drm/i915: Fix GT_MODE default value

commit f8f2ac9a upstream.

I can't even find how I figured this might be needed anymore. But sure
enough, the value I'm reading back on platforms doesn't match what the
docs recommends.

It seemed to fix Chris' GT1 in limited testing as well.
Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[bwh: Backported to 3.2: open-code _MASKED_BIT_{ENABLE,DISABLE}]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/gpu/drm/i915/i915_reg.h

@@ -401,6 +401,9 @@
 # define VS_TIMER_DISPATCH				(1 << 6)
 # define MI_FLUSH_ENABLE				(1 << 11)
 
+#define GEN6_GT_MODE	0x20d0
+#define   GEN6_GT_MODE_HI	(1 << 9)
+
 #define GFX_MODE	0x02520
 #define GFX_MODE_GEN7	0x0229c
 #define   GFX_RUN_LIST_ENABLE		(1<<15)

drivers/gpu/drm/i915/intel_display.c

@@ -8298,6 +8298,11 @@ static void gen6_init_clock_gating(struct drm_device *dev)
 			   DISPPLANE_TRICKLE_FEED_DISABLE);
 		intel_flush_display_plane(dev_priv, pipe);
 	}
+
+	/* The default value should be 0x200 according to docs, but the two
+	 * platforms I checked have a 0 for this. (Maybe BIOS overrides?) */
+	I915_WRITE(GEN6_GT_MODE, 0xffff << 16);
+	I915_WRITE(GEN6_GT_MODE, GEN6_GT_MODE_HI << 16 | GEN6_GT_MODE_HI);
 }
 
 static void gen7_setup_fixed_func_scheduler(struct drm_i915_private *dev_priv)