1 // SPDX-License-Identifier: GPL-2.0
2 /*
3  * Sample kobject implementation
4  *
5  * Copyright (C) 2004-2007 Greg Kroah-Hartman <greg@kroah.com>
6  * Copyright (C) 2007 Novell Inc.
7  */
8 #include <linux/kobject.h>
9 #include <linux/string.h>
10 #include <linux/sysfs.h>
11 #include <linux/module.h>
12 #include <linux/init.h>
13 
14 /*
15  * This module shows how to create a simple subdirectory in sysfs called
16  * /sys/kernel/kobject-example  In that directory, 3 files are created:
17  * "foo", "baz", and "bar".  If an integer is written to these files, it can be
18  * later read out of it.
19  */
20 
21 static int foo;
22 static int baz;
23 static int bar;
24 
25 /*
26  * The "foo" file where a static variable is read from and written to.
27  */
28 static ssize_t foo_show(struct kobject *kobj, struct kobj_attribute *attr,
29 			char *buf)
30 {
31 	return sysfs_emit(buf, "%d\n", foo);
32 }
33 
34 static ssize_t foo_store(struct kobject *kobj, struct kobj_attribute *attr,
35 			 const char *buf, size_t count)
36 {
37 	int ret;
38 
39 	ret = kstrtoint(buf, 10, &foo);
40 	if (ret < 0)
41 		return ret;
42 
43 	return count;
44 }
45 
46 /* Sysfs attributes cannot be world-writable. */
47 static struct kobj_attribute foo_attribute =
48 	__ATTR(foo, 0664, foo_show, foo_store);
49 
50 /*
51  * More complex function where we determine which variable is being accessed by
52  * looking at the attribute for the "baz" and "bar" files.
53  */
54 static ssize_t b_show(struct kobject *kobj, struct kobj_attribute *attr,
55 		      char *buf)
56 {
57 	int var;
58 
59 	if (strcmp(attr->attr.name, "baz") == 0)
60 		var = baz;
61 	else
62 		var = bar;
63 	return sysfs_emit(buf, "%d\n", var);
64 }
65 
66 static ssize_t b_store(struct kobject *kobj, struct kobj_attribute *attr,
67 		       const char *buf, size_t count)
68 {
69 	int var, ret;
70 
71 	ret = kstrtoint(buf, 10, &var);
72 	if (ret < 0)
73 		return ret;
74 
75 	if (strcmp(attr->attr.name, "baz") == 0)
76 		baz = var;
77 	else
78 		bar = var;
79 	return count;
80 }
81 
82 static struct kobj_attribute baz_attribute =
83 	__ATTR(baz, 0664, b_show, b_store);
84 static struct kobj_attribute bar_attribute =
85 	__ATTR(bar, 0664, b_show, b_store);
86 
87 
88 /*
89  * Create a group of attributes so that we can create and destroy them all
90  * at once.
91  */
92 static struct attribute *attrs[] = {
93 	&foo_attribute.attr,
94 	&baz_attribute.attr,
95 	&bar_attribute.attr,
96 	NULL,	/* need to NULL terminate the list of attributes */
97 };
98 
99 /*
100  * An unnamed attribute group will put all of the attributes directly in
101  * the kobject directory.  If we specify a name, a subdirectory will be
102  * created for the attributes with the directory being the name of the
103  * attribute group.
104  */
105 static struct attribute_group attr_group = {
106 	.attrs = attrs,
107 };
108 
109 static struct kobject *example_kobj;
110 
111 static int __init example_init(void)
112 {
113 	int retval;
114 
115 	/*
116 	 * Create a simple kobject with the name of "kobject_example",
117 	 * located under /sys/kernel/
118 	 *
119 	 * As this is a simple directory, no uevent will be sent to
120 	 * userspace.  That is why this function should not be used for
121 	 * any type of dynamic kobjects, where the name and number are
122 	 * not known ahead of time.
123 	 */
124 	example_kobj = kobject_create_and_add("kobject_example", kernel_kobj);
125 	if (!example_kobj)
126 		return -ENOMEM;
127 
128 	/* Create the files associated with this kobject */
129 	retval = sysfs_create_group(example_kobj, &attr_group);
130 	if (retval)
131 		kobject_put(example_kobj);
132 
133 	return retval;
134 }
135 
136 static void __exit example_exit(void)
137 {
138 	kobject_put(example_kobj);
139 }
140 
141 module_init(example_init);
142 module_exit(example_exit);
143 MODULE_LICENSE("GPL v2");
144 MODULE_AUTHOR("Greg Kroah-Hartman <greg@kroah.com>");
145