[PATCH 6.17 075/175] drm/amd/display: Fix pbn to kbps Conversion

27 Nov 2025

6.17-stable review patch.  If anyone has any objections, please let me know.
------------------
From: Fangzhi Zuo Jerry.Zuo@amd.com
commit 1788ef30725da53face7e311cdf62ad65fababcd upstream.
[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 aurabindo.pillai@amd.com
Signed-off-by: Fangzhi Zuo Jerry.Zuo@amd.com
Signed-off-by: Ivan Lipski ivan.lipski@amd.com
Tested-by: Dan Wheeler daniel.wheeler@amd.com
Signed-off-by: Alex Deucher alexander.deucher@amd.com
(cherry picked from commit e0dec00f3d05e8c0eceaaebfdca217f8d10d380c)
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
---
 drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c |   59 ++++--------
 1 file changed, 23 insertions(+), 36 deletions(-)

--- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c
+++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c
@@ -852,26 +852,28 @@ struct dsc_mst_fairness_params {
 };
#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;
+
+	if (with_margin)	// deduct 0.6% (994/1000) overhead from effective pbn
+		pbn_effective *= (1000000 / PEAK_FACTOR_X1000);
+	else
+		pbn_effective *= 1000;
-	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));
+	return DIV_U64_ROUND_UP(pbn_effective * 8 * 54, 64);
 }
static void set_dsc_configs_from_fairness_vars(struct dsc_mst_fairness_params *params,
@@ -942,7 +944,7 @@ static int bpp_x16_from_pbn(struct dsc_m
    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,
@@ -971,12 +973,11 @@ static int increase_dsc_bpp(struct drm_a
    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 {
@@ -1072,7 +1073,6 @@ static int try_disable_dsc(struct drm_at
    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++) {
@@ -1105,7 +1105,7 @@ static int try_disable_dsc(struct drm_at
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,
@@ -1165,7 +1165,6 @@ static int compute_mst_dsc_configs_for_l
    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));
@@ -1246,7 +1245,7 @@ static int compute_mst_dsc_configs_for_l
    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,
@@ -1268,7 +1267,7 @@ static int compute_mst_dsc_configs_for_l
    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,
@@ -1276,7 +1275,7 @@ static int compute_mst_dsc_configs_for_l
    		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,
@@ -1731,18 +1730,6 @@ clean_exit:
    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)
 {
@@ -1835,7 +1822,7 @@ enum dc_status dm_dp_mst_is_port_support
    		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);
@@ -1886,7 +1873,7 @@ enum dc_status dm_dp_mst_is_port_support
    			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*/

    

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[PATCH 6.17 075/175] drm/amd/display: Fix pbn to kbps Conversion