drm/i915/gen4: Extra CRT hotplug paranoia

Disable the CRT plug interrupt while doing the force cycle, explicitly
clear any CRT interrupt we may have generated, and restore when done.
Should mitigate interrupt storms from hotplug detection.
Signed-off-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Eric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/i915_reg.h

@@ -1067,7 +1067,6 @@
 #define CRT_HOTPLUG_DETECT_VOLTAGE_325MV	(0 << 2)
 #define CRT_HOTPLUG_DETECT_VOLTAGE_475MV	(1 << 2)
 #define CRT_HOTPLUG_MASK			(0x3fc) /* Bits 9-2 */
-#define CRT_FORCE_HOTPLUG_MASK			0xfffffe1f
 
 #define PORT_HOTPLUG_STAT	0x61114
 #define   HDMIB_HOTPLUG_INT_STATUS		(1 << 29)

drivers/gpu/drm/i915/intel_crt.c

@@ -217,7 +217,8 @@ static bool intel_crt_detect_hotplug(struct drm_connector *connector)
 {
 	struct drm_device *dev = connector->dev;
 	struct drm_i915_private *dev_priv = dev->dev_private;
-	u32 hotplug_en;
+	u32 hotplug_en, orig, stat;
+	bool ret = false;
 	int i, tries = 0;
 
 	if (HAS_PCH_SPLIT(dev))
@@ -232,8 +233,8 @@ static bool intel_crt_detect_hotplug(struct drm_connector *connector)
 		tries = 2;
 	else
 		tries = 1;
-	hotplug_en = I915_READ(PORT_HOTPLUG_EN);
-	hotplug_en &= CRT_FORCE_HOTPLUG_MASK;
+	hotplug_en = orig = I915_READ(PORT_HOTPLUG_EN);
+	hotplug_en &= CRT_HOTPLUG_MASK;
 	hotplug_en |= CRT_HOTPLUG_FORCE_DETECT;
 
 	if (IS_G4X(dev))
@@ -255,11 +256,17 @@ static bool intel_crt_detect_hotplug(struct drm_connector *connector)
 		} while (time_after(timeout, jiffies));
 	}
 
-	if ((I915_READ(PORT_HOTPLUG_STAT) & CRT_HOTPLUG_MONITOR_MASK) !=
-	    CRT_HOTPLUG_MONITOR_NONE)
-		return true;
+	stat = I915_READ(PORT_HOTPLUG_STAT);
+	if ((stat & CRT_HOTPLUG_MONITOR_MASK) != CRT_HOTPLUG_MONITOR_NONE)
+		ret = true;
 
-	return false;
+	/* clear the interrupt we just generated, if any */
+	I915_WRITE(PORT_HOTPLUG_STAT, CRT_HOTPLUG_INT_STATUS);
+
+	/* and put the bits back */
+	I915_WRITE(PORT_HOTPLUG_EN, orig);
+
+	return ret;
 }
 
 static bool intel_crt_detect_ddc(struct drm_encoder *encoder)