drm/i915: Set persistent-mode for ILK/SNB framebuffer compression

Persistent mode is intended for use with front-buffer rendering, such as
X, where it is necessary to detect writes to the scanout either by the
GPU or through the CPU's fence, and recompress the dirty regions on the
fly. (By comparison to the back-buffer rendering, the scanout is always
recompressed after a page-flip.)

References: https://bugs.freedesktop.org/show_bug.cgi?id=33487
References: https://bugs.freedesktop.org/show_bug.cgi?id=31742
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Keith Packard <keithp@keithp.com>

drivers/gpu/drm/i915/i915_reg.h

@@ -579,6 +579,7 @@
 #define   DPFC_CTL_PLANEA	(0<<30)
 #define   DPFC_CTL_PLANEB	(1<<30)
 #define   DPFC_CTL_FENCE_EN	(1<<29)
+#define   DPFC_CTL_PERSISTENT_MODE	(1<<25)
 #define   DPFC_SR_EN		(1<<10)
 #define   DPFC_CTL_LIMIT_1X	(0<<6)
 #define   DPFC_CTL_LIMIT_2X	(1<<6)

drivers/gpu/drm/i915/intel_display.c

@@ -1581,6 +1581,8 @@ static void ironlake_enable_fbc(struct drm_crtc *crtc, unsigned long interval)
 
 	dpfc_ctl &= DPFC_RESERVED;
 	dpfc_ctl |= (plane | DPFC_CTL_LIMIT_1X);
+	/* Set persistent mode for front-buffer rendering, ala X. */
+	dpfc_ctl |= DPFC_CTL_PERSISTENT_MODE;
 	dpfc_ctl |= (DPFC_CTL_FENCE_EN | dev_priv->cfb_fence);
 	I915_WRITE(ILK_DPFC_CHICKEN, DPFC_HT_MODIFY);