i915: make vbl interrupts work properly on i965g/gm hw.

This code is ported from the DRM git tree and allows the vblank interrupts
to function on the i965 hw. It also requires a change in Mesa's 965 driver
to actually use them.

[ Without this patch, my 965GM drops vblank interrupts  - Jesse ]
Signed-off-by: Dave Airlie <airlied@linux.ie>
Acked-by: Jesse Barnes <jesse.barnes@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/char/drm/i915_drv.h

@@ -210,6 +210,12 @@ extern int i915_wait_ring(struct drm_device * dev, int n, const char *caller);
 #define I915REG_INT_MASK_R 	0x020a8
 #define I915REG_INT_ENABLE_R	0x020a0
 
+#define I915REG_PIPEASTAT	0x70024
+#define I915REG_PIPEBSTAT	0x71024
+
+#define I915_VBLANK_INTERRUPT_ENABLE	(1UL<<17)
+#define I915_VBLANK_CLEAR		(1UL<<1)
+
 #define SRX_INDEX		0x3c4
 #define SRX_DATA		0x3c5
 #define SR01			1

drivers/char/drm/i915_irq.c

@@ -214,6 +214,10 @@ irqreturn_t i915_driver_irq_handler(DRM_IRQ_ARGS)
 	struct drm_device *dev = (struct drm_device *) arg;
 	drm_i915_private_t *dev_priv = (drm_i915_private_t *) dev->dev_private;
 	u16 temp;
+	u32 pipea_stats, pipeb_stats;
+
+	pipea_stats = I915_READ(I915REG_PIPEASTAT);
+	pipeb_stats = I915_READ(I915REG_PIPEBSTAT);
 
 	temp = I915_READ16(I915REG_INT_IDENTITY_R);
 
@@ -225,6 +229,8 @@ irqreturn_t i915_driver_irq_handler(DRM_IRQ_ARGS)
 		return IRQ_NONE;
 
 	I915_WRITE16(I915REG_INT_IDENTITY_R, temp);
+	(void) I915_READ16(I915REG_INT_IDENTITY_R);
+	DRM_READMEMORYBARRIER();
 
 	dev_priv->sarea_priv->last_dispatch = READ_BREADCRUMB(dev_priv);
 
@@ -252,6 +258,12 @@ irqreturn_t i915_driver_irq_handler(DRM_IRQ_ARGS)
 
 		if (dev_priv->swaps_pending > 0)
 			drm_locked_tasklet(dev, i915_vblank_tasklet);
+		I915_WRITE(I915REG_PIPEASTAT,
+			pipea_stats|I915_VBLANK_INTERRUPT_ENABLE|
+			I915_VBLANK_CLEAR);
+		I915_WRITE(I915REG_PIPEBSTAT,
+			pipeb_stats|I915_VBLANK_INTERRUPT_ENABLE|
+			I915_VBLANK_CLEAR);
 	}
 
 	return IRQ_HANDLED;