[PATCH v3 1/3] serdev: handle devices without an ACPI or device tree node

[ChunAn Wu <an.wu@canonical.com>]

Serdev clients instantiated by a driver have no ACPI companion and no
device tree node. Their modalias read returns -ENODEV, their add uevent
is suppressed, and they cannot be driver-matched, so creators force a
bind with device_driver_attach().

Resolve both TODOs in the serdev core by modelling matching on the SPI
bus: add a creator-set modalias to struct serdev_device and an id_table
to struct serdev_device_driver. ACPI and OF are still tried first;
clients without their own modalias keep returning -ENODEV. The device
name, derived from the controller, is unchanged.

Signed-off-by: ChunAn Wu <an.wu@canonical.com>
---
Changes in v3:
- Rename struct serdev_device::name to ::modalias; dev_name() is unchanged
- Model matching on the SPI bus: add struct serdev_device_id and
  id_table; drop driver->name matching
- Add serdev_device_set_modalias() helper and SERDEV_MODULE_PREFIX
- No MODULE_DEVICE_TABLE(serdev, ...) support: both in-tree firmware-less
  clients are created by the same module that provides their driver, so
  udev has nothing to autoload

Changes in v2:
- Instead of silently returning success with no modalias, use a
  creator-supplied name as a "serial:<name>" modalias fallback and as
  the matching key, so clients can bind through normal serdev matching

 MAINTAINERS                      |  1 +
 drivers/tty/serdev/core.c        | 54 ++++++++++++++++++++++++++++----
 include/linux/device-id/serdev.h | 16 ++++++++++
 include/linux/serdev.h           | 34 ++++++++++++++++++++
 4 files changed, 99 insertions(+), 6 deletions(-)
 create mode 100644 include/linux/device-id/serdev.h

diff --git a/MAINTAINERS b/MAINTAINERS
index 917f3f72e31b..06e67aa5d9b0 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -24905,6 +24905,7 @@ L:	linux-serial@vger.kernel.org
 S:	Maintained
 F:	Documentation/devicetree/bindings/serial/serial.yaml
 F:	drivers/tty/serdev/
+F:	include/linux/device-id/serdev.h
 F:	include/linux/serdev.h
 F:	rust/helpers/serdev.c
 F:	rust/kernel/serdev.rs
diff --git a/drivers/tty/serdev/core.c b/drivers/tty/serdev/core.c
index 7500efcdfc21..e178dc57d31b 100644
--- a/drivers/tty/serdev/core.c
+++ b/drivers/tty/serdev/core.c
@@ -29,13 +29,21 @@ static DEFINE_IDA(ctrl_ida);
 static ssize_t modalias_show(struct device *dev,
 			     struct device_attribute *attr, char *buf)
 {
+	const struct serdev_device *serdev = to_serdev_device(dev);
 	int len;
 
 	len = acpi_device_modalias(dev, buf, PAGE_SIZE - 1);
 	if (len != -ENODEV)
 		return len;
 
-	return of_device_modalias(dev, buf, PAGE_SIZE);
+	len = of_device_modalias(dev, buf, PAGE_SIZE);
+	if (len != -ENODEV)
+		return len;
+
+	if (!serdev->modalias[0])
+		return -ENODEV;
+
+	return sysfs_emit(buf, "%s%s\n", SERDEV_MODULE_PREFIX, serdev->modalias);
 }
 static DEVICE_ATTR_RO(modalias);
 
@@ -47,15 +55,22 @@ ATTRIBUTE_GROUPS(serdev_device);
 
 static int serdev_device_uevent(const struct device *dev, struct kobj_uevent_env *env)
 {
+	const struct serdev_device *serdev = to_serdev_device(dev);
 	int rc;
 
-	/* TODO: platform modalias */
-
 	rc = acpi_device_uevent_modalias(dev, env);
 	if (rc != -ENODEV)
 		return rc;
 
-	return of_device_uevent_modalias(dev, env);
+	rc = of_device_uevent_modalias(dev, env);
+	if (rc != -ENODEV)
+		return rc;
+
+	if (!serdev->modalias[0])
+		return -ENODEV;
+
+	return add_uevent_var(env, "MODALIAS=%s%s", SERDEV_MODULE_PREFIX,
+			      serdev->modalias);
 }
 
 static void serdev_device_release(struct device *dev)
@@ -86,16 +101,43 @@ static const struct device_type serdev_ctrl_type = {
 	.release	= serdev_ctrl_release,
 };
 
+static const struct serdev_device_id *serdev_match_id(const struct serdev_device_id *id,
+						      const char *modalias)
+{
+	if (!id || !modalias[0])
+		return NULL;
+
+	while (id->name[0]) {
+		if (!strcmp(modalias, id->name))
+			return id;
+		id++;
+	}
+
+	return NULL;
+}
+
 static int serdev_device_match(struct device *dev, const struct device_driver *drv)
 {
+	const struct serdev_device *serdev;
+
 	if (!is_serdev_device(dev))
 		return 0;
 
-	/* TODO: platform matching */
 	if (acpi_driver_match_device(dev, drv))
 		return 1;
 
-	return of_driver_match_device(dev, drv);
+	if (of_driver_match_device(dev, drv))
+		return 1;
+
+	/*
+	 * Devices which are instantiated by a driver instead of by firmware
+	 * have no ACPI companion and no device tree node, match them by the
+	 * modalias their creator gave them.
+	 */
+	serdev = to_serdev_device(dev);
+
+	return !!serdev_match_id(to_serdev_device_driver(drv)->id_table,
+				 serdev->modalias);
 }
 
 /**
diff --git a/include/linux/device-id/serdev.h b/include/linux/device-id/serdev.h
new file mode 100644
index 000000000000..1c3b03b7d501
--- /dev/null
+++ b/include/linux/device-id/serdev.h
@@ -0,0 +1,16 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef LINUX_DEVICE_ID_SERDEV_H
+#define LINUX_DEVICE_ID_SERDEV_H
+
+/* serdev */
+
+/* Large enough to hold the KBUILD_MODNAME of any serdev client driver */
+#define SERDEV_NAME_SIZE	40
+#define SERDEV_MODULE_PREFIX	"serial:"
+
+struct serdev_device_id {
+	char name[SERDEV_NAME_SIZE];
+	unsigned long driver_data;	/* Data private to the driver */
+};
+
+#endif /* ifndef LINUX_DEVICE_ID_SERDEV_H */
diff --git a/include/linux/serdev.h b/include/linux/serdev.h
index b6c3d957ec15..9f91aa3e875d 100644
--- a/include/linux/serdev.h
+++ b/include/linux/serdev.h
@@ -7,7 +7,9 @@
 
 #include <linux/types.h>
 #include <linux/device.h>
+#include <linux/device-id/serdev.h>
 #include <linux/iopoll.h>
+#include <linux/string.h>
 #include <linux/uaccess.h>
 #include <linux/termios.h>
 #include <linux/delay.h>
@@ -35,6 +37,10 @@ struct serdev_device_ops {
  * struct serdev_device - Basic representation of an serdev device
  * @dev:	Driver model representation of the device.
  * @nr:		Device number on serdev bus.
+ * @modalias:	Name of the driver to use with this device, or the alias
+ *		for it. Empty for devices enumerated from firmware, which
+ *		are identified through their ACPI companion or their device
+ *		tree node instead. See serdev_device_set_modalias().
  * @ctrl:	serdev controller managing this device.
  * @ops:	Device operations.
  * @write_comp:	Completion used by serdev_device_write() internally
@@ -43,6 +49,7 @@ struct serdev_device_ops {
 struct serdev_device {
 	struct device dev;
 	int nr;
+	char modalias[SERDEV_NAME_SIZE];
 	struct serdev_controller *ctrl;
 	const struct serdev_device_ops *ops;
 	struct completion write_comp;
@@ -58,12 +65,15 @@ struct serdev_device {
  * @probe:	binds this driver to a serdev device.
  * @remove:	unbinds this driver from the serdev device.
  * @shutdown:	shut down this serdev device.
+ * @id_table:	List of serdev devices supported by this driver, used to
+ *		match devices which are not enumerated from firmware.
  */
 struct serdev_device_driver {
 	struct device_driver driver;
 	int	(*probe)(struct serdev_device *);
 	void	(*remove)(struct serdev_device *);
 	void	(*shutdown)(struct serdev_device *);
+	const struct serdev_device_id *id_table;
 };
 
 #define to_serdev_device_driver(d) container_of_const(d, struct serdev_device_driver, driver)
@@ -135,6 +145,30 @@ static inline void serdev_device_set_client_ops(struct serdev_device *serdev,
 	serdev->ops = ops;
 }
 
+/**
+ * serdev_device_set_modalias() - set the modalias of a serdev device
+ * @serdev:	serdev device.
+ * @modalias:	non-empty modalias to use, typically KBUILD_MODNAME of the
+ *		client driver.
+ *
+ * A serdev device is identified by its ACPI companion, by its device tree
+ * node, or by a modalias set by its creator. Serdev devices which are
+ * instantiated by a driver rather than by firmware have neither of the
+ * former, so they must set a modalias before serdev_device_add() to be
+ * identifiable. Doing so reports a SERDEV_MODULE_PREFIX modalias to
+ * userspace and makes the device match an id_table entry of a serdev
+ * device driver.
+ *
+ * Devices enumerated from firmware have no modalias of their own and keep
+ * deriving their modalias and their driver match from firmware only. The
+ * device name is not affected either way.
+ */
+static inline void serdev_device_set_modalias(struct serdev_device *serdev,
+					      const char *modalias)
+{
+	strscpy(serdev->modalias, modalias);
+}
+
 static inline
 void *serdev_controller_get_drvdata(const struct serdev_controller *ctrl)
 {
-- 
2.34.1