drm/i915: Ack interrupts before handling them (GEN5 - GEN7)

Otherwise, we might receive a new interrupt before we have time to ack the first
one, eventually missing it.

According to BSPec, the right order should be:

1 - Disable Master Interrupt Control.
2 - Find the source(s) of the interrupt.
3 - Clear the Interrupt Identity bits (IIR).
4 - Process the interrupt(s) that had bits set in the IIRs.
5 - Re-enable Master Interrupt Control.

Without an atomic XCHG operation with mmio space, the above merely reduces the window
in which we can miss an interrupt (especially when you consider how heavyweight the
I915_READ/I915_WRITE operations are).

We maintain the "disable SDE interrupts when handling" hack since apparently it works.

Spotted by Bob Beckett <robert.beckett@intel.com>.

v2: Add warning to commit message and comments to the code as per Chris Wilson's request.
v3: Improve the source comments.
Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_irq.c

@@ -2139,6 +2139,14 @@ static void ivb_display_irq_handler(struct drm_device *dev, u32 de_iir)
 	}
 }
 
+/*
+ * To handle irqs with the minimum potential races with fresh interrupts, we:
+ * 1 - Disable Master Interrupt Control.
+ * 2 - Find the source(s) of the interrupt.
+ * 3 - Clear the Interrupt Identity bits (IIR).
+ * 4 - Process the interrupt(s) that had bits set in the IIRs.
+ * 5 - Re-enable Master Interrupt Control.
+ */
 static irqreturn_t ironlake_irq_handler(int irq, void *arg)
 {
 	struct drm_device *dev = arg;
@@ -2166,32 +2174,34 @@ static irqreturn_t ironlake_irq_handler(int irq, void *arg)
 		POSTING_READ(SDEIER);
 	}
 
+	/* Find, clear, then process each source of interrupt */
+
 	gt_iir = I915_READ(GTIIR);
 	if (gt_iir) {
+		I915_WRITE(GTIIR, gt_iir);
+		ret = IRQ_HANDLED;
 		if (INTEL_INFO(dev)->gen >= 6)
 			snb_gt_irq_handler(dev, dev_priv, gt_iir);
 		else
 			ilk_gt_irq_handler(dev, dev_priv, gt_iir);
-		I915_WRITE(GTIIR, gt_iir);
-		ret = IRQ_HANDLED;
 	}
 
 	de_iir = I915_READ(DEIIR);
 	if (de_iir) {
+		I915_WRITE(DEIIR, de_iir);
+		ret = IRQ_HANDLED;
 		if (INTEL_INFO(dev)->gen >= 7)
 			ivb_display_irq_handler(dev, de_iir);
 		else
 			ilk_display_irq_handler(dev, de_iir);
-		I915_WRITE(DEIIR, de_iir);
-		ret = IRQ_HANDLED;
 	}
 
 	if (INTEL_INFO(dev)->gen >= 6) {
 		u32 pm_iir = I915_READ(GEN6_PMIIR);
 		if (pm_iir) {
-			gen6_rps_irq_handler(dev_priv, pm_iir);
 			I915_WRITE(GEN6_PMIIR, pm_iir);
 			ret = IRQ_HANDLED;
+			gen6_rps_irq_handler(dev_priv, pm_iir);
 		}
 	}