drm/i915/display: Add vblank_start adjustment logic for always-on VRR TG

As we move towards using a shorter, optimized guardband, we need to adjust
how the delayed vblank start is computed.

Adjust the crtc_vblank_start using Vmin Vtotal - guardband only when
intel_vrr_always_use_vrr_tg() is true. Also update the
pipe_mode->crtc_vblank_start which is derived from
adjusted_mode->crtc_vblank_start in intel_crtc_compute_pipe_mode().

To maintain consistency between the computed and readout paths, update
the readout logic in intel_vrr_get_config() to overwrite crtc_vblank_start
with the same value (vtotal - guardband) on platforms with always-on
VRR TG.

This also paves way for guardband optimization, by handling the movement of
the crtc_vblank_start for platforms that have VRR TG always active.

v2: Drop the helper and add the adjustment directly to
    intel_vrr_compute_guardband(). (Ville)
v3: Use adjusted_mode.crtc_vtotal instead of vmin and include the readout
    logic to keep the compute and readout paths in sync. (Ville)
v4: Also set pipe_mode->crtc_vblank_start as its derived from
    adjusted_mode. (Ville)
v5: Add a comment about rationale behind updating
    pipe_mode->crtc_vblank_start. (Ville)

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/20251016055415.2101347-10-ankit.k.nautiyal@intel.com

1 files changed, 26 insertions, 1 deletions

diff --git a/drivers/gpu/drm/i915/display/intel_vrr.c b/drivers/gpu/drm/i915/display/intel_vrr.c
index 8d71d7dc9d12..597008a6c744 100644
--- a/drivers/gpu/drm/i915/display/intel_vrr.c
+++ b/drivers/gpu/drm/i915/display/intel_vrr.c
@@ -436,7 +436,8 @@ intel_vrr_max_guardband(struct intel_crtc_state *crtc_state)
 void intel_vrr_compute_guardband(struct intel_crtc_state *crtc_state)
 {
 	struct intel_display *display = to_intel_display(crtc_state);
-	const struct drm_display_mode *adjusted_mode = &crtc_state->hw.adjusted_mode;
+	struct drm_display_mode *adjusted_mode = &crtc_state->hw.adjusted_mode;
+	struct drm_display_mode *pipe_mode = &crtc_state->hw.pipe_mode;
 
 	if (!intel_vrr_possible(crtc_state))
 		return;
@@ -444,6 +445,17 @@ void intel_vrr_compute_guardband(struct intel_crtc_state *crtc_state)
 	crtc_state->vrr.guardband = min(crtc_state->vrr.vmin - adjusted_mode->crtc_vdisplay,
 					intel_vrr_max_guardband(crtc_state));
 
+	if (intel_vrr_always_use_vrr_tg(display)) {
+		adjusted_mode->crtc_vblank_start  =
+			adjusted_mode->crtc_vtotal - crtc_state->vrr.guardband;
+		/*
+		 * pipe_mode has already been derived from the
+		 * original adjusted_mode, keep the two in sync.
+		 */
+		pipe_mode->crtc_vblank_start =
+			adjusted_mode->crtc_vblank_start;
+	}
+
 	if (DISPLAY_VER(display) < 13)
 		crtc_state->vrr.pipeline_full =
 			intel_vrr_guardband_to_pipeline_full(crtc_state,
@@ -821,6 +833,19 @@ 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)