drm/i915/fbc: don't flush for operations on the wrong frontbuffer

If frontbuffer_bits doesn't match the current frontbuffer, there's no
reason to recompress or update FBC.

There was a plan to make the FBC test suite catch this type of
problem, but it never got implemented due to being low priority.

While at it, also implement Ville's suggestion and use
plane->frontbuffer_bit instead of INTEL_FRONTBUFFER_PRIMARY.
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453210558-7875-8-git-send-email-paulo.r.zanoni@intel.com

drivers/gpu/drm/i915/intel_fbc.c

@@ -979,12 +979,19 @@ void intel_fbc_update(struct intel_crtc *crtc)
 	mutex_unlock(&fbc->lock);
 }
 
+static unsigned int intel_fbc_get_frontbuffer_bit(struct intel_fbc *fbc)
+{
+	if (fbc->enabled)
+		return to_intel_plane(fbc->crtc->base.primary)->frontbuffer_bit;
+	else
+		return fbc->possible_framebuffer_bits;
+}
+
 void intel_fbc_invalidate(struct drm_i915_private *dev_priv,
 			  unsigned int frontbuffer_bits,
 			  enum fb_op_origin origin)
 {
 	struct intel_fbc *fbc = &dev_priv->fbc;
-	unsigned int fbc_bits;
 
 	if (!fbc_supported(dev_priv))
 		return;
@@ -994,12 +1001,7 @@ void intel_fbc_invalidate(struct drm_i915_private *dev_priv,
 
 	mutex_lock(&fbc->lock);
 
-	if (fbc->enabled)
-		fbc_bits = INTEL_FRONTBUFFER_PRIMARY(fbc->crtc->pipe);
-	else
-		fbc_bits = fbc->possible_framebuffer_bits;
-
-	fbc->busy_bits |= (fbc_bits & frontbuffer_bits);
+	fbc->busy_bits |= intel_fbc_get_frontbuffer_bit(fbc) & frontbuffer_bits;
 
 	if (fbc->busy_bits)
 		__intel_fbc_deactivate(dev_priv);
@@ -1022,7 +1024,8 @@ void intel_fbc_flush(struct drm_i915_private *dev_priv,
 
 	fbc->busy_bits &= ~frontbuffer_bits;
 
-	if (!fbc->busy_bits && fbc->enabled) {
+	if (!fbc->busy_bits && fbc->enabled &&
+	    (frontbuffer_bits & intel_fbc_get_frontbuffer_bit(fbc))) {
 		if (fbc->active)
 			intel_fbc_recompress(dev_priv);
 		else