[Intel-gfx] [PATCH 1/2] drm/i915/display/fbc: Make fences a nice-to-have for GEN9+

["José Roberto de Souza" <jose.souza@intel.com>]

dGFX have local memory so it do not have aperture and do not support
CPU fences but even for iGFX it have a small number of fences.

As replacement for fences to track frontbuffer modifications by CPU
we have a software tracking that is already in used by FBC and PSR.
PSR don't support fences so it shows that this tracking is reliable.

So lets make fences a nice-to-have to activate FBC for GEN9+(as we
only have a good CI coverage for GEN9+), this will allow us to enable
FBC for dGFXs and iGFXs even when there is no available fence.

intel_fbc_hw_tracking_covers_screen() maybe can also have the same
treatment as fences but BSpec is not clear if the size limitation is
for hardware tracking or general use of FBC and I don't have a 5K
display to test it, so keeping as is for safety.

Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
---
 drivers/gpu/drm/i915/display/intel_fbc.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_fbc.c b/drivers/gpu/drm/i915/display/intel_fbc.c
index 2a3f1333c8ff..1f0d24a1dec1 100644
--- a/drivers/gpu/drm/i915/display/intel_fbc.c
+++ b/drivers/gpu/drm/i915/display/intel_fbc.c
@@ -717,11 +717,15 @@ static bool intel_fbc_can_activate(struct intel_crtc *crtc)
 		return false;
 	}
 
-	/* The use of a CPU fence is mandatory in order to detect writes
-	 * by the CPU to the scanout and trigger updates to the FBC.
+	/* The use of a CPU fence is one of two ways to detect writes by the
+	 * CPU to the scanout and trigger updates to the FBC.
+	 *
+	 * The other method is by software tracking(see
+	 * intel_fbc_invalidate/flush()), it will manually notify FBC and nuke
+	 * the current compressed buffer and recompress it.
 	 *
 	 * Note that is possible for a tiled surface to be unmappable (and
-	 * so have no fence associated with it) due to aperture constaints
+	 * so have no fence associated with it) due to aperture constraints
 	 * at the time of pinning.
 	 *
 	 * FIXME with 90/270 degree rotation we should use the fence on
@@ -730,7 +734,7 @@ static bool intel_fbc_can_activate(struct intel_crtc *crtc)
 	 * For now this will effecively disable FBC with 90/270 degree
 	 * rotation.
 	 */
-	if (cache->fence_id < 0) {
+	if (cache->fence_id < 0 && INTEL_GEN(dev_priv) < 9) {
 		fbc->no_fbc_reason = "framebuffer not tiled or fenced";
 		return false;
 	}
-- 
2.25.0

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