drm/i915/fbc: FBC causes display flicker when VT-d is enabled on Skylake

Erratum SKL075: Display Flicker May Occur When Both VT-d And FBC Are Enabled

"Display flickering may occur when both FBC (Frame Buffer Compression)
and VT - d (Intel® Virtualization Technology for Directed I/O) are enabled
and in use by the display controller."

Ville found the w/a name in the database:
WaFbcTurnOffFbcWhenHyperVisorIsUsed:skl,bxt and also dug out that it
affects Broxton.

v2: Log when the quirk is applied.
v3: Ensure i915.enable_fbc is false when !HAS_FBC()
v4: Fix function name after rebase
v5: Add Broxton to the workaround

Note for backporting to stable, we need to add
  #define mkwrite_device_info(ptr) \
	((struct intel_device_info *)INTEL_INFO(ptr))
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: stable@vger.kernel.org
Link: http://patchwork.freedesktop.org/patch/msgid/1470296633-20388-1-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_fbc.c

@@ -1229,12 +1229,29 @@ static int intel_sanitize_fbc_option(struct drm_i915_private *dev_priv)
 	if (i915.enable_fbc >= 0)
 		return !!i915.enable_fbc;
 
+	if (!HAS_FBC(dev_priv))
+		return 0;
+
 	if (IS_BROADWELL(dev_priv))
 		return 1;
 
 	return 0;
 }
 
+static bool need_fbc_vtd_wa(struct drm_i915_private *dev_priv)
+{
+#ifdef CONFIG_INTEL_IOMMU
+	/* WaFbcTurnOffFbcWhenHyperVisorIsUsed:skl,bxt */
+	if (intel_iommu_gfx_mapped &&
+	    (IS_SKYLAKE(dev_priv) || IS_BROXTON(dev_priv))) {
+		DRM_INFO("Disabling framebuffer compression (FBC) to prevent screen flicker with VT-d enabled\n");
+		return true;
+	}
+#endif
+
+	return false;
+}
+
 /**
  * intel_fbc_init - Initialize FBC
  * @dev_priv: the i915 device
@@ -1252,6 +1269,9 @@ void intel_fbc_init(struct drm_i915_private *dev_priv)
 	fbc->active = false;
 	fbc->work.scheduled = false;
 
+	if (need_fbc_vtd_wa(dev_priv))
+		mkwrite_device_info(dev_priv)->has_fbc = false;
+
 	i915.enable_fbc = intel_sanitize_fbc_option(dev_priv);
 	DRM_DEBUG_KMS("Sanitized enable_fbc value: %d\n", i915.enable_fbc);