commit f8b48b21cb7b587ad918d4ce8dcf28ed7ca0a631
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Fri Feb 17 11:07:29 2012 +0100

    Revert "drm/i915: enable forcewake voodoo also for gen6"
    
    Well, only partially, we keep the forcewake voodoo but reinstate the
    hwstam crap ...
    
    Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

diff --git a/drivers/gpu/drm/i915/i915_irq.c b/drivers/gpu/drm/i915/i915_irq.c
index afd4e03..909b81f 100644
--- a/drivers/gpu/drm/i915/i915_irq.c
+++ b/drivers/gpu/drm/i915/i915_irq.c
@@ -1815,6 +1815,18 @@ static void ironlake_irq_preinstall(struct drm_device *dev)
 
 	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);
+		I915_WRITE(GEN6_BSD_HWSTAM, ~GEN6_BSD_USER_INTERRUPT);
+	}
+
 	/* XXX hotplug from PCH */
 
 	I915_WRITE(DEIMR, 0xffffffff);