drm/i915: Replace vblank PM QoS with "Interrupt-Based AGPBUSY#"

I stumbled over this magic bit in the gen3 INSTPM:

Bit11 Interrupt-Based AGPBUSY# Enable:

‘0’ = Pending GMCH interrupts will not cause AGPBUSY# assertion.
‘1’ = Pending GMCH interrupts will cause AGPBUSY# assertion and hence
      can cause the CPU to exit C3.  There is no suppression of cacheable
      writes.

Note that in either case in C3 the interrupts are not lost. They will be
forwarded to the ICH when the GMCH is out of C3.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: stable@kernel.org

drivers/gpu/drm/i915/i915_irq.c

@@ -1353,7 +1353,12 @@ int i915_enable_vblank(struct drm_device *dev, int pipe)
 	else
 		i915_enable_pipestat(dev_priv, pipe,
 				     PIPE_VBLANK_INTERRUPT_ENABLE);
+
+	/* maintain vblank delivery even in deep C-states */
+	if (dev_priv->info->gen == 3)
+		I915_WRITE(INSTPM, INSTPM_AGPBUSY_DIS << 16);
 	spin_unlock_irqrestore(&dev_priv->irq_lock, irqflags);
+
 	return 0;
 }
 
@@ -1366,6 +1371,10 @@ void i915_disable_vblank(struct drm_device *dev, int pipe)
 	unsigned long irqflags;
 
 	spin_lock_irqsave(&dev_priv->irq_lock, irqflags);
+	if (dev_priv->info->gen == 3)
+		I915_WRITE(INSTPM,
+			   INSTPM_AGPBUSY_DIS << 16 | INSTPM_AGPBUSY_DIS);
+
 	if (HAS_PCH_SPLIT(dev))
 		ironlake_disable_display_irq(dev_priv, (pipe == 0) ?
 					     DE_PIPEA_VBLANK: DE_PIPEB_VBLANK);

drivers/gpu/drm/i915/i915_reg.h

@@ -405,6 +405,9 @@
 #define   I915_ERROR_INSTRUCTION			(1<<0)
 #define INSTPM	        0x020c0
 #define   INSTPM_SELF_EN (1<<12) /* 915GM only */
+#define   INSTPM_AGPBUSY_DIS (1<<11) /* gen3: when disabled, pending interrupts
+					will not assert AGPBUSY# and will only
+					be delivered when out of C3. */
 #define ACTHD	        0x020c8
 #define FW_BLC		0x020d8
 #define FW_BLC2		0x020dc