usb: typec: Implement mode selection

The mode selection process is controlled by the following API functions,
which allow to initiate and complete mode entry based on the priority of
each mode:

`typec_mode_selection_start` function compiles a priority list of supported
Alternate Modes.
`typec_altmode_state_update` function is invoked by the port driver to
communicate the current mode of the Type-C connector.
`typec_mode_selection_delete` function stops the currently running mode
selection process and releases all associated system resources.

`mode_selection_work_fn` task attempts to activate modes. The process stops
on success; otherwise, it proceeds to the next mode after a timeout or
error.

Signed-off-by: Andrei Kuchynski <akuchynski@chromium.org>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Link: https://patch.msgid.link/20260119131824.2529334-5-akuchynski@chromium.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/typec/Makefile

@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0
 obj-$(CONFIG_TYPEC)		+= typec.o
-typec-y				:= class.o mux.o bus.o pd.o retimer.o
+typec-y				:= class.o mux.o bus.o pd.o retimer.o mode_selection.o
 typec-$(CONFIG_ACPI)		+= port-mapper.o
 obj-$(CONFIG_TYPEC)		+= altmodes/
 obj-$(CONFIG_TYPEC_TCPM)	+= tcpm/

drivers/usb/typec/class.h

@@ -9,6 +9,7 @@
 struct typec_mux;
 struct typec_switch;
 struct usb_device;
+struct mode_selection;
 
 struct typec_plug {
 	struct device			dev;
@@ -39,6 +40,7 @@ struct typec_partner {
 	u8				usb_capability;
 
 	struct usb_power_delivery	*pd;
+	struct mode_selection	*sel;
 
 	void (*attach)(struct typec_partner *partner, struct device *dev);
 	void (*deattach)(struct typec_partner *partner, struct device *dev);

drivers/usb/typec/mode_selection.c

@@ -0,0 +1,283 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright 2025 Google LLC.
+ */
+
+#include <linux/types.h>
+#include <linux/list_sort.h>
+#include <linux/slab.h>
+#include <linux/mutex.h>
+#include <linux/workqueue.h>
+#include <linux/usb/typec_altmode.h>
+
+#include "class.h"
+
+/**
+ * struct mode_state - State tracking for a specific Type-C alternate mode
+ * @svid: Standard or Vendor ID of the Alternate Mode
+ * @priority: Mode priority
+ * @error: Outcome of the last attempt to enter the mode
+ * @list: List head to link this mode state into a prioritized list
+ */
+struct mode_state {
+	u16 svid;
+	u8 priority;
+	int error;
+	struct list_head list;
+};
+
+/**
+ * struct mode_selection - Manages the selection and state of Alternate Modes
+ * @mode_list: Prioritized list of available Alternate Modes
+ * @lock: Mutex to protect mode_list
+ * @work: Work structure
+ * @partner: Handle to the Type-C partner device
+ * @active_svid: svid of currently active mode
+ * @timeout: Timeout for a mode entry attempt, ms
+ * @delay: Delay between mode entry/exit attempts, ms
+ */
+struct mode_selection {
+	struct list_head mode_list;
+	/* Protects the mode_list*/
+	struct mutex lock;
+	struct delayed_work work;
+	struct typec_partner *partner;
+	u16 active_svid;
+	unsigned int timeout;
+	unsigned int delay;
+};
+
+/**
+ * struct mode_order - Mode activation tracking
+ * @svid: Standard or Vendor ID of the Alternate Mode
+ * @enter: Flag indicating if the driver is currently attempting to enter or
+ * exit the mode
+ * @result: Outcome of the attempt to activate the mode
+ */
+struct mode_order {
+	u16 svid;
+	int enter;
+	int result;
+};
+
+static int activate_altmode(struct device *dev, void *data)
+{
+	if (is_typec_partner_altmode(dev)) {
+		struct typec_altmode *alt = to_typec_altmode(dev);
+		struct mode_order *order = (struct mode_order *)data;
+
+		if (order->svid == alt->svid) {
+			if (alt->ops && alt->ops->activate)
+				order->result = alt->ops->activate(alt, order->enter);
+			else
+				order->result = -EOPNOTSUPP;
+			return 1;
+		}
+	}
+	return 0;
+}
+
+static int mode_selection_activate(struct mode_selection *sel,
+				   const u16 svid, const int enter)
+
+	__must_hold(&sel->lock)
+{
+	struct mode_order order = {.svid = svid, .enter = enter, .result = -ENODEV};
+
+	/*
+	 * The port driver may acquire its internal mutex during alternate mode
+	 * activation. Since this is the same mutex that may be held during the
+	 * execution of typec_altmode_state_update(), it is crucial to release
+	 * sel->mutex before activation to avoid potential deadlock.
+	 * Note that sel->mode_list must remain invariant throughout this unlocked
+	 * interval.
+	 */
+	mutex_unlock(&sel->lock);
+	device_for_each_child(&sel->partner->dev, &order, activate_altmode);
+	mutex_lock(&sel->lock);
+
+	return order.result;
+}
+
+static void mode_list_clean(struct mode_selection *sel)
+{
+	struct mode_state *ms, *tmp;
+
+	list_for_each_entry_safe(ms, tmp, &sel->mode_list, list) {
+		list_del(&ms->list);
+		kfree(ms);
+	}
+}
+
+/**
+ * mode_selection_work_fn() - Alternate mode activation task
+ * @work: work structure
+ *
+ * - If the Alternate Mode currently prioritized at the top of the list is already
+ * active, the entire selection process is considered finished.
+ * - If a different Alternate Mode is currently active, the system must exit that
+ * active mode first before attempting any new entry.
+ *
+ * The function then checks the result of the attempt to entre the current mode,
+ * stored in the `ms->error` field:
+ * - if the attempt FAILED, the mode is deactivated and removed from the list.
+ * - `ms->error` value of 0 signifies that the mode has not yet been activated.
+ *
+ * Once successfully activated, the task is scheduled for subsequent entry after
+ * a timeout period. The alternate mode driver is expected to call back with the
+ * actual mode entry result via `typec_altmode_state_update()`.
+ */
+static void mode_selection_work_fn(struct work_struct *work)
+{
+	struct mode_selection *sel = container_of(work,
+				struct mode_selection, work.work);
+	struct mode_state *ms;
+	unsigned int delay = sel->delay;
+	int result;
+
+	guard(mutex)(&sel->lock);
+
+	ms = list_first_entry_or_null(&sel->mode_list, struct mode_state, list);
+	if (!ms)
+		return;
+
+	if (sel->active_svid == ms->svid) {
+		dev_dbg(&sel->partner->dev, "%x altmode is active\n", ms->svid);
+		mode_list_clean(sel);
+	} else if (sel->active_svid != 0) {
+		result = mode_selection_activate(sel, sel->active_svid, 0);
+		if (result)
+			mode_list_clean(sel);
+		else
+			sel->active_svid = 0;
+	} else if (ms->error) {
+		dev_err(&sel->partner->dev, "%x: entry error %pe\n",
+			ms->svid, ERR_PTR(ms->error));
+		mode_selection_activate(sel, ms->svid, 0);
+		list_del(&ms->list);
+		kfree(ms);
+	} else {
+		result = mode_selection_activate(sel, ms->svid, 1);
+		if (result) {
+			dev_err(&sel->partner->dev, "%x: activation error %pe\n",
+				ms->svid, ERR_PTR(result));
+			list_del(&ms->list);
+			kfree(ms);
+		} else {
+			delay = sel->timeout;
+			ms->error = -ETIMEDOUT;
+		}
+	}
+
+	if (!list_empty(&sel->mode_list))
+		schedule_delayed_work(&sel->work, msecs_to_jiffies(delay));
+}
+
+void typec_altmode_state_update(struct typec_partner *partner, const u16 svid,
+				const int error)
+{
+	struct mode_selection *sel = partner->sel;
+	struct mode_state *ms;
+
+	if (sel) {
+		mutex_lock(&sel->lock);
+		ms = list_first_entry_or_null(&sel->mode_list, struct mode_state, list);
+		if (ms && ms->svid == svid) {
+			ms->error = error;
+			if (cancel_delayed_work(&sel->work))
+				schedule_delayed_work(&sel->work, 0);
+		}
+		if (!error)
+			sel->active_svid = svid;
+		else
+			sel->active_svid = 0;
+		mutex_unlock(&sel->lock);
+	}
+}
+EXPORT_SYMBOL_GPL(typec_altmode_state_update);
+
+static int compare_priorities(void *priv,
+			      const struct list_head *a, const struct list_head *b)
+{
+	const struct mode_state *msa = container_of(a, struct mode_state, list);
+	const struct mode_state *msb = container_of(b, struct mode_state, list);
+
+	if (msa->priority < msb->priority)
+		return -1;
+	return 1;
+}
+
+static int altmode_add_to_list(struct device *dev, void *data)
+{
+	if (is_typec_partner_altmode(dev)) {
+		struct list_head *list = (struct list_head *)data;
+		struct typec_altmode *altmode = to_typec_altmode(dev);
+		const struct typec_altmode *pdev = typec_altmode_get_partner(altmode);
+		struct mode_state *ms;
+
+		if (pdev && altmode->ops && altmode->ops->activate) {
+			ms = kzalloc(sizeof(*ms), GFP_KERNEL);
+			if (!ms)
+				return -ENOMEM;
+			ms->svid = pdev->svid;
+			ms->priority = pdev->priority;
+			INIT_LIST_HEAD(&ms->list);
+			list_add_tail(&ms->list, list);
+		}
+	}
+	return 0;
+}
+
+int typec_mode_selection_start(struct typec_partner *partner,
+			       const unsigned int delay, const unsigned int timeout)
+{
+	struct mode_selection *sel;
+	int ret;
+
+	if (partner->usb_mode == USB_MODE_USB4)
+		return -EBUSY;
+
+	if (partner->sel)
+		return -EALREADY;
+
+	sel = kzalloc(sizeof(*sel), GFP_KERNEL);
+	if (!sel)
+		return -ENOMEM;
+
+	INIT_LIST_HEAD(&sel->mode_list);
+
+	ret = device_for_each_child(&partner->dev, &sel->mode_list,
+				    altmode_add_to_list);
+
+	if (ret || list_empty(&sel->mode_list)) {
+		mode_list_clean(sel);
+		kfree(sel);
+		return ret;
+	}
+
+	list_sort(NULL, &sel->mode_list, compare_priorities);
+	sel->partner = partner;
+	sel->delay = delay;
+	sel->timeout = timeout;
+	mutex_init(&sel->lock);
+	INIT_DELAYED_WORK(&sel->work, mode_selection_work_fn);
+	schedule_delayed_work(&sel->work, msecs_to_jiffies(delay));
+	partner->sel = sel;
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(typec_mode_selection_start);
+
+void typec_mode_selection_delete(struct typec_partner *partner)
+{
+	struct mode_selection *sel = partner->sel;
+
+	if (sel) {
+		partner->sel = NULL;
+		cancel_delayed_work_sync(&sel->work);
+		mode_list_clean(sel);
+		mutex_destroy(&sel->lock);
+		kfree(sel);
+	}
+}
+EXPORT_SYMBOL_GPL(typec_mode_selection_delete);

include/linux/usb/typec_altmode.h

@@ -240,4 +240,44 @@ void typec_altmode_unregister_driver(struct typec_altmode_driver *drv);
 	module_driver(__typec_altmode_driver, typec_altmode_register_driver, \
 		      typec_altmode_unregister_driver)
 
+/**
+ * typec_mode_selection_start - Start an alternate mode selection process
+ * @partner: Handle to the Type-C partner device
+ * @delay: Delay between mode entry/exit attempts, ms
+ * @timeout: Timeout for a mode entry attempt, ms
+ *
+ * This function initiates the process of attempting to enter an Alternate Mode
+ * supported by the connected Type-C partner.
+ * Returns 0 on success, or a negative error code on failure.
+ */
+int typec_mode_selection_start(struct typec_partner *partner,
+			       const unsigned int delay, const unsigned int timeout);
+
+/**
+ * typec_altmode_state_update - Report the current status of an Alternate Mode
+ * negotiation
+ * @partner: Handle to the Type-C partner device
+ * @svid: Standard or Vendor ID of the Alternate Mode. A value of 0 should be
+ * passed if no mode is currently active
+ * @result: Result of the entry operation. This should be 0 on success, or a
+ * negative error code if the negotiation failed
+ *
+ * This function should be called by an Alternate Mode driver to report the
+ * result of an asynchronous alternate mode entry request. It signals what the
+ * current active SVID is (or 0 if none) and the success or failure status of
+ * the last attempt.
+ */
+void typec_altmode_state_update(struct typec_partner *partner, const u16 svid,
+				const int result);
+
+/**
+ * typec_mode_selection_delete - Delete an alternate mode selection instance
+ * @partner: Handle to the Type-C partner device.
+ *
+ * This function cancels a pending alternate mode selection request that was
+ * previously started with typec_mode_selection_start().
+ * This is typically called when the partner disconnects.
+ */
+void typec_mode_selection_delete(struct typec_partner *partner);
+
 #endif /* __USB_TYPEC_ALTMODE_H */