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1da177e4 LT |
1 | |
2 | sysfs - _The_ filesystem for exporting kernel objects. | |
3 | ||
4 | Patrick Mochel <mochel@osdl.org> | |
f8a1af6b | 5 | Mike Murphy <mamurph@cs.clemson.edu> |
1da177e4 | 6 | |
f8a1af6b MM |
7 | Revised: 22 February 2009 |
8 | Original: 10 January 2003 | |
1da177e4 LT |
9 | |
10 | ||
11 | What it is: | |
12 | ~~~~~~~~~~~ | |
13 | ||
14 | sysfs is a ram-based filesystem initially based on ramfs. It provides | |
15 | a means to export kernel data structures, their attributes, and the | |
16 | linkages between them to userspace. | |
17 | ||
18 | sysfs is tied inherently to the kobject infrastructure. Please read | |
19 | Documentation/kobject.txt for more information concerning the kobject | |
20 | interface. | |
21 | ||
22 | ||
23 | Using sysfs | |
24 | ~~~~~~~~~~~ | |
25 | ||
a39ea210 LAG |
26 | sysfs is always compiled in if CONFIG_SYSFS is defined. You can access |
27 | it by doing: | |
1da177e4 LT |
28 | |
29 | mount -t sysfs sysfs /sys | |
30 | ||
31 | ||
32 | Directory Creation | |
33 | ~~~~~~~~~~~~~~~~~~ | |
34 | ||
35 | For every kobject that is registered with the system, a directory is | |
36 | created for it in sysfs. That directory is created as a subdirectory | |
37 | of the kobject's parent, expressing internal object hierarchies to | |
38 | userspace. Top-level directories in sysfs represent the common | |
39 | ancestors of object hierarchies; i.e. the subsystems the objects | |
40 | belong to. | |
41 | ||
42 | Sysfs internally stores the kobject that owns the directory in the | |
43 | ->d_fsdata pointer of the directory's dentry. This allows sysfs to do | |
44 | reference counting directly on the kobject when the file is opened and | |
45 | closed. | |
46 | ||
47 | ||
48 | Attributes | |
49 | ~~~~~~~~~~ | |
50 | ||
51 | Attributes can be exported for kobjects in the form of regular files in | |
52 | the filesystem. Sysfs forwards file I/O operations to methods defined | |
53 | for the attributes, providing a means to read and write kernel | |
54 | attributes. | |
55 | ||
56 | Attributes should be ASCII text files, preferably with only one value | |
f8c34f98 | 57 | per file. It is noted that it may not be efficient to contain only one |
1da177e4 LT |
58 | value per file, so it is socially acceptable to express an array of |
59 | values of the same type. | |
60 | ||
61 | Mixing types, expressing multiple lines of data, and doing fancy | |
62 | formatting of data is heavily frowned upon. Doing these things may get | |
63 | you publically humiliated and your code rewritten without notice. | |
64 | ||
65 | ||
66 | An attribute definition is simply: | |
67 | ||
68 | struct attribute { | |
69 | char * name; | |
f8a1af6b | 70 | struct module *owner; |
1da177e4 LT |
71 | mode_t mode; |
72 | }; | |
73 | ||
74 | ||
f8a1af6b MM |
75 | int sysfs_create_file(struct kobject * kobj, const struct attribute * attr); |
76 | void sysfs_remove_file(struct kobject * kobj, const struct attribute * attr); | |
1da177e4 LT |
77 | |
78 | ||
79 | A bare attribute contains no means to read or write the value of the | |
80 | attribute. Subsystems are encouraged to define their own attribute | |
81 | structure and wrapper functions for adding and removing attributes for | |
82 | a specific object type. | |
83 | ||
84 | For example, the driver model defines struct device_attribute like: | |
85 | ||
86 | struct device_attribute { | |
f8a1af6b MM |
87 | struct attribute attr; |
88 | ssize_t (*show)(struct device *dev, struct device_attribute *attr, | |
89 | char *buf); | |
90 | ssize_t (*store)(struct device *dev, struct device_attribute *attr, | |
91 | const char *buf, size_t count); | |
1da177e4 LT |
92 | }; |
93 | ||
94 | int device_create_file(struct device *, struct device_attribute *); | |
95 | void device_remove_file(struct device *, struct device_attribute *); | |
96 | ||
97 | It also defines this helper for defining device attributes: | |
98 | ||
f8a1af6b MM |
99 | #define DEVICE_ATTR(_name, _mode, _show, _store) \ |
100 | struct device_attribute dev_attr_##_name = __ATTR(_name, _mode, _show, _store) | |
1da177e4 LT |
101 | |
102 | For example, declaring | |
103 | ||
91e49001 | 104 | static DEVICE_ATTR(foo, S_IWUSR | S_IRUGO, show_foo, store_foo); |
1da177e4 LT |
105 | |
106 | is equivalent to doing: | |
107 | ||
108 | static struct device_attribute dev_attr_foo = { | |
109 | .attr = { | |
110 | .name = "foo", | |
91e49001 | 111 | .mode = S_IWUSR | S_IRUGO, |
f8a1af6b MM |
112 | .show = show_foo, |
113 | .store = store_foo, | |
1da177e4 | 114 | }, |
1da177e4 LT |
115 | }; |
116 | ||
117 | ||
118 | Subsystem-Specific Callbacks | |
119 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
120 | ||
121 | When a subsystem defines a new attribute type, it must implement a | |
122 | set of sysfs operations for forwarding read and write calls to the | |
123 | show and store methods of the attribute owners. | |
124 | ||
125 | struct sysfs_ops { | |
f8d825bf JV |
126 | ssize_t (*show)(struct kobject *, struct attribute *, char *); |
127 | ssize_t (*store)(struct kobject *, struct attribute *, const char *); | |
1da177e4 LT |
128 | }; |
129 | ||
130 | [ Subsystems should have already defined a struct kobj_type as a | |
131 | descriptor for this type, which is where the sysfs_ops pointer is | |
132 | stored. See the kobject documentation for more information. ] | |
133 | ||
134 | When a file is read or written, sysfs calls the appropriate method | |
135 | for the type. The method then translates the generic struct kobject | |
136 | and struct attribute pointers to the appropriate pointer types, and | |
137 | calls the associated methods. | |
138 | ||
139 | ||
140 | To illustrate: | |
141 | ||
f8d825bf | 142 | #define to_dev_attr(_attr) container_of(_attr, struct device_attribute, attr) |
1da177e4 LT |
143 | #define to_dev(d) container_of(d, struct device, kobj) |
144 | ||
145 | static ssize_t | |
146 | dev_attr_show(struct kobject * kobj, struct attribute * attr, char * buf) | |
147 | { | |
148 | struct device_attribute * dev_attr = to_dev_attr(attr); | |
149 | struct device * dev = to_dev(kobj); | |
150 | ssize_t ret = 0; | |
151 | ||
152 | if (dev_attr->show) | |
f8d825bf | 153 | ret = dev_attr->show(dev, buf); |
1da177e4 LT |
154 | return ret; |
155 | } | |
156 | ||
157 | ||
158 | ||
159 | Reading/Writing Attribute Data | |
160 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
161 | ||
162 | To read or write attributes, show() or store() methods must be | |
163 | specified when declaring the attribute. The method types should be as | |
164 | simple as those defined for device attributes: | |
165 | ||
f8a1af6b MM |
166 | ssize_t (*show)(struct device * dev, struct device_attribute * attr, |
167 | char * buf); | |
168 | ssize_t (*store)(struct device * dev, struct device_attribute * attr, | |
169 | const char * buf); | |
1da177e4 | 170 | |
f8a1af6b | 171 | IOW, they should take only an object, an attribute, and a buffer as parameters. |
1da177e4 LT |
172 | |
173 | ||
174 | sysfs allocates a buffer of size (PAGE_SIZE) and passes it to the | |
175 | method. Sysfs will call the method exactly once for each read or | |
176 | write. This forces the following behavior on the method | |
177 | implementations: | |
178 | ||
179 | - On read(2), the show() method should fill the entire buffer. | |
180 | Recall that an attribute should only be exporting one value, or an | |
181 | array of similar values, so this shouldn't be that expensive. | |
182 | ||
2424b5dd DW |
183 | This allows userspace to do partial reads and forward seeks |
184 | arbitrarily over the entire file at will. If userspace seeks back to | |
185 | zero or does a pread(2) with an offset of '0' the show() method will | |
186 | be called again, rearmed, to fill the buffer. | |
1da177e4 LT |
187 | |
188 | - On write(2), sysfs expects the entire buffer to be passed during the | |
189 | first write. Sysfs then passes the entire buffer to the store() | |
190 | method. | |
191 | ||
192 | When writing sysfs files, userspace processes should first read the | |
193 | entire file, modify the values it wishes to change, then write the | |
194 | entire buffer back. | |
195 | ||
196 | Attribute method implementations should operate on an identical | |
197 | buffer when reading and writing values. | |
198 | ||
199 | Other notes: | |
200 | ||
2424b5dd DW |
201 | - Writing causes the show() method to be rearmed regardless of current |
202 | file position. | |
203 | ||
1da177e4 LT |
204 | - The buffer will always be PAGE_SIZE bytes in length. On i386, this |
205 | is 4096. | |
206 | ||
207 | - show() methods should return the number of bytes printed into the | |
208 | buffer. This is the return value of snprintf(). | |
209 | ||
210 | - show() should always use snprintf(). | |
211 | ||
212 | - store() should return the number of bytes used from the buffer. This | |
213 | can be done using strlen(). | |
214 | ||
215 | - show() or store() can always return errors. If a bad value comes | |
216 | through, be sure to return an error. | |
217 | ||
218 | - The object passed to the methods will be pinned in memory via sysfs | |
219 | referencing counting its embedded object. However, the physical | |
220 | entity (e.g. device) the object represents may not be present. Be | |
221 | sure to have a way to check this, if necessary. | |
222 | ||
223 | ||
224 | A very simple (and naive) implementation of a device attribute is: | |
225 | ||
3eb8c783 | 226 | static ssize_t show_name(struct device *dev, struct device_attribute *attr, char *buf) |
1da177e4 | 227 | { |
f8d825bf | 228 | return snprintf(buf, PAGE_SIZE, "%s\n", dev->name); |
1da177e4 LT |
229 | } |
230 | ||
231 | static ssize_t store_name(struct device * dev, const char * buf) | |
232 | { | |
f8d825bf JV |
233 | sscanf(buf, "%20s", dev->name); |
234 | return strnlen(buf, PAGE_SIZE); | |
1da177e4 LT |
235 | } |
236 | ||
f8d825bf | 237 | static DEVICE_ATTR(name, S_IRUGO, show_name, store_name); |
1da177e4 LT |
238 | |
239 | ||
240 | (Note that the real implementation doesn't allow userspace to set the | |
241 | name for a device.) | |
242 | ||
243 | ||
244 | Top Level Directory Layout | |
245 | ~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
246 | ||
247 | The sysfs directory arrangement exposes the relationship of kernel | |
248 | data structures. | |
249 | ||
fff9289b | 250 | The top level sysfs directory looks like: |
1da177e4 LT |
251 | |
252 | block/ | |
253 | bus/ | |
254 | class/ | |
e105b8bf | 255 | dev/ |
1da177e4 LT |
256 | devices/ |
257 | firmware/ | |
258 | net/ | |
c86d90df | 259 | fs/ |
1da177e4 LT |
260 | |
261 | devices/ contains a filesystem representation of the device tree. It maps | |
262 | directly to the internal kernel device tree, which is a hierarchy of | |
263 | struct device. | |
264 | ||
265 | bus/ contains flat directory layout of the various bus types in the | |
266 | kernel. Each bus's directory contains two subdirectories: | |
267 | ||
268 | devices/ | |
269 | drivers/ | |
270 | ||
271 | devices/ contains symlinks for each device discovered in the system | |
272 | that point to the device's directory under root/. | |
273 | ||
274 | drivers/ contains a directory for each device driver that is loaded | |
275 | for devices on that particular bus (this assumes that drivers do not | |
276 | span multiple bus types). | |
277 | ||
c86d90df MS |
278 | fs/ contains a directory for some filesystems. Currently each |
279 | filesystem wanting to export attributes must create its own hierarchy | |
280 | below fs/ (see ./fuse.txt for an example). | |
281 | ||
e105b8bf DW |
282 | dev/ contains two directories char/ and block/. Inside these two |
283 | directories there are symlinks named <major>:<minor>. These symlinks | |
284 | point to the sysfs directory for the given device. /sys/dev provides a | |
285 | quick way to lookup the sysfs interface for a device from the result of | |
286 | a stat(2) operation. | |
1da177e4 LT |
287 | |
288 | More information can driver-model specific features can be found in | |
289 | Documentation/driver-model/. | |
290 | ||
291 | ||
292 | TODO: Finish this section. | |
293 | ||
294 | ||
295 | Current Interfaces | |
296 | ~~~~~~~~~~~~~~~~~~ | |
297 | ||
298 | The following interface layers currently exist in sysfs: | |
299 | ||
300 | ||
301 | - devices (include/linux/device.h) | |
302 | ---------------------------------- | |
303 | Structure: | |
304 | ||
305 | struct device_attribute { | |
f8a1af6b MM |
306 | struct attribute attr; |
307 | ssize_t (*show)(struct device *dev, struct device_attribute *attr, | |
308 | char *buf); | |
309 | ssize_t (*store)(struct device *dev, struct device_attribute *attr, | |
310 | const char *buf, size_t count); | |
1da177e4 LT |
311 | }; |
312 | ||
313 | Declaring: | |
314 | ||
f8a1af6b | 315 | DEVICE_ATTR(_name, _mode, _show, _store); |
1da177e4 LT |
316 | |
317 | Creation/Removal: | |
318 | ||
319 | int device_create_file(struct device *device, struct device_attribute * attr); | |
320 | void device_remove_file(struct device * dev, struct device_attribute * attr); | |
321 | ||
322 | ||
323 | - bus drivers (include/linux/device.h) | |
324 | -------------------------------------- | |
325 | Structure: | |
326 | ||
327 | struct bus_attribute { | |
328 | struct attribute attr; | |
329 | ssize_t (*show)(struct bus_type *, char * buf); | |
330 | ssize_t (*store)(struct bus_type *, const char * buf); | |
331 | }; | |
332 | ||
333 | Declaring: | |
334 | ||
f8d825bf | 335 | BUS_ATTR(_name, _mode, _show, _store) |
1da177e4 LT |
336 | |
337 | Creation/Removal: | |
338 | ||
339 | int bus_create_file(struct bus_type *, struct bus_attribute *); | |
340 | void bus_remove_file(struct bus_type *, struct bus_attribute *); | |
341 | ||
342 | ||
343 | - device drivers (include/linux/device.h) | |
344 | ----------------------------------------- | |
345 | ||
346 | Structure: | |
347 | ||
348 | struct driver_attribute { | |
349 | struct attribute attr; | |
350 | ssize_t (*show)(struct device_driver *, char * buf); | |
f8a1af6b MM |
351 | ssize_t (*store)(struct device_driver *, const char * buf, |
352 | size_t count); | |
1da177e4 LT |
353 | }; |
354 | ||
355 | Declaring: | |
356 | ||
f8d825bf | 357 | DRIVER_ATTR(_name, _mode, _show, _store) |
1da177e4 LT |
358 | |
359 | Creation/Removal: | |
360 | ||
361 | int driver_create_file(struct device_driver *, struct driver_attribute *); | |
362 | void driver_remove_file(struct device_driver *, struct driver_attribute *); | |
363 | ||
364 |