Re: [PATCH v5 1/3] serdev: handle devices without an ACPI or device tree node

[Markus Probst <markus.probst@posteo.de>]

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On Sat, 2026-10-24 at 20:04 +0800, ChunAn Wu wrote:
> 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: let the creator pass a modalias to serdev_device_add() and give
> struct serdev_device_driver an id_table. ACPI and OF are still tried
> first; clients without their own modalias keep returning -ENODEV. The
> device name, derived from the controller, is unchanged.
> 
> Split struct serdev_device_id and its defines into a new
> include/linux/device-id/serdev.h, following the pattern already used by
> the SPI and I2C buses.
> 
> Signed-off-by: ChunAn Wu <an.wu@canonical.com>
> ---
>  MAINTAINERS                                   |  1 +
>  .../platform/x86/x86-android-tablets/core.c   |  2 +-
>  drivers/power/sequencing/pwrseq-pcie-m2.c     |  2 +-
>  drivers/tty/serdev/core.c                     | 62 ++++++++++++++++---
>  include/linux/device-id/serdev.h              | 19 ++++++
>  include/linux/serdev.h                        | 12 +++-
>  6 files changed, 86 insertions(+), 12 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/platform/x86/x86-android-tablets/core.c b/drivers/platform/x86/x86-android-tablets/core.c
> index cfff7f5eac5d..853aa6ff5d50 100644
> --- a/drivers/platform/x86/x86-android-tablets/core.c
> +++ b/drivers/platform/x86/x86-android-tablets/core.c
> @@ -317,7 +317,7 @@ static __init int x86_instantiate_serdev(const struct x86_dev_info *dev_info, in
>  	ACPI_COMPANION_SET(&serdev->dev, serdev_adev);
>  	acpi_device_set_enumerated(serdev_adev);
>  
> -	ret = serdev_device_add(serdev);
> +	ret = serdev_device_add(serdev, NULL);
>  	if (ret) {
>  		dev_err(&serdev->dev, "error %d adding serdev\n", ret);
>  		serdev_device_put(serdev);
> diff --git a/drivers/power/sequencing/pwrseq-pcie-m2.c b/drivers/power/sequencing/pwrseq-pcie-m2.c
> index e42e85f47629..e0482f2ef39f 100644
> --- a/drivers/power/sequencing/pwrseq-pcie-m2.c
> +++ b/drivers/power/sequencing/pwrseq-pcie-m2.c
> @@ -309,7 +309,7 @@ static int pwrseq_pcie_m2_create_serdev_one(struct pwrseq_pcie_m2_ctx *ctx,
>  	if (ret)
>  		goto err_free_serdev;
>  
> -	ret = serdev_device_add(pci_dev->serdev);
> +	ret = serdev_device_add(pci_dev->serdev, NULL);
>  	if (ret) {
>  		dev_err(dev, "Failed to add serdev for PCI device (%s): %d\n",
>  			pci_name(pdev), ret);
> diff --git a/drivers/tty/serdev/core.c b/drivers/tty/serdev/core.c
> index 7500efcdfc21..e0517d5a48da 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)
> @@ -86,28 +101,57 @@ 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;
> +	const struct serdev_device_driver *sdrv;
> +
>  	if (!is_serdev_device(dev))
>  		return 0;
>  
> -	/* TODO: platform matching */
> +	serdev = to_serdev_device(dev);
> +	sdrv = to_serdev_device_driver(drv);
> +
>  	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;
> +
> +	return !!serdev_match_id(sdrv->id_table, serdev->modalias);
>  }
>  
>  /**
>   * serdev_device_add() - add a device previously constructed via serdev_device_alloc()
>   * @serdev:	serdev_device to be added
> + * @modalias:	Driver name or alias, used to match devices which have neither
> + *		an ACPI companion nor a device tree node. NULL for devices
> + *		enumerated from firmware.
>   */
> -int serdev_device_add(struct serdev_device *serdev)
> +int serdev_device_add(struct serdev_device *serdev, const char *modalias)
>  {
>  	struct serdev_controller *ctrl = serdev->ctrl;
>  	struct device *parent = serdev->dev.parent;
>  	int err;
>  
> +	if (modalias)
> +		strscpy(serdev->modalias, modalias);
> +
>  	dev_set_name(&serdev->dev, "%s-%d", dev_name(parent), serdev->nr);
>  
>  	/* Only a single slave device is currently supported. */
> @@ -553,7 +597,7 @@ static int of_serdev_register_devices(struct serdev_controller *ctrl)
>  
>  		device_set_node(&serdev->dev, of_fwnode_handle(node));
>  
> -		err = serdev_device_add(serdev);
> +		err = serdev_device_add(serdev, NULL);
>  		if (err) {
>  			dev_err(&serdev->dev,
>  				"failure adding device. status %pe\n",
> @@ -707,7 +751,7 @@ static acpi_status acpi_serdev_register_device(struct serdev_controller *ctrl,
>  	ACPI_COMPANION_SET(&serdev->dev, adev);
>  	acpi_device_set_enumerated(adev);
>  
> -	err = serdev_device_add(serdev);
> +	err = serdev_device_add(serdev, NULL);
>  	if (err) {
>  		dev_err(&serdev->dev,
>  			"failure adding ACPI serdev device. status %pe\n",
> diff --git a/include/linux/device-id/serdev.h b/include/linux/device-id/serdev.h
> new file mode 100644
> index 000000000000..96644953cece
> --- /dev/null
> +++ b/include/linux/device-id/serdev.h
> @@ -0,0 +1,19 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef LINUX_DEVICE_ID_SERDEV_H
> +#define LINUX_DEVICE_ID_SERDEV_H
> +
> +#ifdef __KERNEL__
> +typedef unsigned long kernel_ulong_t;
> +#endif
> +
> +/* serdev */
> +
> +#define SERDEV_NAME_SIZE	40
> +#define SERDEV_MODULE_PREFIX	"serial:"
> +
> +struct serdev_device_id {
> +	char name[SERDEV_NAME_SIZE];
> +	kernel_ulong_t 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..96a441abc1ac 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>
Unused include.

>  #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_add().
>   * @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;
> @@ -55,12 +62,15 @@ struct serdev_device {
>   * struct serdev_device_driver - serdev slave device driver
>   * @driver:	serdev device drivers should initialize name field of this
>   *		structure.
> + * @id_table:	List of serdev devices supported by this driver, used to
> + *		match devices which are not enumerated from firmware.
>   * @probe:	binds this driver to a serdev device.
>   * @remove:	unbinds this driver from the serdev device.
>   * @shutdown:	shut down this serdev device.
>   */
>  struct serdev_device_driver {
>  	struct device_driver driver;
> +	const struct serdev_device_id *id_table;
>  	int	(*probe)(struct serdev_device *);
>  	void	(*remove)(struct serdev_device *);
>  	void	(*shutdown)(struct serdev_device *);
> @@ -158,7 +168,7 @@ static inline void serdev_controller_put(struct serdev_controller *ctrl)
>  }
>  
>  struct serdev_device *serdev_device_alloc(struct serdev_controller *);
> -int serdev_device_add(struct serdev_device *);
> +int serdev_device_add(struct serdev_device *serdev, const char *modalias);
>  void serdev_device_remove(struct serdev_device *);
>  
>  struct serdev_controller *serdev_controller_alloc(struct device *host,

You will need to address the sashiko report (both are true positives).

In the scripts/mod/file2alias.c file, you can copy the spi entry.
scripts/mod/devicetable-offsets.{h,c,s} and
include/linux/mod_devicetable.h are also relevant for this.
(Didn't know myself those files existed, so thx Sashiko)

Also add serdev_device_id to scripts/coccinelle/misc/of_table.cocci .


With that fixed,

Reviewed-by: Markus Probst <markus.probst@posteo.de>


Thanks
- Markus Probst

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