drm/i915: Clear VLV_IER around irq processing

On VLV/CHV the master interrupt enable bit only affects GT/PM
interrupts. Display interrupts are not affected by the master
irq control.

Also it seems that the CPU interrupt will only be generated when
the combined result of all GT/PM/display interrupts has a 0->1
edge. We already use the master interrupt enable bit to make sure
GT/PM interrupt can generate such an edge if we don't end up clearing
all IIR bits. We must do the same for display interrupts, and for
that we can simply clear out VLV_IER, and restore after we've acked
all the interrupts we are about to process.

So with both master interrupt enable and VLV_IER cleared out, we will
guarantee that there will be a 0->1 edge if any IIR bits are still set
at the end, and thus another CPU interrupt will be generated.

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: 579de73b048a ("drm/i915: Exit cherryview_irq_handler() after one pass")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1460571598-24452-6-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_irq.c

@@ -1778,7 +1778,7 @@ static irqreturn_t valleyview_irq_handler(int irq, void *arg)
 	disable_rpm_wakeref_asserts(dev_priv);
 
 	while (true) {
-		/* Find, clear, then process each source of interrupt */
+		u32 ier = 0;
 
 		gt_iir = I915_READ(GTIIR);
 		pm_iir = I915_READ(GEN6_PMIIR);
@@ -1789,7 +1789,22 @@ static irqreturn_t valleyview_irq_handler(int irq, void *arg)
 
 		ret = IRQ_HANDLED;
 
+		/*
+		 * Theory on interrupt generation, based on empirical evidence:
+		 *
+		 * x = ((VLV_IIR & VLV_IER) ||
+		 *      (((GT_IIR & GT_IER) || (GEN6_PMIIR & GEN6_PMIER)) &&
+		 *       (VLV_MASTER_IER & MASTER_INTERRUPT_ENABLE)));
+		 *
+		 * A CPU interrupt will only be raised when 'x' has a 0->1 edge.
+		 * Hence we clear MASTER_INTERRUPT_ENABLE and VLV_IER to
+		 * guarantee the CPU interrupt will be raised again even if we
+		 * don't end up clearing all the VLV_IIR, GT_IIR, GEN6_PMIIR
+		 * bits this time around.
+		 */
 		I915_WRITE(VLV_MASTER_IER, 0);
+		ier = I915_READ(VLV_IER);
+		I915_WRITE(VLV_IER, 0);
 
 		if (gt_iir)
 			I915_WRITE(GTIIR, gt_iir);
@@ -1815,6 +1830,7 @@ static irqreturn_t valleyview_irq_handler(int irq, void *arg)
 		if (iir)
 			I915_WRITE(VLV_IIR, iir);
 
+		I915_WRITE(VLV_IER, ier);
 		I915_WRITE(VLV_MASTER_IER, MASTER_INTERRUPT_ENABLE);
 		POSTING_READ(VLV_MASTER_IER);
 	}
@@ -1839,6 +1855,8 @@ static irqreturn_t cherryview_irq_handler(int irq, void *arg)
 	disable_rpm_wakeref_asserts(dev_priv);
 
 	do {
+		u32 ier = 0;
+
 		master_ctl = I915_READ(GEN8_MASTER_IRQ) & ~GEN8_MASTER_IRQ_CONTROL;
 		iir = I915_READ(VLV_IIR);
 
@@ -1847,7 +1865,22 @@ static irqreturn_t cherryview_irq_handler(int irq, void *arg)
 
 		ret = IRQ_HANDLED;
 
+		/*
+		 * Theory on interrupt generation, based on empirical evidence:
+		 *
+		 * x = ((VLV_IIR & VLV_IER) ||
+		 *      ((GEN8_MASTER_IRQ & ~GEN8_MASTER_IRQ_CONTROL) &&
+		 *       (GEN8_MASTER_IRQ & GEN8_MASTER_IRQ_CONTROL)));
+		 *
+		 * A CPU interrupt will only be raised when 'x' has a 0->1 edge.
+		 * Hence we clear GEN8_MASTER_IRQ_CONTROL and VLV_IER to
+		 * guarantee the CPU interrupt will be raised again even if we
+		 * don't end up clearing all the VLV_IIR and GEN8_MASTER_IRQ_CONTROL
+		 * bits this time around.
+		 */
 		I915_WRITE(GEN8_MASTER_IRQ, 0);
+		ier = I915_READ(VLV_IER);
+		I915_WRITE(VLV_IER, 0);
 
 		gen8_gt_irq_handler(dev_priv, master_ctl);
 
@@ -1865,6 +1898,7 @@ static irqreturn_t cherryview_irq_handler(int irq, void *arg)
 		if (iir)
 			I915_WRITE(VLV_IIR, iir);
 
+		I915_WRITE(VLV_IER, ier);
 		I915_WRITE(GEN8_MASTER_IRQ, GEN8_MASTER_IRQ_CONTROL);
 		POSTING_READ(GEN8_MASTER_IRQ);
 	} while (0);