[Intel-gfx] [PATCH] drm/i915/fbc: FBC causes display flicker when VT-d is enabled on Skylake

[Chris Wilson]

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

v2: Log when the quirk is applied.
v3: Ensure i915.enable_fbc is false when !HAS_FBC()

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Paulo Zanoni <paulo.r.zanoni at intel.com>
Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
---

Paulo, I was under the impression you wanted to ack this as a known
issue?
-Chris

---
 drivers/gpu/drm/i915/intel_fbc.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/drivers/gpu/drm/i915/intel_fbc.c b/drivers/gpu/drm/i915/intel_fbc.c
index 06d3a86828fc..17343baf25ea 100644
--- a/drivers/gpu/drm/i915/intel_fbc.c
+++ b/drivers/gpu/drm/i915/intel_fbc.c
@@ -1240,12 +1240,28 @@ 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_wa(struct drm_i915_private *dev_priv)
+{
+#ifdef CONFIG_INTEL_IOMMU
+	/* WaFbcTurnOffFbcWhenHyperVisorIsUsed:skl */
+	if (intel_iommu_gfx_mapped && IS_SKYLAKE(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
@@ -1263,6 +1279,9 @@ void intel_fbc_init(struct drm_i915_private *dev_priv)
 	fbc->active = false;
 	fbc->work.scheduled = false;
 
+	if (need_fbc_wa(dev_priv))
+		mkwrite_intel_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);
 
-- 
2.8.1