linux-kernel - Re: [PATCH 1/3] driver core: add a faux bus for use when a simple device/bus is needed

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Message-ID: <Z6DchyPieQKBJ0SN@smile.fi.intel.com>
Date: Mon, 3 Feb 2025 17:11:03 +0200
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: "Rafael J. Wysocki" <rafael@...nel.org>,
	Danilo Krummrich <dakr@...nel.org>, Lyude Paul <lyude@...hat.com>,
	Alexander Lobakin <aleksander.lobakin@...el.com>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Jonathan Cameron <Jonathan.Cameron@...wei.com>,
	Liam Girdwood <lgirdwood@...il.com>, Lukas Wunner <lukas@...ner.de>,
	Mark Brown <broonie@...nel.org>,
	Maíra Canal <mairacanal@...eup.net>,
	Robin Murphy <robin.murphy@....com>,
	Simona Vetter <simona.vetter@...ll.ch>,
	Zijun Hu <quic_zijuhu@...cinc.com>, linux-kernel@...r.kernel.org,
	linux-usb@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH 1/3] driver core: add a faux bus for use when a simple
 device/bus is needed

On Mon, Feb 03, 2025 at 03:25:17PM +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.

...

> +#include <linux/device/faux.h>

I would rather think that this goes after generic inclusions...

> +#include <linux/err.h>
> +#include <linux/init.h>
> +#include <linux/slab.h>
> +#include <linux/string.h>

...somewhere here.

But looking into organisation of device.h and device/*.h,
I would rather think of the linux/faux_device.h.

> +#include "base.h"

I don't remember by heart what it does include, I would go with IWYU principle
and list above all what we use.

container_of.h
device.h
export.h
printk.h
types.h

...

> +static int faux_match(struct device *dev, const struct device_driver *drv)
> +{
> +	struct faux_object *faux_obj = to_faux_object(dev);
> +
> +	/* Match is simple, strcmp()! */
> +	return (strcmp(faux_obj->name, drv->name) == 0);

Outer parentheses are not needed.

> +}

...

> +/**
> + * __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
> + * @owner: module owner of the device/driver
> + *
> + * 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.

The kernel-doc will complain on missing Return: section.

> + */
> +struct faux_device *__faux_device_create(const char *name,
> +					       struct faux_driver_ops *faux_ops,
> +					       struct module *owner)
> +{
> +	struct device_driver *drv;
> +	struct device *dev;
> +	struct faux_object *faux_obj;
> +	struct faux_device *faux_dev;
> +	int ret;

> +	faux_obj = kzalloc(sizeof(*faux_obj) + strlen(name) + 1, GFP_KERNEL);

Potential overflow. To avoid one may use struct_size() from overflow.h.

> +	if (!faux_obj)
> +		return NULL;
> +
> +	/* Save off the name of the object into local memory */
> +	strcpy(faux_obj->name, name);
> +
> +	/* Initialize the driver portion and register it with the driver core */
> +	faux_obj->faux_ops = faux_ops;
> +	drv = &faux_obj->driver;
> +
> +	drv->owner = owner;
> +	drv->name = faux_obj->name;
> +	drv->bus = &faux_bus_type;
> +	drv->probe_type = PROBE_PREFER_ASYNCHRONOUS;
> +
> +	ret = driver_register(drv);
> +	if (ret) {
> +		pr_err("%s: driver_register for %s faux driver failed with %d\n",
> +		       __func__, name, ret);
> +		kfree(faux_obj);
> +		return NULL;
> +	}
> +
> +	/* 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;
> +	dev->bus = &faux_bus_type;
> +	dev_set_name(dev, "%s", name);
> +
> +	ret = device_add(dev);
> +	if (ret) {
> +		pr_err("%s: device_add for %s faux device failed with %d\n",
> +		       __func__, name, ret);
> +		put_device(dev);
> +		return NULL;
> +	}
> +
> +	return faux_dev;
> +}
> +EXPORT_SYMBOL_GPL(__faux_device_create);

...

> +#ifndef _FAUX_DEVICE_H_
> +#define _FAUX_DEVICE_H_

> +#include <linux/module.h>

+ 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);
> +};
> +
> +#define faux_device_create(name, faux_ops) __faux_device_create(name, faux_ops, THIS_MODULE)
> +struct faux_device *__faux_device_create(const char *name,
> +					       struct faux_driver_ops *faux_ops,
> +					       struct module *module);
> +void faux_device_destroy(struct faux_device *faux_dev);
> +
> +#endif /* _FAUX_DEVICE_H_ */

-- 
With Best Regards,
Andy Shevchenko