drm/amd/display: Fix pbn to kbps Conversion

[Why]
Existing routine has two conversion sequence,
pbn_to_kbps and kbps_to_pbn with margin.
Non of those has without-margin calculation.

kbps_to_pbn with margin conversion includes
fec overhead which has already been included in
pbn_div calculation with 0.994 factor considered.
It is a double counted fec overhead factor that causes
potential bw loss.

[How]
Add without-margin calculation.
Fix fec overhead double counted issue.

Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3735
Reviewed-by: Aurabindo Pillai <[e-mail]>
Signed-off-by: Fangzhi Zuo <[e-mail]>
Signed-off-by: Ivan Lipski <[e-mail]>
Tested-by: Dan Wheeler <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c

···
 };
 
 #if defined(CONFIG_DRM_AMD_DC_FP)
-static uint16_t get_fec_overhead_multiplier(struct dc_link *dc_link)
+static uint64_t kbps_to_pbn(int kbps, bool is_peak_pbn)
 {
-	u8 link_coding_cap;
-	uint16_t fec_overhead_multiplier_x1000 = PBN_FEC_OVERHEAD_MULTIPLIER_8B_10B;
+	uint64_t effective_kbps = (uint64_t)kbps;
 
-	link_coding_cap = dc_link_dp_mst_decide_link_encoding_format(dc_link);
-	if (link_coding_cap == DP_128b_132b_ENCODING)
-		fec_overhead_multiplier_x1000 = PBN_FEC_OVERHEAD_MULTIPLIER_128B_132B;
+	if (is_peak_pbn) {	// add 0.6% (1006/1000) overhead into effective kbps
+		effective_kbps *= 1006;
+		effective_kbps = div_u64(effective_kbps, 1000);
+	}
 
-	return fec_overhead_multiplier_x1000;
+	return (uint64_t) DIV64_U64_ROUND_UP(effective_kbps * 64, (54 * 8 * 1000));
 }
 
-static int kbps_to_peak_pbn(int kbps, uint16_t fec_overhead_multiplier_x1000)
+static uint32_t pbn_to_kbps(unsigned int pbn, bool with_margin)
 {
-	u64 peak_kbps = kbps;
+	uint64_t pbn_effective = (uint64_t)pbn;
 
-	peak_kbps *= 1006;
-	peak_kbps *= fec_overhead_multiplier_x1000;
-	peak_kbps = div_u64(peak_kbps, 1000 * 1000);
-	return (int) DIV64_U64_ROUND_UP(peak_kbps * 64, (54 * 8 * 1000));
+	if (with_margin)	// deduct 0.6% (994/1000) overhead from effective pbn
+		pbn_effective *= (1000000 / PEAK_FACTOR_X1000);
+	else
+		pbn_effective *= 1000;
+
+	return DIV_U64_ROUND_UP(pbn_effective * 8 * 54, 64);
 }
 
 static void set_dsc_configs_from_fairness_vars(struct dsc_mst_fairness_params *params,
···
 	dc_dsc_get_default_config_option(param.sink->ctx->dc, &dsc_options);
 	dsc_options.max_target_bpp_limit_override_x16 = drm_connector->display_info.max_dsc_bpp * 16;
 
-	kbps = div_u64((u64)pbn * 994 * 8 * 54, 64);
+	kbps = pbn_to_kbps(pbn, false);
 	dc_dsc_compute_config(
 			param.sink->ctx->dc->res_pool->dscs[0],
 			&param.sink->dsc_caps.dsc_dec_caps,
···
 	int link_timeslots_used;
 	int fair_pbn_alloc;
 	int ret = 0;
-	uint16_t fec_overhead_multiplier_x1000 = get_fec_overhead_multiplier(dc_link);
 
 	for (i = 0; i < count; i++) {
 		if (vars[i + k].dsc_enabled) {
 			initial_slack[i] =
-			kbps_to_peak_pbn(params[i].bw_range.max_kbps, fec_overhead_multiplier_x1000) - vars[i + k].pbn;
+			kbps_to_pbn(params[i].bw_range.max_kbps, false) - vars[i + k].pbn;
 			bpp_increased[i] = false;
 			remaining_to_increase += 1;
 		} else {
···
 	int next_index;
 	int remaining_to_try = 0;
 	int ret;
-	uint16_t fec_overhead_multiplier_x1000 = get_fec_overhead_multiplier(dc_link);
 	int var_pbn;
 
 	for (i = 0; i < count; i++) {
···
 
 		DRM_DEBUG_DRIVER("MST_DSC index #%d, try no compression\n", next_index);
 		var_pbn = vars[next_index].pbn;
-		vars[next_index].pbn = kbps_to_peak_pbn(params[next_index].bw_range.stream_kbps, fec_overhead_multiplier_x1000);
+		vars[next_index].pbn = kbps_to_pbn(params[next_index].bw_range.stream_kbps, true);
 		ret = drm_dp_atomic_find_time_slots(state,
 						    params[next_index].port->mgr,
 						    params[next_index].port,
···
 	int count = 0;
 	int i, k, ret;
 	bool debugfs_overwrite = false;
-	uint16_t fec_overhead_multiplier_x1000 = get_fec_overhead_multiplier(dc_link);
 	struct drm_connector_state *new_conn_state;
 
 	memset(params, 0, sizeof(params));
···
 	DRM_DEBUG_DRIVER("MST_DSC Try no compression\n");
 	for (i = 0; i < count; i++) {
 		vars[i + k].aconnector = params[i].aconnector;
-		vars[i + k].pbn = kbps_to_peak_pbn(params[i].bw_range.stream_kbps, fec_overhead_multiplier_x1000);
+		vars[i + k].pbn = kbps_to_pbn(params[i].bw_range.stream_kbps, false);
 		vars[i + k].dsc_enabled = false;
 		vars[i + k].bpp_x16 = 0;
 		ret = drm_dp_atomic_find_time_slots(state, params[i].port->mgr, params[i].port,
···
 	DRM_DEBUG_DRIVER("MST_DSC Try max compression\n");
 	for (i = 0; i < count; i++) {
 		if (params[i].compression_possible && params[i].clock_force_enable != DSC_CLK_FORCE_DISABLE) {
-			vars[i + k].pbn = kbps_to_peak_pbn(params[i].bw_range.min_kbps, fec_overhead_multiplier_x1000);
+			vars[i + k].pbn = kbps_to_pbn(params[i].bw_range.min_kbps, false);
 			vars[i + k].dsc_enabled = true;
 			vars[i + k].bpp_x16 = params[i].bw_range.min_target_bpp_x16;
 			ret = drm_dp_atomic_find_time_slots(state, params[i].port->mgr,
···
 			if (ret < 0)
 				return ret;
 		} else {
-			vars[i + k].pbn = kbps_to_peak_pbn(params[i].bw_range.stream_kbps, fec_overhead_multiplier_x1000);
+			vars[i + k].pbn = kbps_to_pbn(params[i].bw_range.stream_kbps, false);
 			vars[i + k].dsc_enabled = false;
 			vars[i + k].bpp_x16 = 0;
 			ret = drm_dp_atomic_find_time_slots(state, params[i].port->mgr,
···
 	return ret;
 }
 
-static uint32_t kbps_from_pbn(unsigned int pbn)
-{
-	uint64_t kbps = (uint64_t)pbn;
-
-	kbps *= (1000000 / PEAK_FACTOR_X1000);
-	kbps *= 8;
-	kbps *= 54;
-	kbps /= 64;
-
-	return (uint32_t)kbps;
-}
-
 static bool is_dsc_common_config_possible(struct dc_stream_state *stream,
 					  struct dc_dsc_bw_range *bw_range)
 {
···
 			dc_link_get_highest_encoding_format(stream->link));
 	cur_link_settings = stream->link->verified_link_cap;
 	root_link_bw_in_kbps = dc_link_bandwidth_kbps(aconnector->dc_link, &cur_link_settings);
-	virtual_channel_bw_in_kbps = kbps_from_pbn(aconnector->mst_output_port->full_pbn);
+	virtual_channel_bw_in_kbps = pbn_to_kbps(aconnector->mst_output_port->full_pbn, true);
 
 	/* pick the end to end bw bottleneck */
 	end_to_end_bw_in_kbps = min(root_link_bw_in_kbps, virtual_channel_bw_in_kbps);
···
 				immediate_upstream_port = aconnector->mst_output_port->parent->port_parent;
 
 			if (immediate_upstream_port) {
-				virtual_channel_bw_in_kbps = kbps_from_pbn(immediate_upstream_port->full_pbn);
+				virtual_channel_bw_in_kbps = pbn_to_kbps(immediate_upstream_port->full_pbn, true);
 				virtual_channel_bw_in_kbps = min(root_link_bw_in_kbps, virtual_channel_bw_in_kbps);
 			} else {
 				/* For topology LCT 1 case - only one mstb*/