[PATCH v2 1/2] drm/i915/dsb: shift delayed-vblank DSL wait start by one scanline

[Ankit Nautiyal <ankit.k.nautiyal@intel.com>]

In intel_dsb_wait_for_delayed_vblank() the VRR path issues a
WAIT_DSL_OUT(safe_window_start, vmin_safe_window_end) followed by a
WAIT_USEC for SCL+1 scanlines to land on the delayed vblank.

Experimentally, DSB appears to observe a slightly stale PIPEDSL value.
When the actual scanline has just reached safe_window_start, the DSB
may still see a value at or before that boundary when WAIT_DSL_OUT is
evaluated, causing the wait to complete immediately.

As a result, the subsequent WAIT_USEC executes too early, and the
flip-done interrupt fires roughly one frame ahead of the delayed vblank.

Shift the scanline start back by one to ensure the wait window is
entered reliably.

v2: Replace explicit one-scanline delay with scanline boundary
    adjustment (safe_window_start - 1). (Ville)

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
---
 drivers/gpu/drm/i915/display/intel_dsb.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/display/intel_dsb.c b/drivers/gpu/drm/i915/display/intel_dsb.c
index fec8a56e21ea..07dd6318d9cc 100644
--- a/drivers/gpu/drm/i915/display/intel_dsb.c
+++ b/drivers/gpu/drm/i915/display/intel_dsb.c
@@ -902,9 +902,17 @@ void intel_dsb_wait_for_delayed_vblank(struct intel_atomic_state *state,
 		 * the hardware itself guarantees that we're SCL lines
 		 * away from the delayed vblank, and we won't be inside
 		 * the vmin safe window so this extra wait does nothing.
+		 *
+		 * Experimentally, DSB may observe a slightly stale
+		 * PIPEDSL value. When the actual scanline has just reached
+		 * safe_window_start, WAIT_DSL_OUT may complete immediately
+		 * due to the stale value.
+		 *
+		 * Shift the start back by one scanline to ensure the wait
+		 * window is entered reliably.
 		 */
 		intel_dsb_wait_scanline_out(state, dsb,
-					    intel_vrr_safe_window_start(crtc_state),
+					    intel_vrr_safe_window_start(crtc_state) - 1,
 					    intel_vrr_vmin_safe_window_end(crtc_state));
 		/*
 		 * When the push is sent during vblank it will trigger
-- 
2.45.2