drm/i915/vrr: Clamp guardband as per hardware and timing constraints

The maximum guardband value is constrained by two factors:
- The actual vblank length minus set context latency (SCL)
- The hardware register field width:
  - 8 bits for ICL/TGL (VRR_CTL_PIPELINE_FULL_MASK -> max 255)
  - 16 bits for ADL+ (XELPD_VRR_CTL_VRR_GUARDBAND_MASK -> max 65535)

Remove the #FIXME and clamp the guardband to the maximum allowed value.

v2:
- Use REG_FIELD_MAX(). (Ville)
- Separate out functions for intel_vrr_max_guardband(),
  intel_vrr_max_vblank_guardband(). (Ville)

v3:
- Fix Typo: Add the missing adjusted_mode->crtc_vdisplay in guardband
  computation. (Ville)
- Refactor intel_vrr_max_hw_guardband() and use else for consistency.
  (Ville)

v4:
- Drop max_guardband from intel_vrr_max_hw_guardband(). (Ville)

Signed-off-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com> (#v2)
Link: https://lore.kernel.org/r/20250924141542.3122126-9-ankit.k.nautiyal@intel.com

drivers/gpu/drm/i915/display/intel_vrr.c

@@ -409,6 +409,38 @@ intel_vrr_compute_config(struct intel_crtc_state *crtc_state,
 	}
 }
 
+static int
+intel_vrr_max_hw_guardband(const struct intel_crtc_state *crtc_state)
+{
+	struct intel_display *display = to_intel_display(crtc_state);
+	int max_pipeline_full = REG_FIELD_MAX(VRR_CTL_PIPELINE_FULL_MASK);
+
+	if (DISPLAY_VER(display) >= 13)
+		return REG_FIELD_MAX(XELPD_VRR_CTL_VRR_GUARDBAND_MASK);
+	else
+		return intel_vrr_pipeline_full_to_guardband(crtc_state,
+							    max_pipeline_full);
+}
+
+static int
+intel_vrr_max_vblank_guardband(const 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;
+
+	return crtc_state->vrr.vmin -
+	       adjusted_mode->crtc_vdisplay -
+	       crtc_state->set_context_latency -
+	       intel_vrr_extra_vblank_delay(display);
+}
+
+static int
+intel_vrr_max_guardband(struct intel_crtc_state *crtc_state)
+{
+	return min(intel_vrr_max_hw_guardband(crtc_state),
+		   intel_vrr_max_vblank_guardband(crtc_state));
+}
+
 void intel_vrr_compute_config_late(struct intel_crtc_state *crtc_state)
 {
 	struct intel_display *display = to_intel_display(crtc_state);
@@ -417,23 +449,14 @@ void intel_vrr_compute_config_late(struct intel_crtc_state *crtc_state)
 	if (!intel_vrr_possible(crtc_state))
 		return;
 
-	crtc_state->vrr.guardband =
+	crtc_state->vrr.guardband = min(crtc_state->vrr.vmin - adjusted_mode->crtc_vdisplay,
-		crtc_state->vrr.vmin -
+					intel_vrr_max_guardband(crtc_state));
-		adjusted_mode->crtc_vdisplay -
-		crtc_state->set_context_latency -
-		intel_vrr_extra_vblank_delay(display);
-
-	if (DISPLAY_VER(display) < 13) {
-		/* FIXME handle the limit in a proper way */
-		crtc_state->vrr.guardband =
-			min(crtc_state->vrr.guardband,
-			    intel_vrr_pipeline_full_to_guardband(crtc_state, 255));
 
+	if (DISPLAY_VER(display) < 13)
 		crtc_state->vrr.pipeline_full =
 			intel_vrr_guardband_to_pipeline_full(crtc_state,
 							     crtc_state->vrr.guardband);
 }
-}
 
 static u32 trans_vrr_ctl(const struct intel_crtc_state *crtc_state)
 {