt);
 
 	if (pch_iir & SDE_AUDIO_POWER_MASK_CPT) {