Re: [PATCH v3 1/8] driver core: add a faux bus for use when a simple device/bus is needed

[Thomas Weißschuh]

On Thu, Feb 06, 2025 at 06:38:15PM +0100, Greg Kroah-Hartman wrote:
> Many drivers abuse the platform driver/bus system as it provides a
> simple way to create and bind a device to a driver-specific set of
> probe/release functions.  Instead of doing that, and wasting all of the
> memory associated with a platform device, here is a "faux" bus that
> can be used instead.
> 
> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
> Reviewed-by: Danilo Krummrich <dakr@xxxxxxxxxx>
> Reviewed-by: Lyude Paul <lyude@xxxxxxxxxx>

Some tiny nitpicks below, but still:

Reviewed-by: Thomas Weißschuh <thomas.weissschuh@xxxxxxxxxxxxx>

> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> ---
> v3: - loads of documentation updates and rewrites
>     - added to the documentation build
>     - removed name[] array as it's no longer needed
>     - added faux_device_create_with_groups()
>     - added functions to get/set devdata
>     - renamed faux_driver_ops -> faux_device_ops
>     - made faux_device_ops a const *
>     - minor cleanups
>     - tested it, again.
> 
> v2: - renamed bus and root device to just "faux" thanks to Thomas
>     - removed the one-driver-per-device and now just have one driver
>       entirely thanks to Danilo
>     - kerneldoc fixups and additions and string handling bounds checks
>       thanks to Andy
>     - coding style fix thanks to Jonathan
>     - tested that the destroy path actually works
>  Documentation/driver-api/infrastructure.rst |   6 +
>  drivers/base/Makefile                       |   2 +-
>  drivers/base/base.h                         |   1 +
>  drivers/base/faux.c                         | 228 ++++++++++++++++++++
>  drivers/base/init.c                         |   1 +
>  include/linux/device/faux.h                 |  65 ++++++
>  6 files changed, 302 insertions(+), 1 deletion(-)
>  create mode 100644 drivers/base/faux.c
>  create mode 100644 include/linux/device/faux.h
 
<snip>

> diff --git a/drivers/base/faux.c b/drivers/base/faux.c
> new file mode 100644
> index 000000000000..27879ae78f53
> --- /dev/null
> +++ b/drivers/base/faux.c
> @@ -0,0 +1,228 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Copyright (c) 2025 Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> + * Copyright (c) 2025 The Linux Foundation
> + *
> + * A "simple" faux bus that allows devices to be created and added
> + * automatically to it.  This is to be used whenever you need to create a
> + * device that is not associated with any "real" system resources, and do
> + * not want to have to deal with a bus/driver binding logic.  It is
> + * intended to be very simple, with only a create and a destroy function
> + * available.
> + */
> +#include <linux/err.h>
> +#include <linux/init.h>
> +#include <linux/slab.h>
> +#include <linux/string.h>
> +#include <linux/container_of.h>
> +#include <linux/device/faux.h>
> +#include "base.h"

Weird order.

> +
> +#define MAX_FAUX_NAME_SIZE	256	/* Max size of a faux_device name */
> +
> +/*
> + * Internal wrapper structure so we can hold a pointer to the
> + * faux_device_ops for this device.
> + */
> +struct faux_object {
> +	struct faux_device faux_dev;
> +	const struct faux_device_ops *faux_ops;
> +};
> +#define to_faux_object(dev) container_of_const(dev, struct faux_object, faux_dev.dev)
> +
> +static struct device faux_bus_root = {
> +	.init_name	= "faux",
> +};
> +
> +static int faux_match(struct device *dev, const struct device_driver *drv)
> +{
> +	/* Match always succeeds, we only have one driver */
> +	return 1;
> +}
> +
> +static int faux_probe(struct device *dev)
> +{
> +	struct faux_object *faux_obj = to_faux_object(dev);
> +	struct faux_device *faux_dev = &faux_obj->faux_dev;
> +	const struct faux_device_ops *faux_ops = faux_obj->faux_ops;
> +	int ret = 0;
> +
> +	if (faux_ops && faux_ops->probe)
> +		ret = faux_ops->probe(faux_dev);
> +
> +	return ret;
> +}
> +
> +static void faux_remove(struct device *dev)
> +{
> +	struct faux_object *faux_obj = to_faux_object(dev);
> +	struct faux_device *faux_dev = &faux_obj->faux_dev;
> +	const struct faux_device_ops *faux_ops = faux_obj->faux_ops;
> +
> +	if (faux_ops && faux_ops->remove)
> +		faux_ops->remove(faux_dev);
> +}
> +
> +static const struct bus_type faux_bus_type = {
> +	.name		= "faux",
> +	.match		= faux_match,
> +	.probe		= faux_probe,
> +	.remove		= faux_remove,
> +};
> +
> +static struct device_driver faux_driver = {
> +	.name		= "faux_driver",
> +	.bus		= &faux_bus_type,
> +	.probe_type	= PROBE_PREFER_ASYNCHRONOUS,
> +};
> +
> +static void faux_device_release(struct device *dev)
> +{
> +	struct faux_object *faux_obj = to_faux_object(dev);
> +
> +	kfree(faux_obj);
> +}
> +
> +/**
> + * faux_device_create_with_groups - create and register with the driver
> + *		core a faux device and populate the device with an initial
> + *		set of sysfs attributes
> + * @name:	The name of the device we are adding, must be unique for
> + *		all faux devices.
> + * @faux_ops:	struct faux_device_ops that the new device will call back
> + *		into, can be NULL.
> + * @groups:	The set of sysfs attributes that will be created for this
> + *		device when it is registered with the driver core.
> + *
> + * Create a new faux device and register it in the driver core properly.
> + * If present, callbacks in @faux_ops will be called with the device that
> + * for the caller to do something with at the proper time given the
> + * device's lifecycle.
> + *
> + * Note, when this function is called, the functions specified in struct
> + * faux_ops can be called before the function returns, so be prepared for
> + * everything to be properly initialized before that point in time.
> + *
> + * Return:
> + * * NULL if an error happened with creating the device
> + * * pointer to a valid struct faux_device that is registered with sysfs
> + */
> +struct faux_device *faux_device_create_with_groups(const char *name,
> +						   const struct faux_device_ops *faux_ops,
> +						   const struct attribute_group **groups)
> +{
> +	struct device *dev;
> +	struct faux_object *faux_obj;
> +	struct faux_device *faux_dev;
> +	int name_size;
> +	int ret;
> +
> +	name_size = strlen(name);
> +	if (name_size > MAX_FAUX_NAME_SIZE)
> +		return NULL;
> +
> +	faux_obj = kzalloc(sizeof(*faux_obj) + name_size + 1, GFP_KERNEL);

The name is not actually stored in the object anymore.

> +	if (!faux_obj)
> +		return NULL;
> +
> +	/* Save off the callbacks so we can use them in the future */
> +	faux_obj->faux_ops = faux_ops;
> +
> +	/* Initialize the device portion and register it with the driver core */
> +	faux_dev = &faux_obj->faux_dev;
> +	dev = &faux_dev->dev;
> +
> +	device_initialize(dev);
> +	dev->release = faux_device_release;
> +	dev->parent = &faux_bus_root;

I guess nobody will want to hang these off a different parent.

> +	dev->bus = &faux_bus_type;
> +	dev->groups = groups;
> +	dev_set_name(dev, "%s", name);
> +
> +	ret = device_add(dev);
> +	if (ret) {
> +		pr_err("%s: device_add for faux device '%s' failed with %d\n",
> +		       __func__, name, ret);
> +		put_device(dev);
> +		return NULL;
> +	}
> +
> +	return faux_dev;
> +}
> +EXPORT_SYMBOL_GPL(faux_device_create_with_groups);
> +
> +/**
> + * faux_device_create - create and register with the driver core a faux device
> + * @name:	name of the device we are adding, must be unique for all
> + *		faux devices.
> + * @faux_ops:	struct faux_device_ops that the new device will call back
> + *		into, can be NULL.
> + *
> + * Create a new faux device and register it in the driver core properly.
> + * If present, callbacks in @faux_ops will be called with the device that
> + * for the caller to do something with at the proper time given the
> + * device's lifecycle.
> + *
> + * Note, when this function is called, the functions specified in struct
> + * faux_ops can be called before the function returns, so be prepared for
> + * everything to be properly initialized before that point in time.
> + *
> + * Return:
> + * * NULL if an error happened with creating the device
> + * * pointer to a valid struct faux_device that is registered with sysfs
> + */
> +struct faux_device *faux_device_create(const char *name,
> +				       const struct faux_device_ops *faux_ops)
> +{
> +	return faux_device_create_with_groups(name, faux_ops, NULL);
> +}
> +EXPORT_SYMBOL_GPL(faux_device_create);
> +
> +/**
> + * faux_device_destroy - destroy a faux device
> + * @faux_dev:	faux device to destroy
> + *
> + * Unregisters and cleans up a device that was created with a call to
> + * faux_device_create()
> + */
> +void faux_device_destroy(struct faux_device *faux_dev)
> +{
> +	struct device *dev = &faux_dev->dev;
> +
> +	if (!faux_dev)
> +		return;
> +
> +	device_del(dev);
> +
> +	/* The final put_device() will clean up the memory we allocated for this device. */
> +	put_device(dev);
> +}
> +EXPORT_SYMBOL_GPL(faux_device_destroy);
> +
> +int __init faux_bus_init(void)
> +{
> +	int ret;
> +
> +	ret = device_register(&faux_bus_root);
> +	if (ret) {
> +		put_device(&faux_bus_root);
> +		return ret;
> +	}
> +
> +	ret = bus_register(&faux_bus_type);
> +	if (ret)
> +		goto error_bus;
> +
> +	ret = driver_register(&faux_driver);
> +	if (ret)
> +		goto error_driver;
> +
> +	return ret;
> +
> +error_driver:
> +	bus_unregister(&faux_bus_type);
> +
> +error_bus:
> +	device_unregister(&faux_bus_root);
> +	return ret;
> +}

<snip>

> diff --git a/include/linux/device/faux.h b/include/linux/device/faux.h
> new file mode 100644
> index 000000000000..2c8ae5bd7ae8
> --- /dev/null
> +++ b/include/linux/device/faux.h
> @@ -0,0 +1,31 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Copyright (c) 2025 Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> + * Copyright (c) 2025 The Linux Foundation
> + *
> + * A "simple" faux bus that allows devices to be created and added
> + * automatically to it.  This is to be used whenever you need to create a
> + * device that is not associated with any "real" system resources, and do
> + * not want to have to deal with a bus/driver binding logic.  It is
> + * intended to be very simple, with only a create and a destroy function
> + * available.
> + */
> +#ifndef _FAUX_DEVICE_H_
> +#define _FAUX_DEVICE_H_
> +

#include <linux/container_of.h>

> +#include <linux/device.h>
> +
> +struct faux_device {
> +	struct device dev;
> +};
> +#define to_faux_device(x) container_of_const((x), struct faux_device, dev)
> +
> +struct faux_driver_ops {
> +	int (*probe)(struct faux_device *faux_dev);
> +	void (*remove)(struct faux_device *faux_dev);
> +};
> +
> +struct faux_device *faux_device_create(const char *name, struct faux_driver_ops *faux_ops);
> +void faux_device_destroy(struct faux_device *faux_dev);
> +
> +#endif /* _FAUX_DEVICE_H_ */