drm/i915: Fix IRQ stall issue on Ironlake

The master irq control in DE must be disabled before irq handling,
and enable after the process. This fixes the irq stall issue on
Ironlake.

Cc: Stable Team <stable@kernel.org>
Signed-off-by: NZou, Nanhai <nanhai.zou@intel.com>
Signed-off-by: NZhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: NEric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/i915_irq.c

@@ -254,10 +254,15 @@ irqreturn_t igdng_irq_handler(struct drm_device *dev)
 {
 	drm_i915_private_t *dev_priv = (drm_i915_private_t *) dev->dev_private;
 	int ret = IRQ_NONE;
-	u32 de_iir, gt_iir;
+	u32 de_iir, gt_iir, de_ier;
 	u32 new_de_iir, new_gt_iir;
 	struct drm_i915_master_private *master_priv;
 
+	/* disable master interrupt before clearing iir  */
+	de_ier = I915_READ(DEIER);
+	I915_WRITE(DEIER, de_ier & ~DE_MASTER_IRQ_CONTROL);
+	(void)I915_READ(DEIER);
+
 	de_iir = I915_READ(DEIIR);
 	gt_iir = I915_READ(GTIIR);
 
@@ -290,6 +295,9 @@ irqreturn_t igdng_irq_handler(struct drm_device *dev)
 		gt_iir = new_gt_iir;
 	}
 
+	I915_WRITE(DEIER, de_ier);
+	(void)I915_READ(DEIER);
+
 	return ret;
 }