[PATCH 6.19 143/844] drm/amd/display: Add USB-C DP Alt Mode lane limitation in DCN32

[Sasha Levin]

From: LinCheng Ku <lincheng.ku@xxxxxxx>

[ Upstream commit cea573a8e1ed83840a2173d153dd68e172849d44 ]

[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@xxxxxxx>
Signed-off-by: LinCheng Ku <lincheng.ku@xxxxxxx>
Signed-off-by: Chenyu Chen <chen-yu.chen@xxxxxxx>
Tested-by: Daniel Wheeler <daniel.wheeler@xxxxxxx>
Signed-off-by: Alex Deucher <alexander.deucher@xxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
---
 .../display/dc/dio/dcn32/dcn32_dio_link_encoder.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

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 06907e8a4eda1..ddc736af776c9 100644
--- 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);
+	}
 }
 
 
-- 
2.51.0