commit 498e720b96379d8ee9c294950a01534a73defcf3
Author: Daniel J Blueman <daniel.blueman@gmail.com>
Date:   Fri Jun 17 11:32:19 2011 -0700

    drm/i915: Fix gen6 (SNB) missed BLT ring interrupts.
    
    The failure appeared in dmesg as:
    
    [drm:i915_hangcheck_ring_idle] *ERROR* Hangcheck timer elapsed... blt
    ring idle [waiting on 35064155, at 35064155], missed IRQ?
    
    This works around that problem on by making the blitter command
    streamer write interrupt state to the Hardware Status Page when a
    MI_USER_INTERRUPT command is decoded, which appears to force the seqno
    out to memory before the interrupt happens.
    
    v1->v2: Moved to prior interrupt handler installation and RMW flags as
    per feedback.
    v2->v3: Removed RMW of flags (by anholt)
    
    Cc: stable@kernel.org
    Signed-off-by: Daniel J Blueman <daniel.blueman@gmail.com>
    Signed-off-by: Eric Anholt <eric@anholt.net>
    Tested-by: Chris Wilson <chris@chris-wilson.co.uk> [v1]
    Tested-by: Eric Anholt <eric@anholt.net> [v1,v3]
    	   (incidence of the bug with a testcase went from avg 2/1000 to
    	   0/12651 in the latest test run (plus more for v1))
    Tested-by: Kenneth Graunke <kenneth@whitecape.org> [v1]
    Tested-by: Robert Hooker <robert.hooker@canonical.com> [v1]
    Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=33394
    Signed-off-by: Dave Airlie <airlied@redhat.com>

diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index b9fafe3..9e34a1a 100644
--- a/drivers/gpu/drm/i915/i915_irq.c
+++ b/drivers/gpu/drm/i915/i915_irq.c
@@ -1740,6 +1740,16 @@ void ironlake_irq_preinstall(struct drm_device *dev)
 		INIT_WORK(&dev_priv->rps_work, gen6_pm_rps_work);
 
 	I915_WRITE(HWSTAM, 0xeffe);
+	if (IS_GEN6(dev)) {
+		/* Workaround stalls observed on Sandy Bridge GPUs by
+		 * making the blitter command streamer generate a
+		 * write to the Hardware Status Page for
+		 * MI_USER_INTERRUPT.  This appears to serialize the
+		 * previous seqno write out before the interrupt
+		 * happens.
+		 */
+		I915_WRITE(GEN6_BLITTER_HWSTAM, ~GEN6_BLITTER_USER_INTERRUPT);
+	}
 
 	/* XXX hotplug from PCH */