drm/i915: Apply B-spec mandated workaround for read flushes on Ironlake.

This is not known to fix any particular bugs we have, but the spec
says to do it, and the BIOS hadn't already set it up on my system.
Signed-off-by: NEric Anholt <eric@anholt.net>
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/i915_reg.h

@@ -316,6 +316,19 @@
 
 #define ERROR_GEN6	0x040a0
 
+/* GM45+ chicken bits -- debug workaround bits that may be required
+ * for various sorts of correct behavior.  The top 16 bits of each are
+ * the enables for writing to the corresponding low bit.
+ */
+#define _3D_CHICKEN	0x02084
+#define _3D_CHICKEN2	0x0208c
+/* Disables pipelining of read flushes past the SF-WIZ interface.
+ * Required on all Ironlake steppings according to the B-Spec, but the
+ * particular danger of not doing so is not specified.
+ */
+# define _3D_CHICKEN2_WM_READ_PIPELINED			(1 << 14)
+#define _3D_CHICKEN3	0x02090
+
 #define MI_MODE		0x0209c
 # define VS_TIMER_DISPATCH				(1 << 6)
 # define MI_FLUSH_ENABLE				(1 << 11)

drivers/gpu/drm/i915/intel_display.c

@@ -5818,6 +5818,12 @@ void intel_init_clock_gating(struct drm_device *dev)
 				   ILK_DPFC_DIS2 |
 				   ILK_CLK_FBC);
 		}
+
+		if (IS_GEN5(dev)) {
+			I915_WRITE(_3D_CHICKEN2,
+				   _3D_CHICKEN2_WM_READ_PIPELINED << 16 |
+				   _3D_CHICKEN2_WM_READ_PIPELINED);
+		}
 		return;
 	} else if (IS_G4X(dev)) {
 		uint32_t dspclk_gate;