2 sysfs - _The_ filesystem for exporting kernel objects.
4 Patrick Mochel <mochel@osdl.org>
5 Mike Murphy <mamurph@cs.clemson.edu>
7 Revised: 16 August 2011
8 Original: 10 January 2003
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.
18 sysfs is tied inherently to the kobject infrastructure. Please read
19 Documentation/kobject.txt for more information concerning the kobject
26 sysfs is always compiled in if CONFIG_SYSFS is defined. You can access
29 mount -t sysfs sysfs /sys
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
42 Sysfs internally stores a pointer to the kobject that implements a
43 directory in the kernfs_node 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().
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
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.
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.
68 An attribute definition is simply:
77 int sysfs_create_file(struct kobject * kobj, const struct attribute * attr);
78 void sysfs_remove_file(struct kobject * kobj, const struct attribute * attr);
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.
86 For example, the driver model defines struct device_attribute like:
88 struct device_attribute {
89 struct attribute attr;
90 ssize_t (*show)(struct device *dev, struct device_attribute *attr,
92 ssize_t (*store)(struct device *dev, struct device_attribute *attr,
93 const char *buf, size_t count);
96 int device_create_file(struct device *, const struct device_attribute *);
97 void device_remove_file(struct device *, const struct device_attribute *);
99 It also defines this helper for defining device attributes:
101 #define DEVICE_ATTR(_name, _mode, _show, _store) \
102 struct device_attribute dev_attr_##_name = __ATTR(_name, _mode, _show, _store)
104 For example, declaring
106 static DEVICE_ATTR(foo, S_IWUSR | S_IRUGO, show_foo, store_foo);
108 is equivalent to doing:
110 static struct device_attribute dev_attr_foo = {
113 .mode = S_IWUSR | S_IRUGO,
119 Note as stated in include/linux/kernel.h "OTHER_WRITABLE? Generally
120 considered a bad idea." so trying to set a sysfs file writable for
121 everyone will fail reverting to RO mode for "Others".
123 For the common cases sysfs.h provides convenience macros to make
124 defining attributes easier as well as making code more concise and
125 readable. The above case could be shortened to:
127 static struct device_attribute dev_attr_foo = __ATTR_RW(foo);
129 the list of helpers available to define your wrapper function is:
130 __ATTR_RO(name): assumes default name_show and mode 0444
131 __ATTR_WO(name): assumes a name_store only and is restricted to mode
132 0200 that is root write access only.
133 __ATTR_RO_MODE(name, mode): fore more restrictive RO access currently
134 only use case is the EFI System Resource Table
135 (see drivers/firmware/efi/esrt.c)
136 __ATTR_RW(name): assumes default name_show, name_store and setting
138 __ATTR_NULL: which sets the name to NULL and is used as end of list
139 indicator (see: kernel/workqueue.c)
141 Subsystem-Specific Callbacks
142 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
144 When a subsystem defines a new attribute type, it must implement a
145 set of sysfs operations for forwarding read and write calls to the
146 show and store methods of the attribute owners.
149 ssize_t (*show)(struct kobject *, struct attribute *, char *);
150 ssize_t (*store)(struct kobject *, struct attribute *, const char *, size_t);
153 [ Subsystems should have already defined a struct kobj_type as a
154 descriptor for this type, which is where the sysfs_ops pointer is
155 stored. See the kobject documentation for more information. ]
157 When a file is read or written, sysfs calls the appropriate method
158 for the type. The method then translates the generic struct kobject
159 and struct attribute pointers to the appropriate pointer types, and
160 calls the associated methods.
165 #define to_dev(obj) container_of(obj, struct device, kobj)
166 #define to_dev_attr(_attr) container_of(_attr, struct device_attribute, attr)
168 static ssize_t dev_attr_show(struct kobject *kobj, struct attribute *attr,
171 struct device_attribute *dev_attr = to_dev_attr(attr);
172 struct device *dev = to_dev(kobj);
176 ret = dev_attr->show(dev, dev_attr, buf);
177 if (ret >= (ssize_t)PAGE_SIZE) {
178 printk("dev_attr_show: %pS returned bad count\n",
186 Reading/Writing Attribute Data
187 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
189 To read or write attributes, show() or store() methods must be
190 specified when declaring the attribute. The method types should be as
191 simple as those defined for device attributes:
193 ssize_t (*show)(struct device *dev, struct device_attribute *attr, char *buf);
194 ssize_t (*store)(struct device *dev, struct device_attribute *attr,
195 const char *buf, size_t count);
197 IOW, they should take only an object, an attribute, and a buffer as parameters.
200 sysfs allocates a buffer of size (PAGE_SIZE) and passes it to the
201 method. Sysfs will call the method exactly once for each read or
202 write. This forces the following behavior on the method
205 - On read(2), the show() method should fill the entire buffer.
206 Recall that an attribute should only be exporting one value, or an
207 array of similar values, so this shouldn't be that expensive.
209 This allows userspace to do partial reads and forward seeks
210 arbitrarily over the entire file at will. If userspace seeks back to
211 zero or does a pread(2) with an offset of '0' the show() method will
212 be called again, rearmed, to fill the buffer.
214 - On write(2), sysfs expects the entire buffer to be passed during the
215 first write. Sysfs then passes the entire buffer to the store() method.
216 A terminating null is added after the data on stores. This makes
217 functions like sysfs_streq() safe to use.
219 When writing sysfs files, userspace processes should first read the
220 entire file, modify the values it wishes to change, then write the
223 Attribute method implementations should operate on an identical
224 buffer when reading and writing values.
228 - Writing causes the show() method to be rearmed regardless of current
231 - The buffer will always be PAGE_SIZE bytes in length. On i386, this
234 - show() methods should return the number of bytes printed into the
237 - show() should only use sysfs_emit() or sysfs_emit_at() when formatting
238 the value to be returned to user space.
240 - store() should return the number of bytes used from the buffer. If the
241 entire buffer has been used, just return the count argument.
243 - show() or store() can always return errors. If a bad value comes
244 through, be sure to return an error.
246 - The object passed to the methods will be pinned in memory via sysfs
247 referencing counting its embedded object. However, the physical
248 entity (e.g. device) the object represents may not be present. Be
249 sure to have a way to check this, if necessary.
252 A very simple (and naive) implementation of a device attribute is:
254 static ssize_t show_name(struct device *dev, struct device_attribute *attr,
257 return scnprintf(buf, PAGE_SIZE, "%s\n", dev->name);
260 static ssize_t store_name(struct device *dev, struct device_attribute *attr,
261 const char *buf, size_t count)
263 snprintf(dev->name, sizeof(dev->name), "%.*s",
264 (int)min(count, sizeof(dev->name) - 1), buf);
268 static DEVICE_ATTR(name, S_IRUGO, show_name, store_name);
271 (Note that the real implementation doesn't allow userspace to set the
275 Top Level Directory Layout
276 ~~~~~~~~~~~~~~~~~~~~~~~~~~
278 The sysfs directory arrangement exposes the relationship of kernel
281 The top level sysfs directory looks like:
292 devices/ contains a filesystem representation of the device tree. It maps
293 directly to the internal kernel device tree, which is a hierarchy of
296 bus/ contains flat directory layout of the various bus types in the
297 kernel. Each bus's directory contains two subdirectories:
302 devices/ contains symlinks for each device discovered in the system
303 that point to the device's directory under root/.
305 drivers/ contains a directory for each device driver that is loaded
306 for devices on that particular bus (this assumes that drivers do not
307 span multiple bus types).
309 fs/ contains a directory for some filesystems. Currently each
310 filesystem wanting to export attributes must create its own hierarchy
311 below fs/ (see ./fuse.txt for an example).
313 dev/ contains two directories char/ and block/. Inside these two
314 directories there are symlinks named <major>:<minor>. These symlinks
315 point to the sysfs directory for the given device. /sys/dev provides a
316 quick way to lookup the sysfs interface for a device from the result of
319 More information can driver-model specific features can be found in
320 Documentation/driver-api/driver-model/.
323 TODO: Finish this section.
329 The following interface layers currently exist in sysfs:
332 - devices (include/linux/device.h)
333 ----------------------------------
336 struct device_attribute {
337 struct attribute attr;
338 ssize_t (*show)(struct device *dev, struct device_attribute *attr,
340 ssize_t (*store)(struct device *dev, struct device_attribute *attr,
341 const char *buf, size_t count);
346 DEVICE_ATTR(_name, _mode, _show, _store);
350 int device_create_file(struct device *dev, const struct device_attribute * attr);
351 void device_remove_file(struct device *dev, const struct device_attribute * attr);
354 - bus drivers (include/linux/device.h)
355 --------------------------------------
358 struct bus_attribute {
359 struct attribute attr;
360 ssize_t (*show)(struct bus_type *, char * buf);
361 ssize_t (*store)(struct bus_type *, const char * buf, size_t count);
366 static BUS_ATTR_RW(name);
367 static BUS_ATTR_RO(name);
368 static BUS_ATTR_WO(name);
372 int bus_create_file(struct bus_type *, struct bus_attribute *);
373 void bus_remove_file(struct bus_type *, struct bus_attribute *);
376 - device drivers (include/linux/device.h)
377 -----------------------------------------
381 struct driver_attribute {
382 struct attribute attr;
383 ssize_t (*show)(struct device_driver *, char * buf);
384 ssize_t (*store)(struct device_driver *, const char * buf,
390 DRIVER_ATTR_RO(_name)
391 DRIVER_ATTR_RW(_name)
395 int driver_create_file(struct device_driver *, const struct driver_attribute *);
396 void driver_remove_file(struct device_driver *, const struct driver_attribute *);
402 The sysfs directory structure and the attributes in each directory define an
403 ABI between the kernel and user space. As for any ABI, it is important that
404 this ABI is stable and properly documented. All new sysfs attributes must be
405 documented in Documentation/ABI. See also Documentation/ABI/README for more