drm/amd/display: Add USB-C DP Alt Mode lane limitation in DCN32

[Why]
USB-C DisplayPort Alt Mode with concurrent USB data needs lane count
limitation to prevent incorrect 4-lane DP configuration when only 2 lanes
are available due to hardware lane sharing between DP and USB3.

[How]
Query DMUB for Alt Mode status (is_dp_alt_disable, is_usb, is_dp4) in
dcn32_link_encoder_get_max_link_cap() and cap DP to 2 lanes when USB is
active on USB-C port. Added inline documentation explaining the USB-C
lane sharing constraint.

Reviewed-by: PeiChen Huang <peichen.huang@amd.com>
Signed-off-by: LinCheng Ku <lincheng.ku@amd.com>
Signed-off-by: Chenyu Chen <chen-yu.chen@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --git a/drivers/gpu/drm/amd/display/dc/dio/dcn32/dcn32_dio_link_encoder.c b/drivers/gpu/drm/amd/display/dc/dio/dcn32/dcn32_dio_link_encoder.c
index 44a4e3c4efb9c8a8edb70ba312ce3310ac08ef19..65d28cb07b0465e78601a453ea4b1cebceffa3bf 100644 (file)
--- a/drivers/gpu/drm/amd/display/dc/dio/dcn32/dcn32_dio_link_encoder.c
+++ b/drivers/gpu/drm/amd/display/dc/dio/dcn32/dcn32_dio_link_encoder.c
@@ -188,9 +188,18 @@ void dcn32_link_encoder_get_max_link_cap(struct link_encoder *enc,
        if (!query_dp_alt_from_dmub(enc, &cmd))
                return;
 
-       if (cmd.query_dp_alt.data.is_usb &&
-                       cmd.query_dp_alt.data.is_dp4 == 0)
-               link_settings->lane_count = MIN(LANE_COUNT_TWO, link_settings->lane_count);
+       /*
+        * USB-C DisplayPort Alt Mode lane count limitation logic:
+        * When USB and DP share the same USB-C connector, hardware must allocate
+        * some lanes for USB data, limiting DP to maximum 2 lanes instead of 4.
+        * This ensures USB functionality remains available while DP is active.
+        */
+       if (cmd.query_dp_alt.data.is_dp_alt_disable == 0 &&
+               cmd.query_dp_alt.data.is_usb &&
+               cmd.query_dp_alt.data.is_dp4 == 0) {
+               link_settings->lane_count =
+                       MIN(LANE_COUNT_TWO, link_settings->lane_count);
+       }
 }