Based on kernel version 6.11
. Page generated on 2024-09-24 08:21 EST
.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 | .. SPDX-License-Identifier: GPL-2.0 ===================================================== sysfs - _The_ filesystem for exporting kernel objects ===================================================== Patrick Mochel <mochel@osdl.org> Mike Murphy <mamurph@cs.clemson.edu> :Revised: 16 August 2011 :Original: 10 January 2003 What it is ~~~~~~~~~~ sysfs is a RAM-based filesystem initially based on ramfs. It provides a means to export kernel data structures, their attributes, and the linkages between them to userspace. sysfs is tied inherently to the kobject infrastructure. Please read Documentation/core-api/kobject.rst for more information concerning the kobject interface. Using sysfs ~~~~~~~~~~~ sysfs is always compiled in if CONFIG_SYSFS is defined. You can access it by doing:: mount -t sysfs sysfs /sys Directory Creation ~~~~~~~~~~~~~~~~~~ For every kobject that is registered with the system, a directory is created for it in sysfs. That directory is created as a subdirectory of the kobject's parent, expressing internal object hierarchies to userspace. Top-level directories in sysfs represent the common ancestors of object hierarchies; i.e. the subsystems the objects belong to. sysfs internally stores a pointer to the kobject that implements a directory in the kernfs_node object associated with the directory. In the past this kobject pointer has been used by sysfs to do reference counting directly on the kobject whenever the file is opened or closed. With the current sysfs implementation the kobject reference count is only modified directly by the function sysfs_schedule_callback(). Attributes ~~~~~~~~~~ Attributes can be exported for kobjects in the form of regular files in the filesystem. sysfs forwards file I/O operations to methods defined for the attributes, providing a means to read and write kernel attributes. Attributes should be ASCII text files, preferably with only one value per file. It is noted that it may not be efficient to contain only one value per file, so it is socially acceptable to express an array of values of the same type. Mixing types, expressing multiple lines of data, and doing fancy formatting of data is heavily frowned upon. Doing these things may get you publicly humiliated and your code rewritten without notice. An attribute definition is simply:: struct attribute { char *name; struct module *owner; umode_t mode; }; int sysfs_create_file(struct kobject * kobj, const struct attribute * attr); void sysfs_remove_file(struct kobject * kobj, const struct attribute * attr); A bare attribute contains no means to read or write the value of the attribute. Subsystems are encouraged to define their own attribute structure and wrapper functions for adding and removing attributes for a specific object type. For example, the driver model defines struct device_attribute like:: struct device_attribute { struct attribute attr; ssize_t (*show)(struct device *dev, struct device_attribute *attr, char *buf); ssize_t (*store)(struct device *dev, struct device_attribute *attr, const char *buf, size_t count); }; int device_create_file(struct device *, const struct device_attribute *); void device_remove_file(struct device *, const struct device_attribute *); It also defines this helper for defining device attributes:: #define DEVICE_ATTR(_name, _mode, _show, _store) \ struct device_attribute dev_attr_##_name = __ATTR(_name, _mode, _show, _store) For example, declaring:: static DEVICE_ATTR(foo, S_IWUSR | S_IRUGO, show_foo, store_foo); is equivalent to doing:: static struct device_attribute dev_attr_foo = { .attr = { .name = "foo", .mode = S_IWUSR | S_IRUGO, }, .show = show_foo, .store = store_foo, }; Note as stated in include/linux/kernel.h "OTHER_WRITABLE? Generally considered a bad idea." so trying to set a sysfs file writable for everyone will fail reverting to RO mode for "Others". For the common cases sysfs.h provides convenience macros to make defining attributes easier as well as making code more concise and readable. The above case could be shortened to: static struct device_attribute dev_attr_foo = __ATTR_RW(foo); the list of helpers available to define your wrapper function is: __ATTR_RO(name): assumes default name_show and mode 0444 __ATTR_WO(name): assumes a name_store only and is restricted to mode 0200 that is root write access only. __ATTR_RO_MODE(name, mode): for more restrictive RO access; currently only use case is the EFI System Resource Table (see drivers/firmware/efi/esrt.c) __ATTR_RW(name): assumes default name_show, name_store and setting mode to 0644. __ATTR_NULL: which sets the name to NULL and is used as end of list indicator (see: kernel/workqueue.c) Subsystem-Specific Callbacks ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When a subsystem defines a new attribute type, it must implement a set of sysfs operations for forwarding read and write calls to the show and store methods of the attribute owners:: struct sysfs_ops { ssize_t (*show)(struct kobject *, struct attribute *, char *); ssize_t (*store)(struct kobject *, struct attribute *, const char *, size_t); }; [ Subsystems should have already defined a struct kobj_type as a descriptor for this type, which is where the sysfs_ops pointer is stored. See the kobject documentation for more information. ] When a file is read or written, sysfs calls the appropriate method for the type. The method then translates the generic struct kobject and struct attribute pointers to the appropriate pointer types, and calls the associated methods. To illustrate:: #define to_dev_attr(_attr) container_of(_attr, struct device_attribute, attr) static ssize_t dev_attr_show(struct kobject *kobj, struct attribute *attr, char *buf) { struct device_attribute *dev_attr = to_dev_attr(attr); struct device *dev = kobj_to_dev(kobj); ssize_t ret = -EIO; if (dev_attr->show) ret = dev_attr->show(dev, dev_attr, buf); if (ret >= (ssize_t)PAGE_SIZE) { printk("dev_attr_show: %pS returned bad count\n", dev_attr->show); } return ret; } Reading/Writing Attribute Data ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ To read or write attributes, show() or store() methods must be specified when declaring the attribute. The method types should be as simple as those defined for device attributes:: ssize_t (*show)(struct device *dev, struct device_attribute *attr, char *buf); ssize_t (*store)(struct device *dev, struct device_attribute *attr, const char *buf, size_t count); IOW, they should take only an object, an attribute, and a buffer as parameters. sysfs allocates a buffer of size (PAGE_SIZE) and passes it to the method. sysfs will call the method exactly once for each read or write. This forces the following behavior on the method implementations: - On read(2), the show() method should fill the entire buffer. Recall that an attribute should only be exporting one value, or an array of similar values, so this shouldn't be that expensive. This allows userspace to do partial reads and forward seeks arbitrarily over the entire file at will. If userspace seeks back to zero or does a pread(2) with an offset of '0' the show() method will be called again, rearmed, to fill the buffer. - On write(2), sysfs expects the entire buffer to be passed during the first write. sysfs then passes the entire buffer to the store() method. A terminating null is added after the data on stores. This makes functions like sysfs_streq() safe to use. When writing sysfs files, userspace processes should first read the entire file, modify the values it wishes to change, then write the entire buffer back. Attribute method implementations should operate on an identical buffer when reading and writing values. Other notes: - Writing causes the show() method to be rearmed regardless of current file position. - The buffer will always be PAGE_SIZE bytes in length. On x86, this is 4096. - show() methods should return the number of bytes printed into the buffer. - show() should only use sysfs_emit() or sysfs_emit_at() when formatting the value to be returned to user space. - store() should return the number of bytes used from the buffer. If the entire buffer has been used, just return the count argument. - show() or store() can always return errors. If a bad value comes through, be sure to return an error. - The object passed to the methods will be pinned in memory via sysfs reference counting its embedded object. However, the physical entity (e.g. device) the object represents may not be present. Be sure to have a way to check this, if necessary. A very simple (and naive) implementation of a device attribute is:: static ssize_t show_name(struct device *dev, struct device_attribute *attr, char *buf) { return sysfs_emit(buf, "%s\n", dev->name); } static ssize_t store_name(struct device *dev, struct device_attribute *attr, const char *buf, size_t count) { snprintf(dev->name, sizeof(dev->name), "%.*s", (int)min(count, sizeof(dev->name) - 1), buf); return count; } static DEVICE_ATTR(name, S_IRUGO, show_name, store_name); (Note that the real implementation doesn't allow userspace to set the name for a device.) Top Level Directory Layout ~~~~~~~~~~~~~~~~~~~~~~~~~~ The sysfs directory arrangement exposes the relationship of kernel data structures. The top level sysfs directory looks like:: block/ bus/ class/ dev/ devices/ firmware/ fs/ hypervisor/ kernel/ module/ net/ power/ devices/ contains a filesystem representation of the device tree. It maps directly to the internal kernel device tree, which is a hierarchy of struct device. bus/ contains flat directory layout of the various bus types in the kernel. Each bus's directory contains two subdirectories:: devices/ drivers/ devices/ contains symlinks for each device discovered in the system that point to the device's directory under root/. drivers/ contains a directory for each device driver that is loaded for devices on that particular bus (this assumes that drivers do not span multiple bus types). fs/ contains a directory for some filesystems. Currently each filesystem wanting to export attributes must create its own hierarchy below fs/ (see ./fuse.rst for an example). module/ contains parameter values and state information for all loaded system modules, for both builtin and loadable modules. dev/ contains two directories: char/ and block/. Inside these two directories there are symlinks named <major>:<minor>. These symlinks point to the sysfs directory for the given device. /sys/dev provides a quick way to lookup the sysfs interface for a device from the result of a stat(2) operation. More information on driver-model specific features can be found in Documentation/driver-api/driver-model/. TODO: Finish this section. Current Interfaces ~~~~~~~~~~~~~~~~~~ The following interface layers currently exist in sysfs. devices (include/linux/device.h) -------------------------------- Structure:: struct device_attribute { struct attribute attr; ssize_t (*show)(struct device *dev, struct device_attribute *attr, char *buf); ssize_t (*store)(struct device *dev, struct device_attribute *attr, const char *buf, size_t count); }; Declaring:: DEVICE_ATTR(_name, _mode, _show, _store); Creation/Removal:: int device_create_file(struct device *dev, const struct device_attribute * attr); void device_remove_file(struct device *dev, const struct device_attribute * attr); bus drivers (include/linux/device.h) ------------------------------------ Structure:: struct bus_attribute { struct attribute attr; ssize_t (*show)(const struct bus_type *, char * buf); ssize_t (*store)(const struct bus_type *, const char * buf, size_t count); }; Declaring:: static BUS_ATTR_RW(name); static BUS_ATTR_RO(name); static BUS_ATTR_WO(name); Creation/Removal:: int bus_create_file(struct bus_type *, struct bus_attribute *); void bus_remove_file(struct bus_type *, struct bus_attribute *); device drivers (include/linux/device.h) --------------------------------------- Structure:: struct driver_attribute { struct attribute attr; ssize_t (*show)(struct device_driver *, char * buf); ssize_t (*store)(struct device_driver *, const char * buf, size_t count); }; Declaring:: DRIVER_ATTR_RO(_name) DRIVER_ATTR_RW(_name) Creation/Removal:: int driver_create_file(struct device_driver *, const struct driver_attribute *); void driver_remove_file(struct device_driver *, const struct driver_attribute *); Documentation ~~~~~~~~~~~~~ The sysfs directory structure and the attributes in each directory define an ABI between the kernel and user space. As for any ABI, it is important that this ABI is stable and properly documented. All new sysfs attributes must be documented in Documentation/ABI. See also Documentation/ABI/README for more information. |