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Linux/Documentation/filesystems/sysfs.txt

  1 
  2 sysfs - _The_ filesystem for exporting kernel objects. 
  3 
  4 Patrick Mochel  <mochel@osdl.org>
  5 Mike Murphy <mamurph@cs.clemson.edu>
  6 
  7 Revised:    16 August 2011
  8 Original:   10 January 2003
  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 if CONFIG_SYSFS is defined. You can access
 27 it by doing:
 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 a pointer to the kobject that implements a
 43 directory in the sysfs_dirent object associated with the directory. In
 44 the past this kobject pointer has been used by sysfs to do reference
 45 counting directly on the kobject whenever the file is opened or closed.
 46 With the current sysfs implementation the kobject reference count is
 47 only modified directly by the function sysfs_schedule_callback().
 48 
 49 
 50 Attributes
 51 ~~~~~~~~~~
 52 
 53 Attributes can be exported for kobjects in the form of regular files in
 54 the filesystem. Sysfs forwards file I/O operations to methods defined
 55 for the attributes, providing a means to read and write kernel
 56 attributes.
 57 
 58 Attributes should be ASCII text files, preferably with only one value
 59 per file. It is noted that it may not be efficient to contain only one
 60 value per file, so it is socially acceptable to express an array of
 61 values of the same type. 
 62 
 63 Mixing types, expressing multiple lines of data, and doing fancy
 64 formatting of data is heavily frowned upon. Doing these things may get
 65 you publicly humiliated and your code rewritten without notice. 
 66 
 67 
 68 An attribute definition is simply:
 69 
 70 struct attribute {
 71         char                    * name;
 72         struct module           *owner;
 73         umode_t                 mode;
 74 };
 75 
 76 
 77 int sysfs_create_file(struct kobject * kobj, const struct attribute * attr);
 78 void sysfs_remove_file(struct kobject * kobj, const struct attribute * attr);
 79 
 80 
 81 A bare attribute contains no means to read or write the value of the
 82 attribute. Subsystems are encouraged to define their own attribute
 83 structure and wrapper functions for adding and removing attributes for
 84 a specific object type. 
 85 
 86 For example, the driver model defines struct device_attribute like:
 87 
 88 struct device_attribute {
 89         struct attribute        attr;
 90         ssize_t (*show)(struct device *dev, struct device_attribute *attr,
 91                         char *buf);
 92         ssize_t (*store)(struct device *dev, struct device_attribute *attr,
 93                          const char *buf, size_t count);
 94 };
 95 
 96 int device_create_file(struct device *, const struct device_attribute *);
 97 void device_remove_file(struct device *, const struct device_attribute *);
 98 
 99 It also defines this helper for defining device attributes: 
100 
101 #define DEVICE_ATTR(_name, _mode, _show, _store) \
102 struct device_attribute dev_attr_##_name = __ATTR(_name, _mode, _show, _store)
103 
104 For example, declaring
105 
106 static DEVICE_ATTR(foo, S_IWUSR | S_IRUGO, show_foo, store_foo);
107 
108 is equivalent to doing:
109 
110 static struct device_attribute dev_attr_foo = {
111         .attr = {
112                 .name = "foo",
113                 .mode = S_IWUSR | S_IRUGO,
114         },
115         .show = show_foo,
116         .store = store_foo,
117 };
118 
119 
120 Subsystem-Specific Callbacks
121 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
122 
123 When a subsystem defines a new attribute type, it must implement a
124 set of sysfs operations for forwarding read and write calls to the
125 show and store methods of the attribute owners. 
126 
127 struct sysfs_ops {
128         ssize_t (*show)(struct kobject *, struct attribute *, char *);
129         ssize_t (*store)(struct kobject *, struct attribute *, const char *, size_t);
130 };
131 
132 [ Subsystems should have already defined a struct kobj_type as a
133 descriptor for this type, which is where the sysfs_ops pointer is
134 stored. See the kobject documentation for more information. ]
135 
136 When a file is read or written, sysfs calls the appropriate method
137 for the type. The method then translates the generic struct kobject
138 and struct attribute pointers to the appropriate pointer types, and
139 calls the associated methods. 
140 
141 
142 To illustrate:
143 
144 #define to_dev(obj) container_of(obj, struct device, kobj)
145 #define to_dev_attr(_attr) container_of(_attr, struct device_attribute, attr)
146 
147 static ssize_t dev_attr_show(struct kobject *kobj, struct attribute *attr,
148                              char *buf)
149 {
150         struct device_attribute *dev_attr = to_dev_attr(attr);
151         struct device *dev = to_dev(kobj);
152         ssize_t ret = -EIO;
153 
154         if (dev_attr->show)
155                 ret = dev_attr->show(dev, dev_attr, buf);
156         if (ret >= (ssize_t)PAGE_SIZE) {
157                 print_symbol("dev_attr_show: %s returned bad count\n",
158                                 (unsigned long)dev_attr->show);
159         }
160         return ret;
161 }
162 
163 
164 
165 Reading/Writing Attribute Data
166 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
167 
168 To read or write attributes, show() or store() methods must be
169 specified when declaring the attribute. The method types should be as
170 simple as those defined for device attributes:
171 
172 ssize_t (*show)(struct device *dev, struct device_attribute *attr, char *buf);
173 ssize_t (*store)(struct device *dev, struct device_attribute *attr,
174                  const char *buf, size_t count);
175 
176 IOW, they should take only an object, an attribute, and a buffer as parameters.
177 
178 
179 sysfs allocates a buffer of size (PAGE_SIZE) and passes it to the
180 method. Sysfs will call the method exactly once for each read or
181 write. This forces the following behavior on the method
182 implementations: 
183 
184 - On read(2), the show() method should fill the entire buffer. 
185   Recall that an attribute should only be exporting one value, or an
186   array of similar values, so this shouldn't be that expensive. 
187 
188   This allows userspace to do partial reads and forward seeks
189   arbitrarily over the entire file at will. If userspace seeks back to
190   zero or does a pread(2) with an offset of '0' the show() method will
191   be called again, rearmed, to fill the buffer.
192 
193 - On write(2), sysfs expects the entire buffer to be passed during the
194   first write. Sysfs then passes the entire buffer to the store()
195   method. 
196   
197   When writing sysfs files, userspace processes should first read the
198   entire file, modify the values it wishes to change, then write the
199   entire buffer back. 
200 
201   Attribute method implementations should operate on an identical
202   buffer when reading and writing values. 
203 
204 Other notes:
205 
206 - Writing causes the show() method to be rearmed regardless of current
207   file position.
208 
209 - The buffer will always be PAGE_SIZE bytes in length. On i386, this
210   is 4096. 
211 
212 - show() methods should return the number of bytes printed into the
213   buffer. This is the return value of scnprintf().
214 
215 - show() should always use scnprintf().
216 
217 - store() should return the number of bytes used from the buffer. If the
218   entire buffer has been used, just return the count argument.
219 
220 - show() or store() can always return errors. If a bad value comes
221   through, be sure to return an error.
222 
223 - The object passed to the methods will be pinned in memory via sysfs
224   referencing counting its embedded object. However, the physical 
225   entity (e.g. device) the object represents may not be present. Be 
226   sure to have a way to check this, if necessary. 
227 
228 
229 A very simple (and naive) implementation of a device attribute is:
230 
231 static ssize_t show_name(struct device *dev, struct device_attribute *attr,
232                          char *buf)
233 {
234         return scnprintf(buf, PAGE_SIZE, "%s\n", dev->name);
235 }
236 
237 static ssize_t store_name(struct device *dev, struct device_attribute *attr,
238                           const char *buf, size_t count)
239 {
240         snprintf(dev->name, sizeof(dev->name), "%.*s",
241                  (int)min(count, sizeof(dev->name) - 1), buf);
242         return count;
243 }
244 
245 static DEVICE_ATTR(name, S_IRUGO, show_name, store_name);
246 
247 
248 (Note that the real implementation doesn't allow userspace to set the 
249 name for a device.)
250 
251 
252 Top Level Directory Layout
253 ~~~~~~~~~~~~~~~~~~~~~~~~~~
254 
255 The sysfs directory arrangement exposes the relationship of kernel
256 data structures. 
257 
258 The top level sysfs directory looks like:
259 
260 block/
261 bus/
262 class/
263 dev/
264 devices/
265 firmware/
266 net/
267 fs/
268 
269 devices/ contains a filesystem representation of the device tree. It maps
270 directly to the internal kernel device tree, which is a hierarchy of
271 struct device. 
272 
273 bus/ contains flat directory layout of the various bus types in the
274 kernel. Each bus's directory contains two subdirectories:
275 
276         devices/
277         drivers/
278 
279 devices/ contains symlinks for each device discovered in the system
280 that point to the device's directory under root/.
281 
282 drivers/ contains a directory for each device driver that is loaded
283 for devices on that particular bus (this assumes that drivers do not
284 span multiple bus types).
285 
286 fs/ contains a directory for some filesystems.  Currently each
287 filesystem wanting to export attributes must create its own hierarchy
288 below fs/ (see ./fuse.txt for an example).
289 
290 dev/ contains two directories char/ and block/. Inside these two
291 directories there are symlinks named <major>:<minor>.  These symlinks
292 point to the sysfs directory for the given device.  /sys/dev provides a
293 quick way to lookup the sysfs interface for a device from the result of
294 a stat(2) operation.
295 
296 More information can driver-model specific features can be found in
297 Documentation/driver-model/. 
298 
299 
300 TODO: Finish this section.
301 
302 
303 Current Interfaces
304 ~~~~~~~~~~~~~~~~~~
305 
306 The following interface layers currently exist in sysfs:
307 
308 
309 - devices (include/linux/device.h)
310 ----------------------------------
311 Structure:
312 
313 struct device_attribute {
314         struct attribute        attr;
315         ssize_t (*show)(struct device *dev, struct device_attribute *attr,
316                         char *buf);
317         ssize_t (*store)(struct device *dev, struct device_attribute *attr,
318                          const char *buf, size_t count);
319 };
320 
321 Declaring:
322 
323 DEVICE_ATTR(_name, _mode, _show, _store);
324 
325 Creation/Removal:
326 
327 int device_create_file(struct device *dev, const struct device_attribute * attr);
328 void device_remove_file(struct device *dev, const struct device_attribute * attr);
329 
330 
331 - bus drivers (include/linux/device.h)
332 --------------------------------------
333 Structure:
334 
335 struct bus_attribute {
336         struct attribute        attr;
337         ssize_t (*show)(struct bus_type *, char * buf);
338         ssize_t (*store)(struct bus_type *, const char * buf, size_t count);
339 };
340 
341 Declaring:
342 
343 BUS_ATTR(_name, _mode, _show, _store)
344 
345 Creation/Removal:
346 
347 int bus_create_file(struct bus_type *, struct bus_attribute *);
348 void bus_remove_file(struct bus_type *, struct bus_attribute *);
349 
350 
351 - device drivers (include/linux/device.h)
352 -----------------------------------------
353 
354 Structure:
355 
356 struct driver_attribute {
357         struct attribute        attr;
358         ssize_t (*show)(struct device_driver *, char * buf);
359         ssize_t (*store)(struct device_driver *, const char * buf,
360                          size_t count);
361 };
362 
363 Declaring:
364 
365 DRIVER_ATTR(_name, _mode, _show, _store)
366 
367 Creation/Removal:
368 
369 int driver_create_file(struct device_driver *, const struct driver_attribute *);
370 void driver_remove_file(struct device_driver *, const struct driver_attribute *);
371 
372 
373 Documentation
374 ~~~~~~~~~~~~~
375 
376 The sysfs directory structure and the attributes in each directory define an
377 ABI between the kernel and user space. As for any ABI, it is important that
378 this ABI is stable and properly documented. All new sysfs attributes must be
379 documented in Documentation/ABI. See also Documentation/ABI/README for more
380 information.

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