1// SPDX-License-Identifier: GPL-2.0-only
2/*
3 * Copyright (c) 2025 Greg Kroah-Hartman <gregkh@linuxfoundation.org>
4 * Copyright (c) 2025 The Linux Foundation
5 *
6 * A "simple" faux bus that allows devices to be created and added
7 * automatically to it. This is to be used whenever you need to create a
8 * device that is not associated with any "real" system resources, and do
9 * not want to have to deal with a bus/driver binding logic. It is
10 * intended to be very simple, with only a create and a destroy function
11 * available.
12 */
13#include <linux/err.h>
14#include <linux/init.h>
15#include <linux/slab.h>
16#include <linux/string.h>
17#include <linux/container_of.h>
18#include <linux/device/faux.h>
19#include "base.h"
20
21/*
22 * Internal wrapper structure so we can hold a pointer to the
23 * faux_device_ops for this device.
24 */
25struct faux_object {
26 struct faux_device faux_dev;
27 const struct faux_device_ops *faux_ops;
28 const struct attribute_group **groups;
29};
30#define to_faux_object(dev) container_of_const(dev, struct faux_object, faux_dev.dev)
31
32static struct device faux_bus_root = {
33 .init_name = "faux",
34};
35
36static int faux_match(struct device *dev, const struct device_driver *drv)
37{
38 /* Match always succeeds, we only have one driver */
39 return 1;
40}
41
42static int faux_probe(struct device *dev)
43{
44 struct faux_object *faux_obj = to_faux_object(dev);
45 struct faux_device *faux_dev = &faux_obj->faux_dev;
46 const struct faux_device_ops *faux_ops = faux_obj->faux_ops;
47 int ret;
48
49 if (faux_ops && faux_ops->probe) {
50 ret = faux_ops->probe(faux_dev);
51 if (ret)
52 return ret;
53 }
54
55 /*
56 * Add groups after the probe succeeds to ensure resources are
57 * initialized correctly
58 */
59 ret = device_add_groups(dev, groups: faux_obj->groups);
60 if (ret && faux_ops && faux_ops->remove)
61 faux_ops->remove(faux_dev);
62
63 return ret;
64}
65
66static void faux_remove(struct device *dev)
67{
68 struct faux_object *faux_obj = to_faux_object(dev);
69 struct faux_device *faux_dev = &faux_obj->faux_dev;
70 const struct faux_device_ops *faux_ops = faux_obj->faux_ops;
71
72 device_remove_groups(dev, groups: faux_obj->groups);
73
74 if (faux_ops && faux_ops->remove)
75 faux_ops->remove(faux_dev);
76}
77
78static const struct bus_type faux_bus_type = {
79 .name = "faux",
80 .match = faux_match,
81 .probe = faux_probe,
82 .remove = faux_remove,
83};
84
85static struct device_driver faux_driver = {
86 .name = "faux_driver",
87 .bus = &faux_bus_type,
88 .probe_type = PROBE_FORCE_SYNCHRONOUS,
89 .suppress_bind_attrs = true,
90};
91
92static void faux_device_release(struct device *dev)
93{
94 struct faux_object *faux_obj = to_faux_object(dev);
95
96 kfree(objp: faux_obj);
97}
98
99/**
100 * faux_device_create_with_groups - Create and register with the driver
101 * core a faux device and populate the device with an initial
102 * set of sysfs attributes.
103 * @name: The name of the device we are adding, must be unique for
104 * all faux devices.
105 * @parent: Pointer to a potential parent struct device. If set to
106 * NULL, the device will be created in the "root" of the faux
107 * device tree in sysfs.
108 * @faux_ops: struct faux_device_ops that the new device will call back
109 * into, can be NULL.
110 * @groups: The set of sysfs attributes that will be created for this
111 * device when it is registered with the driver core.
112 *
113 * Create a new faux device and register it in the driver core properly.
114 * If present, callbacks in @faux_ops will be called with the device that
115 * for the caller to do something with at the proper time given the
116 * device's lifecycle.
117 *
118 * Note, when this function is called, the functions specified in struct
119 * faux_ops can be called before the function returns, so be prepared for
120 * everything to be properly initialized before that point in time. If the
121 * probe callback (if one is present) does NOT succeed, the creation of the
122 * device will fail and NULL will be returned.
123 *
124 * Return:
125 * * NULL if an error happened with creating the device
126 * * pointer to a valid struct faux_device that is registered with sysfs
127 */
128struct faux_device *faux_device_create_with_groups(const char *name,
129 struct device *parent,
130 const struct faux_device_ops *faux_ops,
131 const struct attribute_group **groups)
132{
133 struct faux_object *faux_obj;
134 struct faux_device *faux_dev;
135 struct device *dev;
136 int ret;
137
138 faux_obj = kzalloc(sizeof(*faux_obj), GFP_KERNEL);
139 if (!faux_obj)
140 return NULL;
141
142 /* Save off the callbacks and groups so we can use them in the future */
143 faux_obj->faux_ops = faux_ops;
144 faux_obj->groups = groups;
145
146 /* Initialize the device portion and register it with the driver core */
147 faux_dev = &faux_obj->faux_dev;
148 dev = &faux_dev->dev;
149
150 device_initialize(dev);
151 dev->release = faux_device_release;
152 if (parent)
153 dev->parent = parent;
154 else
155 dev->parent = &faux_bus_root;
156 dev->bus = &faux_bus_type;
157 dev_set_name(dev, name: "%s", name);
158 device_set_pm_not_required(dev);
159
160 ret = device_add(dev);
161 if (ret) {
162 pr_err("%s: device_add for faux device '%s' failed with %d\n",
163 __func__, name, ret);
164 put_device(dev);
165 return NULL;
166 }
167
168 /*
169 * Verify that we did bind the driver to the device (i.e. probe worked),
170 * if not, let's fail the creation as trying to guess if probe was
171 * successful is almost impossible to determine by the caller.
172 */
173 if (!dev->driver) {
174 dev_dbg(dev, "probe did not succeed, tearing down the device\n");
175 faux_device_destroy(faux_dev);
176 faux_dev = NULL;
177 }
178
179 return faux_dev;
180}
181EXPORT_SYMBOL_GPL(faux_device_create_with_groups);
182
183/**
184 * faux_device_create - create and register with the driver core a faux device
185 * @name: The name of the device we are adding, must be unique for all
186 * faux devices.
187 * @parent: Pointer to a potential parent struct device. If set to
188 * NULL, the device will be created in the "root" of the faux
189 * device tree in sysfs.
190 * @faux_ops: struct faux_device_ops that the new device will call back
191 * into, can be NULL.
192 *
193 * Create a new faux device and register it in the driver core properly.
194 * If present, callbacks in @faux_ops will be called with the device that
195 * for the caller to do something with at the proper time given the
196 * device's lifecycle.
197 *
198 * Note, when this function is called, the functions specified in struct
199 * faux_ops can be called before the function returns, so be prepared for
200 * everything to be properly initialized before that point in time.
201 *
202 * Return:
203 * * NULL if an error happened with creating the device
204 * * pointer to a valid struct faux_device that is registered with sysfs
205 */
206struct faux_device *faux_device_create(const char *name,
207 struct device *parent,
208 const struct faux_device_ops *faux_ops)
209{
210 return faux_device_create_with_groups(name, parent, faux_ops, NULL);
211}
212EXPORT_SYMBOL_GPL(faux_device_create);
213
214/**
215 * faux_device_destroy - destroy a faux device
216 * @faux_dev: faux device to destroy
217 *
218 * Unregisters and cleans up a device that was created with a call to
219 * faux_device_create()
220 */
221void faux_device_destroy(struct faux_device *faux_dev)
222{
223 struct device *dev = &faux_dev->dev;
224
225 if (!faux_dev)
226 return;
227
228 device_del(dev);
229
230 /* The final put_device() will clean up the memory we allocated for this device. */
231 put_device(dev);
232}
233EXPORT_SYMBOL_GPL(faux_device_destroy);
234
235int __init faux_bus_init(void)
236{
237 int ret;
238
239 ret = device_register(dev: &faux_bus_root);
240 if (ret) {
241 put_device(dev: &faux_bus_root);
242 return ret;
243 }
244
245 ret = bus_register(bus: &faux_bus_type);
246 if (ret)
247 goto error_bus;
248
249 ret = driver_register(drv: &faux_driver);
250 if (ret)
251 goto error_driver;
252
253 return ret;
254
255error_driver:
256 bus_unregister(bus: &faux_bus_type);
257
258error_bus:
259 device_unregister(dev: &faux_bus_root);
260 return ret;
261}
262