[PATCH v5 1/2] drm/dp/mst: recognize DP-to-HDMI PCON as virtual DPCD in DP-to-DP topology

[Xiao Lu <xiaolu.xie@intel.com>]

drm_dp_mst_is_virtual_dpcd() checks if a branch port qualifies as a
virtual DPCD device by looking for a downstream port with
Peer_Device_Type=3 (DP_PEER_DEVICE_SST_SINK).

Some PCON devices advertise their downstream HDMI sink port using
Peer_Device_Type=4 (DP_PEER_DEVICE_DP_LEGACY_CONV) with a non-zero
DPCD_Revision in the link address reply, indicating a virtual DPCD is
present (VESA SCR Option 2). The current check misses this case, so
passthrough_aux is never set and DSC passthrough modes are rejected.

Accept DP_PEER_DEVICE_DP_LEGACY_CONV as a valid downstream peer type
in the DP-to-DP branch so these PCON topologies are correctly
identified.

Signed-off-by: Xiao Lu <xiaolu.xie@intel.com>
---
 drivers/gpu/drm/display/drm_dp_mst_topology.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/display/drm_dp_mst_topology.c b/drivers/gpu/drm/display/drm_dp_mst_topology.c
index 4de36fda0..7d1128bcd 100644
--- a/drivers/gpu/drm/display/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/display/drm_dp_mst_topology.c
@@ -6045,7 +6045,8 @@ static bool drm_dp_mst_is_virtual_dpcd(struct drm_dp_mst_port *port)
 	    port->mstb &&
 	    port->mstb->num_ports == 2) {
 		list_for_each_entry(downstream_port, &port->mstb->ports, next) {
-			if (downstream_port->pdt == DP_PEER_DEVICE_SST_SINK &&
+			if ((downstream_port->pdt == DP_PEER_DEVICE_SST_SINK ||
+			     downstream_port->pdt == DP_PEER_DEVICE_DP_LEGACY_CONV) &&
 			    !downstream_port->input) {
 				mutex_unlock(&port->mgr->lock);
 				return true;
-- 
2.43.0