[PATCH] usb: typec: displayport: defer instead of failing when the port is not DFP

[Jean-Francois Bobier]

dp_altmode_probe() rejects a port that is not yet TYPEC_HOST with
-EPROTO. That is a permanent failure: the altmode device stays bound to
nothing and the driver core never retries it.

The role is not stable at that point. With a dock, the partner's
alternate modes are registered while the port is still UFP and the
data-role swap happens afterwards, so whether DisplayPort comes up at
all depends on the altmode happening to be registered a second time
after the swap. On the OnePlus 8T this made DP-over-USB-C work on some
attaches and not others, with "dp_altmode: probe ... failed with error
-71" as the only clue.

Return -EPROBE_DEFER so the core retries once the role settles.

Signed-off-by: Jean-Francois Bobier <jean-francois.bobier@xxxxxxxxxxx>
---
 drivers/usb/typec/altmodes/displayport.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/typec/altmodes/displayport.c b/drivers/usb/typec/altmodes/displayport.c
index 51c92dd88..d84785309 100644
--- a/drivers/usb/typec/altmodes/displayport.c
+++ b/drivers/usb/typec/altmodes/displayport.c
@@ -766,9 +766,19 @@ int dp_altmode_probe(struct typec_altmode *alt)
 	struct dp_altmode *dp;
 	u32 cap = DP_CAP_CAPABILITY(alt->vdo);
 
-	/* Port can only be DFP_U. */
+	/*
+	 * Port can only be DFP_U.
+	 *
+	 * Defer rather than reject: on a dock the partner registers its
+	 * altmodes while the port is still UFP, so a hard -EPROTO here drops
+	 * the DisplayPort altmode permanently and nothing retries it --
+	 * binding then depends on the altmode happening to be re-registered
+	 * after the data-role swap. -EPROBE_DEFER makes the driver core retry
+	 * once the role settles, which is deterministic and stops logging a
+	 * failure for an ordinary ordering race.
+	 */
 	if (typec_altmode_get_data_role(alt) != TYPEC_HOST)
-		return -EPROTO;
+		return -EPROBE_DEFER;
 
 	/* Make sure we have compatible pin configurations */
 	if (!(DP_CAP_PIN_ASSIGN_DFP_D(port->vdo) &
-- 
2.55.0