// 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;	const struct attribute_group **groups;};#define to_faux_object(dev) container_of_const(dev, struct faux_object, faux_dev.dev) static struct device *faux_bus_root; 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; 	if (faux_ops && faux_ops->probe) {		ret = faux_ops->probe(faux_dev);		if (ret)			return ret;	} 	/*	 * Add groups after the probe succeeds to ensure resources are	 * initialized correctly	 */	ret = device_add_groups(dev, faux_obj->groups);	if (ret && faux_ops && faux_ops->remove)		faux_ops->remove(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; 	device_remove_groups(dev, faux_obj->groups); 	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,	.suppress_bind_attrs = true,}; 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.  If the * probe callback (if one is present) does NOT succeed, the creation of the * device will fail and NULL will be returned. * * 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; 	if (!faux_bus_root)		return NULL; 	faux_obj = kzalloc_obj(*faux_obj);	if (!faux_obj)		return NULL; 	/* Save off the callbacks and groups so we can use them in the future */	faux_obj->faux_ops = faux_ops;	faux_obj->groups = groups; 	/* 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_set_name(dev, "%s", name);	device_set_pm_not_required(dev); 	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;	} 	/*	 * Verify that we did bind the driver to the device (i.e. probe worked),	 * if not, let's fail the creation as trying to guess if probe was	 * successful is almost impossible to determine by the caller.	 */	if (!dev->driver) {		dev_dbg(dev, "probe did not succeed, tearing down the device\n");		faux_device_destroy(faux_dev);		faux_dev = 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:	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. * * 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,				       struct device *parent,				       const struct faux_device_ops *faux_ops){	return faux_device_create_with_groups(name, parent, 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){	struct device *root;	int ret; 	root = root_device_register("faux");	if (IS_ERR(root))		return PTR_ERR(root); 	ret = bus_register(&faux_bus_type);	if (ret)		goto err_deregister_root; 	ret = driver_register(&faux_driver);	if (ret)		goto err_deregister_bus; 	faux_bus_root = root; 	return 0; err_deregister_bus:	bus_unregister(&faux_bus_type);err_deregister_root:	root_device_unregister(root); 	return ret;}