[Intel-gfx] [PATCH] drm: i915: Preserve old FBC status for update without 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 in the previous commit, if the
current commit doesn't include new plane update, it tries to overwrite
no_fbc_reason to "no suitable CRTC for FBC".  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, which should skip, to fail on Haswell.

Changes since v1:
  - Update commit message (Manasi)

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