[PATCH] drm/i915/gen9: Disable FBC on planes with a misaligned Y-offset

[Imre Deak <imre.deak@intel.com>]

Enabling FBC on a plane having a Y-offset that isn't dividable by 4 may
cause pipe FIFO underruns and flickers, so disable FBC on such a config.

I tried to the followings to work around the issue:
- enable each HW work around in ILK_DPFC_CHICKEN
- disable each compression algorithm in ILK_DPFC_CONTROL
- disable low-power watermarks
None of the above got rid of the problem. I haven't found this issue in
the Bspec/WA database either.

Besides the igt testcase below (yet to be merged) an easy way to
reproduce the issue is to enable a plane with FBC and a plane Y-offset
not aligned to 4 and then just enable/disable FBC in a loop, keeping the
plane enabled.

I could trigger the problem on BXT/GLK/SKL, so assume for now that it's
only present on GEN9.

Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Testcase: igt/kms_plane/plane-clipping-pipe-A-planes
Signed-off-by: Imre Deak <imre.deak@intel.com>
---
 drivers/gpu/drm/i915/intel_fbc.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/gpu/drm/i915/intel_fbc.c b/drivers/gpu/drm/i915/intel_fbc.c
index 38b036c499d9..224c6a795f70 100644
--- a/drivers/gpu/drm/i915/intel_fbc.c
+++ b/drivers/gpu/drm/i915/intel_fbc.c
@@ -859,6 +859,17 @@ static bool intel_fbc_can_activate(struct intel_crtc *crtc)
 		return false;
 	}
 
+	/*
+	 * Work around a problem on GEN9 HW, where enabling FBC on a plane
+	 * having a Y offset that isn't dividable by 4 causes FIFO underrun
+	 * and screen ficker.
+	 */
+	if (INTEL_GEN(dev_priv) == 9 &&
+	    (fbc->state_cache.plane.adjusted_y & 3)) {
+		fbc->no_fbc_reason = "plane Y offset is misaligned";
+		return false;
+	}
+
 	return true;
 }
 
-- 
2.13.2

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