Re: [PATCH v2 1/5] driver core: add a faux bus for use when a simple device/bus is needed

[Rob Herring <robh@kernel.org>]

On Tue, Feb 04, 2025 at 12:09:13PM +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.
> 
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> ---
>  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
>        hanks to Andy
>      - coding style fix thanks to Jonathan
>      - tested that the destroy path actually works
> 
>  drivers/base/Makefile       |   2 +-
>  drivers/base/base.h         |   1 +
>  drivers/base/faux.c         | 196 ++++++++++++++++++++++++++++++++++++
>  drivers/base/init.c         |   1 +
>  include/linux/device/faux.h |  31 ++++++
>  5 files changed, 230 insertions(+), 1 deletion(-)
>  create mode 100644 drivers/base/faux.c
>  create mode 100644 include/linux/device/faux.h
> 
> diff --git a/drivers/base/Makefile b/drivers/base/Makefile
> index 7fb21768ca36..8074a10183dc 100644
> --- a/drivers/base/Makefile
> +++ b/drivers/base/Makefile
> @@ -6,7 +6,7 @@ obj-y			:= component.o core.o bus.o dd.o syscore.o \
>  			   cpu.o firmware.o init.o map.o devres.o \
>  			   attribute_container.o transport_class.o \
>  			   topology.o container.o property.o cacheinfo.o \
> -			   swnode.o
> +			   swnode.o faux.o
>  obj-$(CONFIG_AUXILIARY_BUS) += auxiliary.o
>  obj-$(CONFIG_DEVTMPFS)	+= devtmpfs.o
>  obj-y			+= power/
> diff --git a/drivers/base/base.h b/drivers/base/base.h
> index 8cf04a557bdb..0042e4774b0c 100644
> --- a/drivers/base/base.h
> +++ b/drivers/base/base.h
> @@ -137,6 +137,7 @@ int hypervisor_init(void);
>  static inline int hypervisor_init(void) { return 0; }
>  #endif
>  int platform_bus_init(void);
> +int faux_bus_init(void);
>  void cpu_dev_init(void);
>  void container_dev_init(void);
>  #ifdef CONFIG_AUXILIARY_BUS
> diff --git a/drivers/base/faux.c b/drivers/base/faux.c
> new file mode 100644
> index 000000000000..9b28643afc45
> --- /dev/null
> +++ b/drivers/base/faux.c
> @@ -0,0 +1,196 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Copyright (c) 2025 Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> + * 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"
> +
> +#define MAX_NAME_SIZE	256	/* Max size of a faux_device name */
> +
> +/*
> + * Internal wrapper structure so we can hold the memory
> + * for the driver and the name string of the faux device.
> + */
> +struct faux_object {
> +	struct faux_device faux_dev;
> +	const struct faux_driver_ops *faux_ops;
> +	char name[];
> +};
> +#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_driver_ops *faux_ops = faux_obj->faux_ops;
> +	int ret = 0;
> +
> +	if (faux_ops && faux_ops->probe)

Is there any use for faux_ops being NULL (or probe being NULL for that 
matter)? I can't think of one. So faux_device_create should check that 
and fail instead of checking here.

> +		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_driver_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 - create and register a faux device and driver
> + * @name: name of the device and driver we are adding
> + * @faux_ops: struct faux_driver_ops that the new device will call back into, can be NULL
> + *
> + * Create a new faux device and driver, both with the same name, and
> + * register them in the driver core properly.  The probe() callback of
> + * @faux_ops will be called with the new device that is created for the
> + * caller to do something with.
> + *
> + * Note, when this function is called, the functions specified in struct
> + * faux_ops will 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, struct faux_driver_ops *faux_ops)

const struct faux_driver_ops

Rob