Re: [PATCH v4 1/9] driver core: add a faux bus for use when a simple device/bus is needed

Kurt Borja
2025-02-10T14:29:56Z

Hi Greg,

On Mon Feb 10, 2025 at 7:30 AM -05, 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@huawei.com>
> Reviewed-by: Danilo Krummrich <dakr@kernel.org>
> Reviewed-by: Lyude Paul <lyude@redhat.com>
> Reviewed-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> ---
> v4: - really removed the name logic
> - added #include <linux/container_of.h> to faux.h
> - added parent pointer to api call
> - minor documentation updates
> - made probe synchronous
> 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 | 232 ++++++++++++++++++++
> drivers/base/init.c | 1 +
> include/linux/device/faux.h | 69 ++++++
> 6 files changed, 310 insertions(+), 1 deletion(-)
> create mode 100644 drivers/base/faux.c
> create mode 100644 include/linux/device/faux.h
>
> diff --git a/Documentation/driver-api/infrastructure.rst b/Documentation/driver-api/infrastructure.rst
> index 3d52dfdfa9fd..35e36fee4238 100644
> --- a/Documentation/driver-api/infrastructure.rst
> +++ b/Documentation/driver-api/infrastructure.rst
> @@ -41,6 +41,12 @@ Device Drivers Base
> .. kernel-doc:: drivers/base/class.c
> :export:
>
> +.. kernel-doc:: include/linux/device/faux.h
> + :internal:
> +
> +.. kernel-doc:: drivers/base/faux.c
> + :export:
> +
> .. kernel-doc:: drivers/base/node.c
> :internal:
>
> 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..531e9d789ee0
> --- /dev/null
> +++ b/drivers/base/faux.c
> @@ -0,0 +1,232 @@
> +// 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"
> +
> +/*
> + * 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_FORCE_SYNCHRONOUS,
> +};
> +
> +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.
> + * @parent: Pointer to a potential parent struct device. If set to
> + * NULL, the device will be created in the "root" of the faux
> + * device tree in sysfs.
> + * @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,
> + struct device *parent,
> + const struct faux_device_ops *faux_ops,
> + const struct attribute_group **groups)
> +{
> + struct faux_object *faux_obj;
> + struct faux_device *faux_dev;
> + struct device *dev;
> + int ret;
> +
> + faux_obj = kzalloc(sizeof(*faux_obj), GFP_KERNEL);
> + 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;
> + if (parent)
> + dev->parent = parent;
> + else
> + dev->parent = &faux_bus_root;
> + 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;
> + }

Now that the probe is synchronous, what do you think about returning
-ENODEV if the device failed to bind to the driver?

This would be useful for modules that may want to unload if the probe
fails.

--
~ Kurt