Documentation/hwmon/sysfs-interface

   1 Naming and data format standards for sysfs files
   2 ------------------------------------------------
   3
   4 The libsensors library offers an interface to the raw sensors data
   5 through the sysfs interface. See libsensors documentation and source for
   6 further information. As of writing this document, libsensors
   7 (from lm_sensors 2.8.3) is heavily chip-dependent. Adding or updating
   8 support for any given chip requires modifying the library's code.
   9 This is because libsensors was written for the procfs interface
  10 older kernel modules were using, which wasn't standardized enough.
  11 Recent versions of libsensors (from lm_sensors 2.8.2 and later) have
  12 support for the sysfs interface, though.
  13
  14 The new sysfs interface was designed to be as chip-independent as
  15 possible.
  16
  17 Note that motherboards vary widely in the connections to sensor chips.
  18 There is no standard that ensures, for example, that the second
  19 temperature sensor is connected to the CPU, or that the second fan is on
  20 the CPU. Also, some values reported by the chips need some computation
  21 before they make full sense. For example, most chips can only measure
  22 voltages between 0 and +4V. Other voltages are scaled back into that
  23 range using external resistors. Since the values of these resistors
  24 can change from motherboard to motherboard, the conversions cannot be
  25 hard coded into the driver and have to be done in user space.
  26
  27 For this reason, even if we aim at a chip-independent libsensors, it will
  28 still require a configuration file (e.g. /etc/sensors.conf) for proper
  29 values conversion, labeling of inputs and hiding of unused inputs.
  30
  31 An alternative method that some programs use is to access the sysfs
  32 files directly. This document briefly describes the standards that the
  33 drivers follow, so that an application program can scan for entries and
  34 access this data in a simple and consistent way. That said, such programs
  35 will have to implement conversion, labeling and hiding of inputs. For
  36 this reason, it is still not recommended to bypass the library.
  37
  38 If you are developing a userspace application please send us feedback on
  39 this standard.
  40
  41 Note that this standard isn't completely established yet, so it is subject
  42 to changes. If you are writing a new hardware monitoring driver those
  43 features can't seem to fit in this interface, please contact us with your
  44 extension proposal. Keep in mind that backward compatibility must be
  45 preserved.
  46
  47 Each chip gets its own directory in the sysfs /sys/devices tree.  To
  48 find all sensor chips, it is easier to follow the device symlinks from
  49 /sys/class/hwmon/hwmon*.
  50
  51 All sysfs values are fixed point numbers.
  52
  53 There is only one value per file, unlike the older /proc specification.
  54 The common scheme for files naming is: <type><number>_<item>. Usual
  55 types for sensor chips are "in" (voltage), "temp" (temperature) and
  56 "fan" (fan). Usual items are "input" (measured value), "max" (high
  57 threshold, "min" (low threshold). Numbering usually starts from 1,
  58 except for voltages which start from 0 (because most data sheets use
  59 this). A number is always used for elements that can be present more
  60 than once, even if there is a single element of the given type on the
  61 specific chip. Other files do not refer to a specific element, so
  62 they have a simple name, and no number.
  63
  64 Alarms are direct indications read from the chips. The drivers do NOT
  65 make comparisons of readings to thresholds. This allows violations
  66 between readings to be caught and alarmed. The exact definition of an
  67 alarm (for example, whether a threshold must be met or must be exceeded
  68 to cause an alarm) is chip-dependent.
  69
  70 When setting values of hwmon sysfs attributes, the string representation of
  71 the desired value must be written, note that strings which are not a number
  72 are interpreted as 0! For more on how written strings are interpreted see the
  73 "sysfs attribute writes interpretation" section at the end of this file.
  74
  75 -------------------------------------------------------------------------
  76
  77 [0-*]   denotes any positive number starting from 0
  78 [1-*]   denotes any positive number starting from 1
  79 RO      read only value
  80 RW      read/write value
  81
  82 Read/write values may be read-only for some chips, depending on the
  83 hardware implementation.
  84
  85 All entries (except name) are optional, and should only be created in a
  86 given driver if the chip has the feature.
  87
  88
  89 ********
  90 * Name *
  91 ********
  92
  93 name            The chip name.
  94                 This should be a short, lowercase string, not containing
  95                 spaces nor dashes, representing the chip name. This is
  96                 the only mandatory attribute.
  97                 I2C devices get this attribute created automatically.
  98                 RO
  99
 100
 101 ************
 102 * Voltages *
 103 ************
 104
 105 in[0-*]_min     Voltage min value.
 106                 Unit: millivolt
 107                 RW
 108
 109 in[0-*]_max     Voltage max value.
 110                 Unit: millivolt
 111                 RW
 112
 113 in[0-*]_input   Voltage input value.
 114                 Unit: millivolt
 115                 RO
 116                 Voltage measured on the chip pin.
 117                 Actual voltage depends on the scaling resistors on the
 118                 motherboard, as recommended in the chip datasheet.
 119                 This varies by chip and by motherboard.
 120                 Because of this variation, values are generally NOT scaled
 121                 by the chip driver, and must be done by the application.
 122                 However, some drivers (notably lm87 and via686a)
 123                 do scale, because of internal resistors built into a chip.
 124                 These drivers will output the actual voltage. Rule of
 125                 thumb: drivers should report the voltage values at the
 126                 "pins" of the chip.
 127
 128 in[0-*]_label   Suggested voltage channel label.
 129                 Text string
 130                 Should only be created if the driver has hints about what
 131                 this voltage channel is being used for, and user-space
 132                 doesn't. In all other cases, the label is provided by
 133                 user-space.
 134                 RO
 135
 136 cpu[0-*]_vid    CPU core reference voltage.
 137                 Unit: millivolt
 138                 RO
 139                 Not always correct.
 140
 141 vrm             Voltage Regulator Module version number.
 142                 RW (but changing it should no more be necessary)
 143                 Originally the VRM standard version multiplied by 10, but now
 144                 an arbitrary number, as not all standards have a version
 145                 number.
 146                 Affects the way the driver calculates the CPU core reference
 147                 voltage from the vid pins.
 148
 149 Also see the Alarms section for status flags associated with voltages.
 150
 151
 152 ********
 153 * Fans *
 154 ********
 155
 156 fan[1-*]_min    Fan minimum value
 157                 Unit: revolution/min (RPM)
 158                 RW
 159
 160 fan[1-*]_input  Fan input value.
 161                 Unit: revolution/min (RPM)
 162                 RO
 163
 164 fan[1-*]_div    Fan divisor.
 165                 Integer value in powers of two (1, 2, 4, 8, 16, 32, 64, 128).
 166                 RW
 167                 Some chips only support values 1, 2, 4 and 8.
 168                 Note that this is actually an internal clock divisor, which
 169                 affects the measurable speed range, not the read value.
 170
 171 fan[1-*]_target
 172                 Desired fan speed
 173                 Unit: revolution/min (RPM)
 174                 RW
 175                 Only makes sense if the chip supports closed-loop fan speed
 176                 control based on the measured fan speed.
 177
 178 fan[1-*]_label  Suggested fan channel label.
 179                 Text string
 180                 Should only be created if the driver has hints about what
 181                 this fan channel is being used for, and user-space doesn't.
 182                 In all other cases, the label is provided by user-space.
 183                 RO
 184
 185 Also see the Alarms section for status flags associated with fans.
 186
 187
 188 *******
 189 * PWM *
 190 *******
 191
 192 pwm[1-*]        Pulse width modulation fan control.
 193                 Integer value in the range 0 to 255
 194                 RW
 195                 255 is max or 100%.
 196
 197 pwm[1-*]_enable
 198                 Fan speed control method:
 199                 0: no fan speed control (i.e. fan at full speed)
 200                 1: manual fan speed control enabled (using pwm[1-*])
 201                 2+: automatic fan speed control enabled
 202                 Check individual chip documentation files for automatic mode
 203                 details.
 204                 RW
 205
 206 pwm[1-*]_mode   0: DC mode (direct current)
 207                 1: PWM mode (pulse-width modulation)
 208                 RW
 209
 210 pwm[1-*]_freq   Base PWM frequency in Hz.
 211                 Only possibly available when pwmN_mode is PWM, but not always
 212                 present even then.
 213                 RW
 214
 215 pwm[1-*]_auto_channels_temp
 216                 Select which temperature channels affect this PWM output in
 217                 auto mode. Bitfield, 1 is temp1, 2 is temp2, 4 is temp3 etc...
 218                 Which values are possible depend on the chip used.
 219                 RW
 220
 221 pwm[1-*]_auto_point[1-*]_pwm
 222 pwm[1-*]_auto_point[1-*]_temp
 223 pwm[1-*]_auto_point[1-*]_temp_hyst
 224                 Define the PWM vs temperature curve. Number of trip points is
 225                 chip-dependent. Use this for chips which associate trip points
 226                 to PWM output channels.
 227                 RW
 228
 229 OR
 230
 231 temp[1-*]_auto_point[1-*]_pwm
 232 temp[1-*]_auto_point[1-*]_temp
 233 temp[1-*]_auto_point[1-*]_temp_hyst
 234                 Define the PWM vs temperature curve. Number of trip points is
 235                 chip-dependent. Use this for chips which associate trip points
 236                 to temperature channels.
 237                 RW
 238
 239
 240 ****************
 241 * Temperatures *
 242 ****************
 243
 244 temp[1-*]_type  Sensor type selection.
 245                 Integers 1 to 6
 246                 RW
 247                 1: PII/Celeron Diode
 248                 2: 3904 transistor
 249                 3: thermal diode
 250                 4: thermistor
 251                 5: AMD AMDSI
 252                 6: Intel PECI
 253                 Not all types are supported by all chips
 254
 255 temp[1-*]_max   Temperature max value.
 256                 Unit: millidegree Celsius (or millivolt, see below)
 257                 RW
 258
 259 temp[1-*]_min   Temperature min value.
 260                 Unit: millidegree Celsius
 261                 RW
 262
 263 temp[1-*]_max_hyst
 264                 Temperature hysteresis value for max limit.
 265                 Unit: millidegree Celsius
 266                 Must be reported as an absolute temperature, NOT a delta
 267                 from the max value.
 268                 RW
 269
 270 temp[1-*]_input Temperature input value.
 271                 Unit: millidegree Celsius
 272                 RO
 273
 274 temp[1-*]_crit  Temperature critical value, typically greater than
 275                 corresponding temp_max values.
 276                 Unit: millidegree Celsius
 277                 RW
 278
 279 temp[1-*]_crit_hyst
 280                 Temperature hysteresis value for critical limit.
 281                 Unit: millidegree Celsius
 282                 Must be reported as an absolute temperature, NOT a delta
 283                 from the critical value.
 284                 RW
 285
 286 temp[1-*]_offset
 287                 Temperature offset which is added to the temperature reading
 288                 by the chip.
 289                 Unit: millidegree Celsius
 290                 Read/Write value.
 291
 292 temp[1-*]_label Suggested temperature channel label.
 293                 Text string
 294                 Should only be created if the driver has hints about what
 295                 this temperature channel is being used for, and user-space
 296                 doesn't. In all other cases, the label is provided by
 297                 user-space.
 298                 RO
 299
 300 Some chips measure temperature using external thermistors and an ADC, and
 301 report the temperature measurement as a voltage. Converting this voltage
 302 back to a temperature (or the other way around for limits) requires
 303 mathematical functions not available in the kernel, so the conversion
 304 must occur in user space. For these chips, all temp* files described
 305 above should contain values expressed in millivolt instead of millidegree
 306 Celsius. In other words, such temperature channels are handled as voltage
 307 channels by the driver.
 308
 309 Also see the Alarms section for status flags associated with temperatures.
 310
 311
 312 ************
 313 * Currents *
 314 ************
 315
 316 Note that no known chip provides current measurements as of writing,
 317 so this part is theoretical, so to say.
 318
 319 curr[1-*]_max   Current max value
 320                 Unit: milliampere
 321                 RW
 322
 323 curr[1-*]_min   Current min value.
 324                 Unit: milliampere
 325                 RW
 326
 327 curr[1-*]_input Current input value
 328                 Unit: milliampere
 329                 RO
 330
 331
 332 **********
 333 * Alarms *
 334 **********
 335
 336 Each channel or limit may have an associated alarm file, containing a
 337 boolean value. 1 means than an alarm condition exists, 0 means no alarm.
 338
 339 Usually a given chip will either use channel-related alarms, or
 340 limit-related alarms, not both. The driver should just reflect the hardware
 341 implementation.
 342
 343 in[0-*]_alarm
 344 fan[1-*]_alarm
 345 temp[1-*]_alarm
 346                 Channel alarm
 347                 0: no alarm
 348                 1: alarm
 349                 RO
 350
 351 OR
 352
 353 in[0-*]_min_alarm
 354 in[0-*]_max_alarm
 355 fan[1-*]_min_alarm
 356 temp[1-*]_min_alarm
 357 temp[1-*]_max_alarm
 358 temp[1-*]_crit_alarm
 359                 Limit alarm
 360                 0: no alarm
 361                 1: alarm
 362                 RO
 363
 364 Each input channel may have an associated fault file. This can be used
 365 to notify open diodes, unconnected fans etc. where the hardware
 366 supports it. When this boolean has value 1, the measurement for that
 367 channel should not be trusted.
 368
 369 in[0-*]_fault
 370 fan[1-*]_fault
 371 temp[1-*]_fault
 372                 Input fault condition
 373                 0: no fault occured
 374                 1: fault condition
 375                 RO
 376
 377 Some chips also offer the possibility to get beeped when an alarm occurs:
 378
 379 beep_enable     Master beep enable
 380                 0: no beeps
 381                 1: beeps
 382                 RW
 383
 384 in[0-*]_beep
 385 fan[1-*]_beep
 386 temp[1-*]_beep
 387                 Channel beep
 388                 0: disable
 389                 1: enable
 390                 RW
 391
 392 In theory, a chip could provide per-limit beep masking, but no such chip
 393 was seen so far.
 394
 395 Old drivers provided a different, non-standard interface to alarms and
 396 beeps. These interface files are deprecated, but will be kept around
 397 for compatibility reasons:
 398
 399 alarms          Alarm bitmask.
 400                 RO
 401                 Integer representation of one to four bytes.
 402                 A '1' bit means an alarm.
 403                 Chips should be programmed for 'comparator' mode so that
 404                 the alarm will 'come back' after you read the register
 405                 if it is still valid.
 406                 Generally a direct representation of a chip's internal
 407                 alarm registers; there is no standard for the position
 408                 of individual bits. For this reason, the use of this
 409                 interface file for new drivers is discouraged. Use
 410                 individual *_alarm and *_fault files instead.
 411                 Bits are defined in kernel/include/sensors.h.
 412
 413 beep_mask       Bitmask for beep.
 414                 Same format as 'alarms' with the same bit locations,
 415                 use discouraged for the same reason. Use individual
 416                 *_beep files instead.
 417                 RW
 418
 419
 420 sysfs attribute writes interpretation
 421 -------------------------------------
 422
 423 hwmon sysfs attributes always contain numbers, so the first thing to do is to
 424 convert the input to a number, there are 2 ways todo this depending whether
 425 the number can be negative or not:
 426 unsigned long u = simple_strtoul(buf, NULL, 10);
 427 long s = simple_strtol(buf, NULL, 10);
 428
 429 With buf being the buffer with the user input being passed by the kernel.
 430 Notice that we do not use the second argument of strto[u]l, and thus cannot
 431 tell when 0 is returned, if this was really 0 or is caused by invalid input.
 432 This is done deliberately as checking this everywhere would add a lot of
 433 code to the kernel.
 434
 435 Notice that it is important to always store the converted value in an
 436 unsigned long or long, so that no wrap around can happen before any further
 437 checking.
 438
 439 After the input string is converted to an (unsigned) long, the value should be
 440 checked if its acceptable. Be careful with further conversions on the value
 441 before checking it for validity, as these conversions could still cause a wrap
 442 around before the check. For example do not multiply the result, and only
 443 add/subtract if it has been divided before the add/subtract.
 444
 445 What to do if a value is found to be invalid, depends on the type of the
 446 sysfs attribute that is being set. If it is a continuous setting like a
 447 tempX_max or inX_max attribute, then the value should be clamped to its
 448 limits using SENSORS_LIMIT(value, min_limit, max_limit). If it is not
 449 continuous like for example a tempX_type, then when an invalid value is
 450 written, -EINVAL should be returned.
 451
 452 Example1, temp1_max, register is a signed 8 bit value (-128 - 127 degrees):
 453
 454         long v = simple_strtol(buf, NULL, 10) / 1000;
 455         v = SENSORS_LIMIT(v, -128, 127);
 456         /* write v to register */
 457
 458 Example2, fan divider setting, valid values 2, 4 and 8:
 459
 460         unsigned long v = simple_strtoul(buf, NULL, 10);
 461
 462         switch (v) {
 463         case 2: v = 1; break;
 464         case 4: v = 2; break;
 465         case 8: v = 3; break;
 466         default:
 467                 return -EINVAL;
 468         }
 469         /* write v to register */