[PATCH 6.12 251/567] Revert "drm/amd/display: Fix pbn to kbps Conversion"

6 Jan 2026

6.12-stable review patch.  If anyone has any objections, please let me know.
------------------
From: Mario Limonciello mario.limonciello@amd.com
commit 72e24456a54fe04710d89626cc5a88703e2f6202 upstream.
Deeply daisy chained DP/MST displays are no longer able to light
up. This reverts commit e0dec00f3d05 ("drm/amd/display: Fix pbn
to kbps Conversion")
Cc: Jerry Zuo jerry.zuo@amd.com
Reported-by: nat@nullable.se
Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/4756
Signed-off-by: Mario Limonciello mario.limonciello@amd.com
Acked-by: Alex Deucher alexander.deucher@amd.com
Signed-off-by: Alex Deucher alexander.deucher@amd.com
(cherry picked from commit e1c94109c76e8a77a21531bd53f6c63356c81158)
Cc: stable@vger.kernel.org # 6.17+
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, 36 insertions(+), 23 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
@@ -846,28 +846,26 @@ struct dsc_mst_fairness_params {
 };
#if defined(CONFIG_DRM_AMD_DC_FP)
-static uint64_t kbps_to_pbn(int kbps, bool is_peak_pbn)
+static uint16_t get_fec_overhead_multiplier(struct dc_link *dc_link)
 {
-	uint64_t effective_kbps = (uint64_t)kbps;
+	u8 link_coding_cap;
+	uint16_t fec_overhead_multiplier_x1000 = PBN_FEC_OVERHEAD_MULTIPLIER_8B_10B;
-	if (is_peak_pbn) {	// add 0.6% (1006/1000) overhead into effective kbps
-		effective_kbps *= 1006;
-		effective_kbps = div_u64(effective_kbps, 1000);
-	}
+	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;
-	return (uint64_t) DIV64_U64_ROUND_UP(effective_kbps * 64, (54 * 8 * 1000));
+	return fec_overhead_multiplier_x1000;
 }
-static uint32_t pbn_to_kbps(unsigned int pbn, bool with_margin)
+static int kbps_to_peak_pbn(int kbps, uint16_t fec_overhead_multiplier_x1000)
 {
-	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;
+	u64 peak_kbps = kbps;
-	return DIV_U64_ROUND_UP(pbn_effective * 8 * 54, 64);
+	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));
 }
static void set_dsc_configs_from_fairness_vars(struct dsc_mst_fairness_params *params,
@@ -938,7 +936,7 @@ 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 = pbn_to_kbps(pbn, false);
+	kbps = div_u64((u64)pbn * 994 * 8 * 54, 64);
    dc_dsc_compute_config(
    		param.sink->ctx->dc->res_pool->dscs[0],
    		&param.sink->dsc_caps.dsc_dec_caps,
@@ -967,11 +965,12 @@ 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_pbn(params[i].bw_range.max_kbps, false) - vars[i + k].pbn;
+			kbps_to_peak_pbn(params[i].bw_range.max_kbps, fec_overhead_multiplier_x1000) - vars[i + k].pbn;
    		bpp_increased[i] = false;
    		remaining_to_increase += 1;
    	} else {
@@ -1067,6 +1066,7 @@ 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++) {
@@ -1099,7 +1099,7 @@ 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_pbn(params[next_index].bw_range.stream_kbps, true);
+		vars[next_index].pbn = kbps_to_peak_pbn(params[next_index].bw_range.stream_kbps, fec_overhead_multiplier_x1000);
    	ret = drm_dp_atomic_find_time_slots(state,
    					    params[next_index].port->mgr,
    					    params[next_index].port,
@@ -1159,6 +1159,7 @@ 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));
@@ -1239,7 +1240,7 @@ 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_pbn(params[i].bw_range.stream_kbps, false);
+		vars[i + k].pbn = kbps_to_peak_pbn(params[i].bw_range.stream_kbps, fec_overhead_multiplier_x1000);
    	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,
@@ -1261,7 +1262,7 @@ 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_pbn(params[i].bw_range.min_kbps, false);
+			vars[i + k].pbn = kbps_to_peak_pbn(params[i].bw_range.min_kbps, fec_overhead_multiplier_x1000);
    		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,
@@ -1269,7 +1270,7 @@ static int compute_mst_dsc_configs_for_l
    		if (ret < 0)
    			return ret;
    	} else {
-			vars[i + k].pbn = kbps_to_pbn(params[i].bw_range.stream_kbps, false);
+			vars[i + k].pbn = kbps_to_peak_pbn(params[i].bw_range.stream_kbps, fec_overhead_multiplier_x1000);
    		vars[i + k].dsc_enabled = false;
    		vars[i + k].bpp_x16 = 0;
    		ret = drm_dp_atomic_find_time_slots(state, params[i].port->mgr,
@@ -1721,6 +1722,18 @@ 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)
 {
@@ -1812,7 +1825,7 @@ 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 = pbn_to_kbps(aconnector->mst_output_port->full_pbn, true);
+	virtual_channel_bw_in_kbps = kbps_from_pbn(aconnector->mst_output_port->full_pbn);
/* pick the end to end bw bottleneck */
    end_to_end_bw_in_kbps = min(root_link_bw_in_kbps, virtual_channel_bw_in_kbps);
@@ -1863,7 +1876,7 @@ 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 = pbn_to_kbps(immediate_upstream_port->full_pbn, true);
+				virtual_channel_bw_in_kbps = kbps_from_pbn(immediate_upstream_port->full_pbn);
    			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 6.12 251/567] Revert "drm/amd/display: Fix pbn to kbps Conversion"