usb: typec: Expose alternate mode priority via sysfs

This patch introduces a priority sysfs attribute to the USB Type-C
alternate mode port interface. This new attribute allows user-space to
configure the numeric priority of alternate modes managing their preferred
order of operation. If a new priority value conflicts with an existing
mode's priority, the priorities of the conflicting mode and all subsequent
modes are automatically incremented to ensure uniqueness.

Signed-off-by: Andrei Kuchynski <akuchynski@chromium.org>
Reviewed-by: Benson Leung <bleung@chromium.org>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Link: https://patch.msgid.link/20260119131824.2529334-4-akuchynski@chromium.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Andrei Kuchynski

Documentation/ABI/testing/sysfs-class-typec

@@ -162,6 +162,17 @@ Description:	Lists the supported USB Modes. The default USB mode that is used
 		- usb3 (USB 3.2)
 		- usb4 (USB4)
 
+What:		/sys/class/typec/<port>/<alt-mode>/priority
+Date:		July 2025
+Contact:	Andrei Kuchynski <akuchynski@chromium.org>
+Description:
+		Displays and allows setting the priority for a specific alternate mode.
+		The priority is an integer in the range 0-255. A lower numerical value
+		indicates a higher priority (0 is the highest).
+		If the new value is already in use by another mode, the priority of the
+		conflicting mode and any subsequent modes will be incremented until they
+		are all unique.
+
 USB Type-C partner devices (eg. /sys/class/typec/port0-partner/)
 
 What:		/sys/class/typec/<port>-partner/accessory_mode

drivers/usb/typec/class.c

@@ -445,11 +445,88 @@ svid_show(struct device *dev, struct device_attribute *attr, char *buf)
 }
 static DEVICE_ATTR_RO(svid);
 
+static int increment_duplicated_priority(struct device *dev, void *data)
+{
+	if (is_typec_port_altmode(dev)) {
+		struct typec_altmode **alt_target = (struct typec_altmode **)data;
+		struct typec_altmode *alt = to_typec_altmode(dev);
+
+		if (alt != *alt_target && alt->priority == (*alt_target)->priority) {
+			alt->priority++;
+			*alt_target = alt;
+			return 1;
+		}
+	}
+	return 0;
+}
+
+static int find_duplicated_priority(struct device *dev, void *data)
+{
+	if (is_typec_port_altmode(dev)) {
+		struct typec_altmode **alt_target = (struct typec_altmode **)data;
+		struct typec_altmode *alt = to_typec_altmode(dev);
+
+		if (alt != *alt_target && alt->priority == (*alt_target)->priority)
+			return 1;
+	}
+	return 0;
+}
+
+static int typec_mode_set_priority(struct typec_altmode *alt, const u8 priority)
+{
+	struct typec_port *port = to_typec_port(alt->dev.parent);
+	const u8 old_priority = alt->priority;
+	int res = 1;
+
+	alt->priority = priority;
+	while (res) {
+		res = device_for_each_child(&port->dev, &alt, find_duplicated_priority);
+		if (res) {
+			alt->priority++;
+			if (alt->priority == 0) {
+				alt->priority = old_priority;
+				return -EOVERFLOW;
+			}
+		}
+	}
+
+	res = 1;
+	alt->priority = priority;
+	while (res)
+		res = device_for_each_child(&port->dev, &alt,
+					    increment_duplicated_priority);
+
+	return 0;
+}
+
+static ssize_t priority_store(struct device *dev,
+			      struct device_attribute *attr,
+			      const char *buf, size_t size)
+{
+	u8 val;
+	int err = kstrtou8(buf, 10, &val);
+
+	if (!err)
+		err = typec_mode_set_priority(to_typec_altmode(dev), val);
+
+	if (!err)
+		return size;
+	return err;
+}
+
+static ssize_t priority_show(struct device *dev,
+			     struct device_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%u\n", to_typec_altmode(dev)->priority);
+}
+static DEVICE_ATTR_RW(priority);
+
 static struct attribute *typec_altmode_attrs[] = {
 	&dev_attr_active.attr,
 	&dev_attr_mode.attr,
 	&dev_attr_svid.attr,
 	&dev_attr_vdo.attr,
+	&dev_attr_priority.attr,
 	NULL
 };
 
@@ -459,11 +536,15 @@ static umode_t typec_altmode_attr_is_visible(struct kobject *kobj,
 	struct typec_altmode *adev = to_typec_altmode(kobj_to_dev(kobj));
 	struct typec_port *port = typec_altmode2port(adev);
 
-	if (attr == &dev_attr_active.attr)
+	if (attr == &dev_attr_active.attr) {
 		if (!is_typec_port(adev->dev.parent)) {
 			if (!port->mode_control || !adev->ops || !adev->ops->activate)
 				return 0444;
 		}
+	} else if (attr == &dev_attr_priority.attr) {
+		if (!is_typec_port(adev->dev.parent) || !port->mode_control)
+			return 0;
+	}
 
 	return attr->mode;
 }
@@ -2498,6 +2579,7 @@ typec_port_register_altmode(struct typec_port *port,
 	struct typec_altmode *adev;
 	struct typec_mux *mux;
 	struct typec_retimer *retimer;
+	int ret;
 
 	mux = typec_mux_get(&port->dev);
 	if (IS_ERR(mux))
@@ -2516,6 +2598,12 @@ typec_port_register_altmode(struct typec_port *port,
 	} else {
 		to_altmode(adev)->mux = mux;
 		to_altmode(adev)->retimer = retimer;
+
+		ret = typec_mode_set_priority(adev, 0);
+		if (ret) {
+			typec_unregister_altmode(adev);
+			return ERR_PTR(ret);
+		}
 	}
 
 	return adev;

include/linux/usb/typec_altmode.h

@@ -36,6 +36,7 @@ struct typec_altmode {
 	int				mode;
 	u32				vdo;
 	unsigned int			active:1;
+	u8				priority;
 
 	char				*desc;
 	const struct typec_altmode_ops	*ops;