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