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