drm/i915: handle FBC and self-refresh better

On 945, we need to avoid entering self-refresh if the compressor is
busy, or we may cause display FIFO underruns leading to ugly flicker.

Fixes fdo bug #24314, kernel bug #15043.

Tested-by: Alexander Lam <lambchop468@gmail.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Tested-by: Julien Cristau <jcristau@debian.org> (fd.o #25371)
Cc: stable@kernel.org
Signed-off-by: Eric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/i915_reg.h

@@ -338,6 +338,7 @@
 #define   FBC_CTL_PERIODIC	(1<<30)
 #define   FBC_CTL_INTERVAL_SHIFT (16)
 #define   FBC_CTL_UNCOMPRESSIBLE (1<<14)
+#define   FBC_C3_IDLE		(1<<13)
 #define   FBC_CTL_STRIDE_SHIFT	(5)
 #define   FBC_CTL_FENCENO	(1<<0)
 #define FBC_COMMAND		0x0320c

drivers/gpu/drm/i915/intel_display.c

@@ -1031,6 +1031,8 @@ static void i8xx_enable_fbc(struct drm_crtc *crtc, unsigned long interval)
 
 	/* enable it... */
 	fbc_ctl = FBC_CTL_EN | FBC_CTL_PERIODIC;
+	if (IS_I945GM(dev))
+		fbc_ctl |= FBC_C3_IDLE; /* 945 needs special SR handling */
 	fbc_ctl |= (dev_priv->cfb_pitch & 0xff) << FBC_CTL_STRIDE_SHIFT;
 	fbc_ctl |= (interval & 0x2fff) << FBC_CTL_INTERVAL_SHIFT;
 	if (obj_priv->tiling_mode != I915_TILING_NONE)