drm/i915: FBC off for ironlake and older, otherwise on by default

Make the default FBC behaviour chipset specific, allowing us to turn
it on by default for Ironlake and older where it has been seen to
cause trouble with screen updates.
Signed-off-by: Keith Packard <keithp@keithp.com>
Tested-by: Francis Moreau <francis.moro@gmail.com>

drivers/gpu/drm/i915/i915_drv.c

@@ -67,11 +67,11 @@ module_param_named(i915_enable_rc6, i915_enable_rc6, int, 0600);
 MODULE_PARM_DESC(i915_enable_rc6,
 		"Enable power-saving render C-state 6 (default: true)");
 
-unsigned int i915_enable_fbc __read_mostly = 1;
+unsigned int i915_enable_fbc __read_mostly = -1;
 module_param_named(i915_enable_fbc, i915_enable_fbc, int, 0600);
 MODULE_PARM_DESC(i915_enable_fbc,
 		"Enable frame buffer compression for power savings "
-		"(default: false)");
+		"(default: -1 (use per-chip default))");
 
 unsigned int i915_lvds_downclock __read_mostly = 0;
 module_param_named(lvds_downclock, i915_lvds_downclock, int, 0400);

drivers/gpu/drm/i915/intel_display.c

@@ -1799,6 +1799,7 @@ static void intel_update_fbc(struct drm_device *dev)
 	struct drm_framebuffer *fb;
 	struct intel_framebuffer *intel_fb;
 	struct drm_i915_gem_object *obj;
+	int enable_fbc;
 
 	DRM_DEBUG_KMS("\n");
 
@@ -1839,8 +1840,15 @@ static void intel_update_fbc(struct drm_device *dev)
 	intel_fb = to_intel_framebuffer(fb);
 	obj = intel_fb->obj;
 
-	if (!i915_enable_fbc) {
-		DRM_DEBUG_KMS("fbc disabled per module param (default off)\n");
+	enable_fbc = i915_enable_fbc;
+	if (enable_fbc < 0) {
+		DRM_DEBUG_KMS("fbc set to per-chip default\n");
+		enable_fbc = 1;
+		if (INTEL_INFO(dev)->gen <= 5)
+			enable_fbc = 0;
+	}
+	if (!enable_fbc) {
+		DRM_DEBUG_KMS("fbc disabled per module param\n");
 		dev_priv->no_fbc_reason = FBC_MODULE_PARAM;
 		goto out_disable;
 	}