[Intel-gfx] [PATCH] drm: i915: Preserve old FBC status if update with no new planes

[Gabriel Krisman Bertazi]

If the atomic commit doesn't include any new plane, there is no need to
choose a new CRTC for FBC, and the intel_fbc_choose_crtc() will bail out
early.  Although, if the FBC setup failed beforehand for whatever reason,
we don't bail early, but we change the no_fbc_reason to "no suitable
CRTC for FBC", which simply hides the real reason why the FBC wasn't
initialized.  For that scenario, it is better that we simply keep the
old message in-place to make debugging easier.

A scenario where this happens is with the
igt at kms_frontbuffer_tracking@fbc-suspend testcase when executed on a
Haswell system with not enough stolen memory.  When enabling the CRTC,
the FBC setup will be correctly initialized to a specific CRTC, but
won't be enabled, since there is not enough memory.  The testcase will
then enable CRC checking, which requires a quirk for Haswell, which
issues a new atomic commit that doesn't update the planes.  Since that
update doesn't include any new planes (and the FBC wasn't enabled),
intel_fbc_choose_crtc() will not find any suitable CRTC, but update the
error message, hiding the lack of memory information, which is the
actual cause of the initialization failure.  As a result, this causes
that test to fail on Haswell.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=100020
Fixes: f7e9b004b8a3 ("drm/i915/fbc: inline intel_fbc_can_choose()")
Reported-by: Dorota Czaplejewicz <dorota.czaplejewicz at collabora.co.uk>
Signed-off-by: Gabriel Krisman Bertazi <krisman at collabora.co.uk>
Cc: Paulo Zanoni <paulo.r.zanoni at intel.com>
---
 drivers/gpu/drm/i915/intel_fbc.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/intel_fbc.c b/drivers/gpu/drm/i915/intel_fbc.c
index ded2add18b26..0c99c9b731ee 100644
--- a/drivers/gpu/drm/i915/intel_fbc.c
+++ b/drivers/gpu/drm/i915/intel_fbc.c
@@ -1045,6 +1045,7 @@ void intel_fbc_choose_crtc(struct drm_i915_private *dev_priv,
 	struct drm_plane *plane;
 	struct drm_plane_state *plane_state;
 	bool crtc_chosen = false;
+	bool new_planes = false;
 	int i;
 
 	mutex_lock(&fbc->lock);
@@ -1066,6 +1067,7 @@ void intel_fbc_choose_crtc(struct drm_i915_private *dev_priv,
 			to_intel_plane_state(plane_state);
 		struct intel_crtc_state *intel_crtc_state;
 		struct intel_crtc *crtc = to_intel_crtc(plane_state->crtc);
+		new_planes = true;
 
 		if (!intel_plane_state->base.visible)
 			continue;
@@ -1084,7 +1086,7 @@ void intel_fbc_choose_crtc(struct drm_i915_private *dev_priv,
 		break;
 	}
 
-	if (!crtc_chosen)
+	if (new_planes && !crtc_chosen)
 		fbc->no_fbc_reason = "no suitable CRTC for FBC";
 
 out:
-- 
2.11.0