usb: typec: altmode/displayport: set displayport signaling rate in configure message

dp_altmode_configure sets the signaling rate to the current
configuration's rate and then shifts the value to the Select
Configuration bitfield. On the initial configuration, dp->data.conf
is 0 to begin with, so the signaling rate field is never set, which
leads to some DisplayPort Alt Mode partners sending NAK to the
Configure message.

Set the signaling rate to the capabilities supported by both the
port and the port partner. If the cable supports DisplayPort Alt Mode,
then include its capabilities as well.

Fixes: a17fae8 ("usb: typec: Add Displayport Alternate Mode 2.1 Support")
Cc: stable <stable@kernel.org>
Signed-off-by: RD Babiera <rdbabiera@google.com>
Acked-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Link: https://patch.msgid.link/20260310204106.3939862-2-rdbabiera@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: RD Babiera

drivers/usb/typec/altmodes/displayport.c

@@ -100,9 +100,14 @@ static int dp_altmode_configure(struct dp_altmode *dp, u8 con)
 {
 	u8 pin_assign = 0;
 	u32 conf;
+	u32 signal;
 
 	/* DP Signalling */
-	conf = (dp->data.conf & DP_CONF_SIGNALLING_MASK) >> DP_CONF_SIGNALLING_SHIFT;
+	signal = DP_CAP_DP_SIGNALLING(dp->port->vdo) & DP_CAP_DP_SIGNALLING(dp->alt->vdo);
+	if (dp->plug_prime)
+		signal &= DP_CAP_DP_SIGNALLING(dp->plug_prime->vdo);
+
+	conf = signal << DP_CONF_SIGNALLING_SHIFT;
 
 	switch (con) {
 	case DP_STATUS_CON_DISABLED: