drm/i915/fbc: sanitize i915.enable_fbc during FBC init

The DDX driver changes its behavior depending on the value it reads
from i915.enable_fbc, so sanitize the value in order to allow it to
know what's going on. It uses this in order to choose the defaults for
the TearFree option. Before this patch, it would read -1 and always
assume that FBC was disabled, so it wouldn't force TearFree.

v2: Extract intel_sanitize_fbc_option() (Chris).
v3: Rebase.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1460574069-14005-1-git-send-email-paulo.r.zanoni@intel.com

drivers/gpu/drm/i915/intel_fbc.c

@@ -818,20 +818,14 @@ static bool intel_fbc_can_choose(struct intel_crtc *crtc)
 {
 	struct drm_i915_private *dev_priv = crtc->base.dev->dev_private;
 	struct intel_fbc *fbc = &dev_priv->fbc;
-	bool enable_by_default = IS_BROADWELL(dev_priv);
 
 	if (intel_vgpu_active(dev_priv)) {
 		fbc->no_fbc_reason = "VGPU is active";
 		return false;
 	}
 
-	if (i915.enable_fbc < 0 && !enable_by_default) {
-		fbc->no_fbc_reason = "disabled per chip default";
-		return false;
-	}
-
 	if (!i915.enable_fbc) {
-		fbc->no_fbc_reason = "disabled per module param";
+		fbc->no_fbc_reason = "disabled per module param or by default";
 		return false;
 	}
 
@@ -1220,6 +1214,26 @@ void intel_fbc_init_pipe_state(struct drm_i915_private *dev_priv)
 			dev_priv->fbc.visible_pipes_mask |= (1 << crtc->pipe);
 }
 
+/*
+ * The DDX driver changes its behavior depending on the value it reads from
+ * i915.enable_fbc, so sanitize it by translating the default value into either
+ * 0 or 1 in order to allow it to know what's going on.
+ *
+ * Notice that this is done at driver initialization and we still allow user
+ * space to change the value during runtime without sanitizing it again. IGT
+ * relies on being able to change i915.enable_fbc at runtime.
+ */
+static int intel_sanitize_fbc_option(struct drm_i915_private *dev_priv)
+{
+	if (i915.enable_fbc >= 0)
+		return !!i915.enable_fbc;
+
+	if (IS_BROADWELL(dev_priv))
+		return 1;
+
+	return 0;
+}
+
 /**
  * intel_fbc_init - Initialize FBC
  * @dev_priv: the i915 device
@@ -1237,6 +1251,9 @@ void intel_fbc_init(struct drm_i915_private *dev_priv)
 	fbc->active = false;
 	fbc->work.scheduled = false;
 
+	i915.enable_fbc = intel_sanitize_fbc_option(dev_priv);
+	DRM_DEBUG_KMS("Sanitized enable_fbc value: %d\n", i915.enable_fbc);
+
 	if (!HAS_FBC(dev_priv)) {
 		fbc->no_fbc_reason = "unsupported by this chipset";
 		return;