Documentation/filesystems/sysfs.txt

   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 one
  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 forward seeks
 180   arbitrarily over the entire file at will. If userspace seeks back to
 181   zero or does a pread(2) with an offset of '0' the show() method will
 182   be called again, rearmed, to fill the buffer.
 183
 184 - On write(2), sysfs expects the entire buffer to be passed during the
 185   first write. Sysfs then passes the entire buffer to the store()
 186   method.
 187
 188   When writing sysfs files, userspace processes should first read the
 189   entire file, modify the values it wishes to change, then write the
 190   entire buffer back.
 191
 192   Attribute method implementations should operate on an identical
 193   buffer when reading and writing values.
 194
 195 Other notes:
 196
 197 - Writing causes the show() method to be rearmed regardless of current
 198   file position.
 199
 200 - The buffer will always be PAGE_SIZE bytes in length. On i386, this
 201   is 4096.
 202
 203 - show() methods should return the number of bytes printed into the
 204   buffer. This is the return value of snprintf().
 205
 206 - show() should always use snprintf().
 207
 208 - store() should return the number of bytes used from the buffer. This
 209   can be done using strlen().
 210
 211 - show() or store() can always return errors. If a bad value comes
 212   through, be sure to return an error.
 213
 214 - The object passed to the methods will be pinned in memory via sysfs
 215   referencing counting its embedded object. However, the physical
 216   entity (e.g. device) the object represents may not be present. Be
 217   sure to have a way to check this, if necessary.
 218
 219
 220 A very simple (and naive) implementation of a device attribute is:
 221
 222 static ssize_t show_name(struct device *dev, struct device_attribute *attr, char *buf)
 223 {
 224         return snprintf(buf, PAGE_SIZE, "%s\n", dev->name);
 225 }
 226
 227 static ssize_t store_name(struct device * dev, const char * buf)
 228 {
 229         sscanf(buf, "%20s", dev->name);
 230         return strnlen(buf, PAGE_SIZE);
 231 }
 232
 233 static DEVICE_ATTR(name, S_IRUGO, show_name, store_name);
 234
 235
 236 (Note that the real implementation doesn't allow userspace to set the
 237 name for a device.)
 238
 239
 240 Top Level Directory Layout
 241 ~~~~~~~~~~~~~~~~~~~~~~~~~~
 242
 243 The sysfs directory arrangement exposes the relationship of kernel
 244 data structures.
 245
 246 The top level sysfs directory looks like:
 247
 248 block/
 249 bus/
 250 class/
 251 devices/
 252 firmware/
 253 net/
 254 fs/
 255
 256 devices/ contains a filesystem representation of the device tree. It maps
 257 directly to the internal kernel device tree, which is a hierarchy of
 258 struct device.
 259
 260 bus/ contains flat directory layout of the various bus types in the
 261 kernel. Each bus's directory contains two subdirectories:
 262
 263         devices/
 264         drivers/
 265
 266 devices/ contains symlinks for each device discovered in the system
 267 that point to the device's directory under root/.
 268
 269 drivers/ contains a directory for each device driver that is loaded
 270 for devices on that particular bus (this assumes that drivers do not
 271 span multiple bus types).
 272
 273 fs/ contains a directory for some filesystems.  Currently each
 274 filesystem wanting to export attributes must create its own hierarchy
 275 below fs/ (see ./fuse.txt for an example).
 276
 277
 278 More information can driver-model specific features can be found in
 279 Documentation/driver-model/.
 280
 281
 282 TODO: Finish this section.
 283
 284
 285 Current Interfaces
 286 ~~~~~~~~~~~~~~~~~~
 287
 288 The following interface layers currently exist in sysfs:
 289
 290
 291 - devices (include/linux/device.h)
 292 ----------------------------------
 293 Structure:
 294
 295 struct device_attribute {
 296         struct attribute        attr;
 297         ssize_t (*show)(struct device * dev, char * buf);
 298         ssize_t (*store)(struct device * dev, const char * buf);
 299 };
 300
 301 Declaring:
 302
 303 DEVICE_ATTR(_name, _str, _mode, _show, _store);
 304
 305 Creation/Removal:
 306
 307 int device_create_file(struct device *device, struct device_attribute * attr);
 308 void device_remove_file(struct device * dev, struct device_attribute * attr);
 309
 310
 311 - bus drivers (include/linux/device.h)
 312 --------------------------------------
 313 Structure:
 314
 315 struct bus_attribute {
 316         struct attribute        attr;
 317         ssize_t (*show)(struct bus_type *, char * buf);
 318         ssize_t (*store)(struct bus_type *, const char * buf);
 319 };
 320
 321 Declaring:
 322
 323 BUS_ATTR(_name, _mode, _show, _store)
 324
 325 Creation/Removal:
 326
 327 int bus_create_file(struct bus_type *, struct bus_attribute *);
 328 void bus_remove_file(struct bus_type *, struct bus_attribute *);
 329
 330
 331 - device drivers (include/linux/device.h)
 332 -----------------------------------------
 333
 334 Structure:
 335
 336 struct driver_attribute {
 337         struct attribute        attr;
 338         ssize_t (*show)(struct device_driver *, char * buf);
 339         ssize_t (*store)(struct device_driver *, const char * buf);
 340 };
 341
 342 Declaring:
 343
 344 DRIVER_ATTR(_name, _mode, _show, _store)
 345
 346 Creation/Removal:
 347
 348 int driver_create_file(struct device_driver *, struct driver_attribute *);
 349 void driver_remove_file(struct device_driver *, struct driver_attribute *);
 350
 351