[PATCH 7/8] drm/i915/vrr: Recompute vblank_start for platforms with always-on VRR TG

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

Currently, crtc_vblank_start is read from either the VBLANK_START register
(on pre-ADL platforms) or computed as vdisplay + set_context_latency in
intel_get_transcoder_timings().

This works when the entire vblank region after vdisplay is treated as
guardband, i.e.:
delayed vblank start = vdisplay + SCL = vtotal - guardband.

However, with optimized guardband, the guardband becomes shorter, and the
delayed vblank_start moves further away from vdisplay.

For platforms where intel_vrr_always_use_vrr_tg == false, the delayed
vblank start is only relevant in VRR mode. We retain the original
crtc_vblank_start and apply adjustments in VRR-specific paths. Evasion
logic, push clear already use vtotal - guardband directly for VRR case.

On platforms where intel_vrr_always_use_vrr_tg == true, the delayed
vblank_start is used in both fixed and VRR modes. So we need to change the
crtc_vblank_start so that fixed rr case works properly.

Therefore for the readout, we need to overwrite crtc_vblank_start
with vtotal - guardband to ensure correct behavior across both modes.

This change prepares the pipeline for optimized guardband usage by ensuring
crtc_vblank_start reflects the correct timing on platforms with always-on
VRR TG.

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
---
 drivers/gpu/drm/i915/display/intel_vrr.c | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/drivers/gpu/drm/i915/display/intel_vrr.c b/drivers/gpu/drm/i915/display/intel_vrr.c
index 8d71d7dc9d12..221b25832e56 100644
--- a/drivers/gpu/drm/i915/display/intel_vrr.c
+++ b/drivers/gpu/drm/i915/display/intel_vrr.c
@@ -821,6 +821,20 @@ void intel_vrr_get_config(struct intel_crtc_state *crtc_state)
 	 */
 	if (crtc_state->vrr.enable)
 		crtc_state->mode_flags |= I915_MODE_FLAG_VRR;
+
+	/*
+	 * For platforms that always use the VRR timing generator, we overwrite
+	 * crtc_vblank_start with vtotal - guardband to reflect the delayed
+	 * vblank start. This works for both default and optimized guardband values.
+	 * On other platforms, we keep the original value from
+	 * intel_get_transcoder_timings() and apply adjustments only in VRR-specific
+	 * paths as needed.
+	 */
+	if (intel_vrr_always_use_vrr_tg(display))
+		crtc_state->hw.adjusted_mode.crtc_vblank_start =
+			crtc_state->hw.adjusted_mode.crtc_vtotal -
+			crtc_state->vrr.guardband;
+
 }
 
 int intel_vrr_safe_window_start(const struct intel_crtc_state *crtc_state)
-- 
2.45.2