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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 | |
a5307032 | 7 | Revised: 15 July 2010 |
f8a1af6b | 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 | ||
26579ab7 PC |
94 | int device_create_file(struct device *, const struct device_attribute *); |
95 | void device_remove_file(struct device *, const struct device_attribute *); | |
1da177e4 LT |
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 | 126 | ssize_t (*show)(struct kobject *, struct attribute *, char *); |
30a69000 | 127 | ssize_t (*store)(struct kobject *, struct attribute *, const char *, size_t); |
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 | ||
30a69000 | 142 | #define to_dev(obj) container_of(obj, struct device, kobj) |
f8d825bf | 143 | #define to_dev_attr(_attr) container_of(_attr, struct device_attribute, attr) |
1da177e4 | 144 | |
30a69000 BVA |
145 | static ssize_t dev_attr_show(struct kobject *kobj, struct attribute *attr, |
146 | char *buf) | |
1da177e4 | 147 | { |
30a69000 BVA |
148 | struct device_attribute *dev_attr = to_dev_attr(attr); |
149 | struct device *dev = to_dev(kobj); | |
150 | ssize_t ret = -EIO; | |
1da177e4 LT |
151 | |
152 | if (dev_attr->show) | |
30a69000 BVA |
153 | ret = dev_attr->show(dev, dev_attr, buf); |
154 | if (ret >= (ssize_t)PAGE_SIZE) { | |
155 | print_symbol("dev_attr_show: %s returned bad count\n", | |
156 | (unsigned long)dev_attr->show); | |
157 | } | |
1da177e4 LT |
158 | return ret; |
159 | } | |
160 | ||
161 | ||
162 | ||
163 | Reading/Writing Attribute Data | |
164 | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
165 | ||
166 | To read or write attributes, show() or store() methods must be | |
167 | specified when declaring the attribute. The method types should be as | |
168 | simple as those defined for device attributes: | |
169 | ||
30a69000 BVA |
170 | ssize_t (*show)(struct device *dev, struct device_attribute *attr, char *buf); |
171 | ssize_t (*store)(struct device *dev, struct device_attribute *attr, | |
172 | const char *buf, size_t count); | |
1da177e4 | 173 | |
f8a1af6b | 174 | IOW, they should take only an object, an attribute, and a buffer as parameters. |
1da177e4 LT |
175 | |
176 | ||
177 | sysfs allocates a buffer of size (PAGE_SIZE) and passes it to the | |
178 | method. Sysfs will call the method exactly once for each read or | |
179 | write. This forces the following behavior on the method | |
180 | implementations: | |
181 | ||
182 | - On read(2), the show() method should fill the entire buffer. | |
183 | Recall that an attribute should only be exporting one value, or an | |
184 | array of similar values, so this shouldn't be that expensive. | |
185 | ||
2424b5dd DW |
186 | This allows userspace to do partial reads and forward seeks |
187 | arbitrarily over the entire file at will. If userspace seeks back to | |
188 | zero or does a pread(2) with an offset of '0' the show() method will | |
189 | be called again, rearmed, to fill the buffer. | |
1da177e4 LT |
190 | |
191 | - On write(2), sysfs expects the entire buffer to be passed during the | |
192 | first write. Sysfs then passes the entire buffer to the store() | |
193 | method. | |
194 | ||
195 | When writing sysfs files, userspace processes should first read the | |
196 | entire file, modify the values it wishes to change, then write the | |
197 | entire buffer back. | |
198 | ||
199 | Attribute method implementations should operate on an identical | |
200 | buffer when reading and writing values. | |
201 | ||
202 | Other notes: | |
203 | ||
2424b5dd DW |
204 | - Writing causes the show() method to be rearmed regardless of current |
205 | file position. | |
206 | ||
1da177e4 LT |
207 | - The buffer will always be PAGE_SIZE bytes in length. On i386, this |
208 | is 4096. | |
209 | ||
210 | - show() methods should return the number of bytes printed into the | |
211 | buffer. This is the return value of snprintf(). | |
212 | ||
213 | - show() should always use snprintf(). | |
214 | ||
30a69000 BVA |
215 | - store() should return the number of bytes used from the buffer. If the |
216 | entire buffer has been used, just return the count argument. | |
1da177e4 LT |
217 | |
218 | - show() or store() can always return errors. If a bad value comes | |
219 | through, be sure to return an error. | |
220 | ||
221 | - The object passed to the methods will be pinned in memory via sysfs | |
222 | referencing counting its embedded object. However, the physical | |
223 | entity (e.g. device) the object represents may not be present. Be | |
224 | sure to have a way to check this, if necessary. | |
225 | ||
226 | ||
227 | A very simple (and naive) implementation of a device attribute is: | |
228 | ||
30a69000 BVA |
229 | static ssize_t show_name(struct device *dev, struct device_attribute *attr, |
230 | char *buf) | |
1da177e4 | 231 | { |
f8d825bf | 232 | return snprintf(buf, PAGE_SIZE, "%s\n", dev->name); |
1da177e4 LT |
233 | } |
234 | ||
30a69000 BVA |
235 | static ssize_t store_name(struct device *dev, struct device_attribute *attr, |
236 | const char *buf, size_t count) | |
1da177e4 | 237 | { |
30a69000 BVA |
238 | snprintf(dev->name, sizeof(dev->name), "%.*s", |
239 | (int)min(count, sizeof(dev->name) - 1), buf); | |
240 | return count; | |
1da177e4 LT |
241 | } |
242 | ||
f8d825bf | 243 | static DEVICE_ATTR(name, S_IRUGO, show_name, store_name); |
1da177e4 LT |
244 | |
245 | ||
246 | (Note that the real implementation doesn't allow userspace to set the | |
247 | name for a device.) | |
248 | ||
249 | ||
250 | Top Level Directory Layout | |
251 | ~~~~~~~~~~~~~~~~~~~~~~~~~~ | |
252 | ||
253 | The sysfs directory arrangement exposes the relationship of kernel | |
254 | data structures. | |
255 | ||
fff9289b | 256 | The top level sysfs directory looks like: |
1da177e4 LT |
257 | |
258 | block/ | |
259 | bus/ | |
260 | class/ | |
e105b8bf | 261 | dev/ |
1da177e4 LT |
262 | devices/ |
263 | firmware/ | |
264 | net/ | |
c86d90df | 265 | fs/ |
1da177e4 LT |
266 | |
267 | devices/ contains a filesystem representation of the device tree. It maps | |
268 | directly to the internal kernel device tree, which is a hierarchy of | |
269 | struct device. | |
270 | ||
271 | bus/ contains flat directory layout of the various bus types in the | |
272 | kernel. Each bus's directory contains two subdirectories: | |
273 | ||
274 | devices/ | |
275 | drivers/ | |
276 | ||
277 | devices/ contains symlinks for each device discovered in the system | |
278 | that point to the device's directory under root/. | |
279 | ||
280 | drivers/ contains a directory for each device driver that is loaded | |
281 | for devices on that particular bus (this assumes that drivers do not | |
282 | span multiple bus types). | |
283 | ||
c86d90df MS |
284 | fs/ contains a directory for some filesystems. Currently each |
285 | filesystem wanting to export attributes must create its own hierarchy | |
286 | below fs/ (see ./fuse.txt for an example). | |
287 | ||
e105b8bf DW |
288 | dev/ contains two directories char/ and block/. Inside these two |
289 | directories there are symlinks named <major>:<minor>. These symlinks | |
290 | point to the sysfs directory for the given device. /sys/dev provides a | |
291 | quick way to lookup the sysfs interface for a device from the result of | |
292 | a stat(2) operation. | |
1da177e4 LT |
293 | |
294 | More information can driver-model specific features can be found in | |
295 | Documentation/driver-model/. | |
296 | ||
297 | ||
298 | TODO: Finish this section. | |
299 | ||
300 | ||
301 | Current Interfaces | |
302 | ~~~~~~~~~~~~~~~~~~ | |
303 | ||
304 | The following interface layers currently exist in sysfs: | |
305 | ||
306 | ||
307 | - devices (include/linux/device.h) | |
308 | ---------------------------------- | |
309 | Structure: | |
310 | ||
311 | struct device_attribute { | |
f8a1af6b MM |
312 | struct attribute attr; |
313 | ssize_t (*show)(struct device *dev, struct device_attribute *attr, | |
314 | char *buf); | |
315 | ssize_t (*store)(struct device *dev, struct device_attribute *attr, | |
316 | const char *buf, size_t count); | |
1da177e4 LT |
317 | }; |
318 | ||
319 | Declaring: | |
320 | ||
f8a1af6b | 321 | DEVICE_ATTR(_name, _mode, _show, _store); |
1da177e4 LT |
322 | |
323 | Creation/Removal: | |
324 | ||
26579ab7 PC |
325 | int device_create_file(struct device *dev, const struct device_attribute * attr); |
326 | void device_remove_file(struct device *dev, const struct device_attribute * attr); | |
1da177e4 LT |
327 | |
328 | ||
329 | - bus drivers (include/linux/device.h) | |
330 | -------------------------------------- | |
331 | Structure: | |
332 | ||
333 | struct bus_attribute { | |
334 | struct attribute attr; | |
335 | ssize_t (*show)(struct bus_type *, char * buf); | |
a5307032 | 336 | ssize_t (*store)(struct bus_type *, const char * buf, size_t count); |
1da177e4 LT |
337 | }; |
338 | ||
339 | Declaring: | |
340 | ||
f8d825bf | 341 | BUS_ATTR(_name, _mode, _show, _store) |
1da177e4 LT |
342 | |
343 | Creation/Removal: | |
344 | ||
345 | int bus_create_file(struct bus_type *, struct bus_attribute *); | |
346 | void bus_remove_file(struct bus_type *, struct bus_attribute *); | |
347 | ||
348 | ||
349 | - device drivers (include/linux/device.h) | |
350 | ----------------------------------------- | |
351 | ||
352 | Structure: | |
353 | ||
354 | struct driver_attribute { | |
355 | struct attribute attr; | |
356 | ssize_t (*show)(struct device_driver *, char * buf); | |
f8a1af6b MM |
357 | ssize_t (*store)(struct device_driver *, const char * buf, |
358 | size_t count); | |
1da177e4 LT |
359 | }; |
360 | ||
361 | Declaring: | |
362 | ||
f8d825bf | 363 | DRIVER_ATTR(_name, _mode, _show, _store) |
1da177e4 LT |
364 | |
365 | Creation/Removal: | |
366 | ||
099c2f21 PC |
367 | int driver_create_file(struct device_driver *, const struct driver_attribute *); |
368 | void driver_remove_file(struct device_driver *, const struct driver_attribute *); | |
1da177e4 LT |
369 | |
370 |