drm/amd/display: Require minimum VBlank size for stutter optimization

If the nominal VBlank is too small, optimizing for stutter can cause
the prefetch bandwidth to increase drasticaly, resulting in higher
clock and power requirements. Only optimize if it is >3x the stutter
latency.

Reviewed-by: Austin Zheng <[email protected]>
Signed-off-by: Dillon Varone <[email protected]>
Signed-off-by: Hamza Mahfooz <[email protected]>
Signed-off-by: Alex Deucher <[email protected]>

Author: Dillon Varone

drivers/gpu/drm/amd/display/dc/dml2/dml21/src/dml2_pmo/dml2_pmo_dcn4_fams2.c

@@ -8,6 +8,7 @@
 #include "dml2_pmo_dcn4_fams2.h"
 
 static const double MIN_VACTIVE_MARGIN_PCT = 0.25; // We need more than non-zero margin because DET buffer granularity can alter vactive latency hiding
+static const double MIN_BLANK_STUTTER_FACTOR = 3.0;
 
 static const struct dml2_pmo_pstate_strategy base_strategy_list_1_display[] = {
 	// VActive Preferred
@@ -2140,6 +2141,7 @@ bool pmo_dcn4_fams2_init_for_stutter(struct dml2_pmo_init_for_stutter_in_out *in
 	struct dml2_pmo_instance *pmo = in_out->instance;
 	bool stutter_period_meets_z8_eco = true;
 	bool z8_stutter_optimization_too_expensive = false;
+	bool stutter_optimization_too_expensive = false;
 	double line_time_us, vblank_nom_time_us;
 
 	unsigned int i;
@@ -2161,10 +2163,15 @@ bool pmo_dcn4_fams2_init_for_stutter(struct dml2_pmo_init_for_stutter_in_out *in
 		line_time_us = (double)in_out->base_display_config->display_config.stream_descriptors[i].timing.h_total / (in_out->base_display_config->display_config.stream_descriptors[i].timing.pixel_clock_khz * 1000) * 1000000;
 		vblank_nom_time_us = line_time_us * in_out->base_display_config->display_config.stream_descriptors[i].timing.vblank_nom;
 
-		if (vblank_nom_time_us < pmo->soc_bb->power_management_parameters.z8_stutter_exit_latency_us) {
+		if (vblank_nom_time_us < pmo->soc_bb->power_management_parameters.z8_stutter_exit_latency_us * MIN_BLANK_STUTTER_FACTOR) {
 			z8_stutter_optimization_too_expensive = true;
 			break;
 		}
+
+		if (vblank_nom_time_us < pmo->soc_bb->power_management_parameters.stutter_enter_plus_exit_latency_us * MIN_BLANK_STUTTER_FACTOR) {
+			stutter_optimization_too_expensive = true;
+			break;
+		}
 	}
 
 	pmo->scratch.pmo_dcn4.num_stutter_candidates = 0;
@@ -2180,7 +2187,7 @@ bool pmo_dcn4_fams2_init_for_stutter(struct dml2_pmo_init_for_stutter_in_out *in
 		pmo->scratch.pmo_dcn4.z8_vblank_optimizable = false;
 	}
 
-	if (pmo->soc_bb->power_management_parameters.stutter_enter_plus_exit_latency_us > 0) {
+	if (!stutter_optimization_too_expensive && pmo->soc_bb->power_management_parameters.stutter_enter_plus_exit_latency_us > 0) {
 		pmo->scratch.pmo_dcn4.optimal_vblank_reserved_time_for_stutter_us[pmo->scratch.pmo_dcn4.num_stutter_candidates] = (unsigned int)pmo->soc_bb->power_management_parameters.stutter_enter_plus_exit_latency_us;
 		pmo->scratch.pmo_dcn4.num_stutter_candidates++;
 	}