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Based on kernel version 4.3. Page generated on 2015-11-02 12:49 EST.

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() must not use snprintf() when formatting the value to be
216	  returned to user space. If you can guarantee that an overflow
217	  will never happen you can use sprintf() otherwise you must use
218	  scnprintf().
219	
220	- store() should return the number of bytes used from the buffer. If the
221	  entire buffer has been used, just return the count argument.
222	
223	- show() or store() can always return errors. If a bad value comes
224	  through, be sure to return an error.
225	
226	- The object passed to the methods will be pinned in memory via sysfs
227	  referencing counting its embedded object. However, the physical 
228	  entity (e.g. device) the object represents may not be present. Be 
229	  sure to have a way to check this, if necessary. 
230	
231	
232	A very simple (and naive) implementation of a device attribute is:
233	
234	static ssize_t show_name(struct device *dev, struct device_attribute *attr,
235	                         char *buf)
236	{
237		return scnprintf(buf, PAGE_SIZE, "%s\n", dev->name);
238	}
239	
240	static ssize_t store_name(struct device *dev, struct device_attribute *attr,
241	                          const char *buf, size_t count)
242	{
243	        snprintf(dev->name, sizeof(dev->name), "%.*s",
244	                 (int)min(count, sizeof(dev->name) - 1), buf);
245		return count;
246	}
247	
248	static DEVICE_ATTR(name, S_IRUGO, show_name, store_name);
249	
250	
251	(Note that the real implementation doesn't allow userspace to set the 
252	name for a device.)
253	
254	
255	Top Level Directory Layout
256	~~~~~~~~~~~~~~~~~~~~~~~~~~
257	
258	The sysfs directory arrangement exposes the relationship of kernel
259	data structures. 
260	
261	The top level sysfs directory looks like:
262	
263	block/
264	bus/
265	class/
266	dev/
267	devices/
268	firmware/
269	net/
270	fs/
271	
272	devices/ contains a filesystem representation of the device tree. It maps
273	directly to the internal kernel device tree, which is a hierarchy of
274	struct device. 
275	
276	bus/ contains flat directory layout of the various bus types in the
277	kernel. Each bus's directory contains two subdirectories:
278	
279		devices/
280		drivers/
281	
282	devices/ contains symlinks for each device discovered in the system
283	that point to the device's directory under root/.
284	
285	drivers/ contains a directory for each device driver that is loaded
286	for devices on that particular bus (this assumes that drivers do not
287	span multiple bus types).
288	
289	fs/ contains a directory for some filesystems.  Currently each
290	filesystem wanting to export attributes must create its own hierarchy
291	below fs/ (see ./fuse.txt for an example).
292	
293	dev/ contains two directories char/ and block/. Inside these two
294	directories there are symlinks named <major>:<minor>.  These symlinks
295	point to the sysfs directory for the given device.  /sys/dev provides a
296	quick way to lookup the sysfs interface for a device from the result of
297	a stat(2) operation.
298	
299	More information can driver-model specific features can be found in
300	Documentation/driver-model/. 
301	
302	
303	TODO: Finish this section.
304	
305	
306	Current Interfaces
307	~~~~~~~~~~~~~~~~~~
308	
309	The following interface layers currently exist in sysfs:
310	
311	
312	- devices (include/linux/device.h)
313	----------------------------------
314	Structure:
315	
316	struct device_attribute {
317		struct attribute	attr;
318		ssize_t (*show)(struct device *dev, struct device_attribute *attr,
319				char *buf);
320		ssize_t (*store)(struct device *dev, struct device_attribute *attr,
321				 const char *buf, size_t count);
322	};
323	
324	Declaring:
325	
326	DEVICE_ATTR(_name, _mode, _show, _store);
327	
328	Creation/Removal:
329	
330	int device_create_file(struct device *dev, const struct device_attribute * attr);
331	void device_remove_file(struct device *dev, const struct device_attribute * attr);
332	
333	
334	- bus drivers (include/linux/device.h)
335	--------------------------------------
336	Structure:
337	
338	struct bus_attribute {
339	        struct attribute        attr;
340	        ssize_t (*show)(struct bus_type *, char * buf);
341	        ssize_t (*store)(struct bus_type *, const char * buf, size_t count);
342	};
343	
344	Declaring:
345	
346	BUS_ATTR(_name, _mode, _show, _store)
347	
348	Creation/Removal:
349	
350	int bus_create_file(struct bus_type *, struct bus_attribute *);
351	void bus_remove_file(struct bus_type *, struct bus_attribute *);
352	
353	
354	- device drivers (include/linux/device.h)
355	-----------------------------------------
356	
357	Structure:
358	
359	struct driver_attribute {
360	        struct attribute        attr;
361	        ssize_t (*show)(struct device_driver *, char * buf);
362	        ssize_t (*store)(struct device_driver *, const char * buf,
363	                         size_t count);
364	};
365	
366	Declaring:
367	
368	DRIVER_ATTR(_name, _mode, _show, _store)
369	
370	Creation/Removal:
371	
372	int driver_create_file(struct device_driver *, const struct driver_attribute *);
373	void driver_remove_file(struct device_driver *, const struct driver_attribute *);
374	
375	
376	Documentation
377	~~~~~~~~~~~~~
378	
379	The sysfs directory structure and the attributes in each directory define an
380	ABI between the kernel and user space. As for any ABI, it is important that
381	this ABI is stable and properly documented. All new sysfs attributes must be
382	documented in Documentation/ABI. See also Documentation/ABI/README for more
383	information.
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