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. Since lm-sensors 3.0.0, libsensors is
   6 completely chip-independent. It assumes that all the kernel drivers
   7 implement the standard sysfs interface described in this document.
   8 This makes adding or updating support for any given chip very easy, as
   9 libsensors, and applications using it, do not need to be modified.
  10 This is a major improvement compared to lm-sensors 2.
  11
  12 Note that motherboards vary widely in the connections to sensor chips.
  13 There is no standard that ensures, for example, that the second
  14 temperature sensor is connected to the CPU, or that the second fan is on
  15 the CPU. Also, some values reported by the chips need some computation
  16 before they make full sense. For example, most chips can only measure
  17 voltages between 0 and +4V. Other voltages are scaled back into that
  18 range using external resistors. Since the values of these resistors
  19 can change from motherboard to motherboard, the conversions cannot be
  20 hard coded into the driver and have to be done in user space.
  21
  22 For this reason, even if we aim at a chip-independent libsensors, it will
  23 still require a configuration file (e.g. /etc/sensors.conf) for proper
  24 values conversion, labeling of inputs and hiding of unused inputs.
  25
  26 An alternative method that some programs use is to access the sysfs
  27 files directly. This document briefly describes the standards that the
  28 drivers follow, so that an application program can scan for entries and
  29 access this data in a simple and consistent way. That said, such programs
  30 will have to implement conversion, labeling and hiding of inputs. For
  31 this reason, it is still not recommended to bypass the library.
  32
  33 Each chip gets its own directory in the sysfs /sys/devices tree.  To
  34 find all sensor chips, it is easier to follow the device symlinks from
  35 /sys/class/hwmon/hwmon*.
  36
  37 Up to lm-sensors 3.0.0, libsensors looks for hardware monitoring attributes
  38 in the "physical" device directory. Since lm-sensors 3.0.1, attributes found
  39 in the hwmon "class" device directory are also supported. Complex drivers
  40 (e.g. drivers for multifunction chips) may want to use this possibility to
  41 avoid namespace pollution. The only drawback will be that older versions of
  42 libsensors won't support the driver in question.
  43
  44 All sysfs values are fixed point numbers.
  45
  46 There is only one value per file, unlike the older /proc specification.
  47 The common scheme for files naming is: <type><number>_<item>. Usual
  48 types for sensor chips are "in" (voltage), "temp" (temperature) and
  49 "fan" (fan). Usual items are "input" (measured value), "max" (high
  50 threshold, "min" (low threshold). Numbering usually starts from 1,
  51 except for voltages which start from 0 (because most data sheets use
  52 this). A number is always used for elements that can be present more
  53 than once, even if there is a single element of the given type on the
  54 specific chip. Other files do not refer to a specific element, so
  55 they have a simple name, and no number.
  56
  57 Alarms are direct indications read from the chips. The drivers do NOT
  58 make comparisons of readings to thresholds. This allows violations
  59 between readings to be caught and alarmed. The exact definition of an
  60 alarm (for example, whether a threshold must be met or must be exceeded
  61 to cause an alarm) is chip-dependent.
  62
  63 When setting values of hwmon sysfs attributes, the string representation of
  64 the desired value must be written, note that strings which are not a number
  65 are interpreted as 0! For more on how written strings are interpreted see the
  66 "sysfs attribute writes interpretation" section at the end of this file.
  67
  68 -------------------------------------------------------------------------
  69
  70 [0-*]   denotes any positive number starting from 0
  71 [1-*]   denotes any positive number starting from 1
  72 RO      read only value
  73 WO      write only value
  74 RW      read/write value
  75
  76 Read/write values may be read-only for some chips, depending on the
  77 hardware implementation.
  78
  79 All entries (except name) are optional, and should only be created in a
  80 given driver if the chip has the feature.
  81
  82
  83 *********************
  84 * Global attributes *
  85 *********************
  86
  87 name            The chip name.
  88                 This should be a short, lowercase string, not containing
  89                 spaces nor dashes, representing the chip name. This is
  90                 the only mandatory attribute.
  91                 I2C devices get this attribute created automatically.
  92                 RO
  93
  94 update_interval The interval at which the chip will update readings.
  95                 Unit: millisecond
  96                 RW
  97                 Some devices have a variable update rate or interval.
  98                 This attribute can be used to change it to the desired value.
  99
 100
 101 ************
 102 * Voltages *
 103 ************
 104
 105 in[0-*]_min     Voltage min value.
 106                 Unit: millivolt
 107                 RW
 108
 109 in[0-*]_lcrit   Voltage critical min value.
 110                 Unit: millivolt
 111                 RW
 112                 If voltage drops to or below this limit, the system may
 113                 take drastic action such as power down or reset. At the very
 114                 least, it should report a fault.
 115
 116 in[0-*]_max     Voltage max value.
 117                 Unit: millivolt
 118                 RW
 119
 120 in[0-*]_crit    Voltage critical max value.
 121                 Unit: millivolt
 122                 RW
 123                 If voltage reaches or exceeds this limit, the system may
 124                 take drastic action such as power down or reset. At the very
 125                 least, it should report a fault.
 126
 127 in[0-*]_input   Voltage input value.
 128                 Unit: millivolt
 129                 RO
 130                 Voltage measured on the chip pin.
 131                 Actual voltage depends on the scaling resistors on the
 132                 motherboard, as recommended in the chip datasheet.
 133                 This varies by chip and by motherboard.
 134                 Because of this variation, values are generally NOT scaled
 135                 by the chip driver, and must be done by the application.
 136                 However, some drivers (notably lm87 and via686a)
 137                 do scale, because of internal resistors built into a chip.
 138                 These drivers will output the actual voltage. Rule of
 139                 thumb: drivers should report the voltage values at the
 140                 "pins" of the chip.
 141
 142 in[0-*]_average
 143                 Average voltage
 144                 Unit: millivolt
 145                 RO
 146
 147 in[0-*]_lowest
 148                 Historical minimum voltage
 149                 Unit: millivolt
 150                 RO
 151
 152 in[0-*]_highest
 153                 Historical maximum voltage
 154                 Unit: millivolt
 155                 RO
 156
 157 in[0-*]_reset_history
 158                 Reset inX_lowest and inX_highest
 159                 WO
 160
 161 in_reset_history
 162                 Reset inX_lowest and inX_highest for all sensors
 163                 WO
 164
 165 in[0-*]_label   Suggested voltage channel label.
 166                 Text string
 167                 Should only be created if the driver has hints about what
 168                 this voltage channel is being used for, and user-space
 169                 doesn't. In all other cases, the label is provided by
 170                 user-space.
 171                 RO
 172
 173 cpu[0-*]_vid    CPU core reference voltage.
 174                 Unit: millivolt
 175                 RO
 176                 Not always correct.
 177
 178 vrm             Voltage Regulator Module version number.
 179                 RW (but changing it should no more be necessary)
 180                 Originally the VRM standard version multiplied by 10, but now
 181                 an arbitrary number, as not all standards have a version
 182                 number.
 183                 Affects the way the driver calculates the CPU core reference
 184                 voltage from the vid pins.
 185
 186 Also see the Alarms section for status flags associated with voltages.
 187
 188
 189 ********
 190 * Fans *
 191 ********
 192
 193 fan[1-*]_min    Fan minimum value
 194                 Unit: revolution/min (RPM)
 195                 RW
 196
 197 fan[1-*]_max    Fan maximum value
 198                 Unit: revolution/min (RPM)
 199                 Only rarely supported by the hardware.
 200                 RW
 201
 202 fan[1-*]_input  Fan input value.
 203                 Unit: revolution/min (RPM)
 204                 RO
 205
 206 fan[1-*]_div    Fan divisor.
 207                 Integer value in powers of two (1, 2, 4, 8, 16, 32, 64, 128).
 208                 RW
 209                 Some chips only support values 1, 2, 4 and 8.
 210                 Note that this is actually an internal clock divisor, which
 211                 affects the measurable speed range, not the read value.
 212
 213 fan[1-*]_pulses Number of tachometer pulses per fan revolution.
 214                 Integer value, typically between 1 and 4.
 215                 RW
 216                 This value is a characteristic of the fan connected to the
 217                 device's input, so it has to be set in accordance with the fan
 218                 model.
 219                 Should only be created if the chip has a register to configure
 220                 the number of pulses. In the absence of such a register (and
 221                 thus attribute) the value assumed by all devices is 2 pulses
 222                 per fan revolution.
 223
 224 fan[1-*]_target
 225                 Desired fan speed
 226                 Unit: revolution/min (RPM)
 227                 RW
 228                 Only makes sense if the chip supports closed-loop fan speed
 229                 control based on the measured fan speed.
 230
 231 fan[1-*]_label  Suggested fan channel label.
 232                 Text string
 233                 Should only be created if the driver has hints about what
 234                 this fan channel is being used for, and user-space doesn't.
 235                 In all other cases, the label is provided by user-space.
 236                 RO
 237
 238 Also see the Alarms section for status flags associated with fans.
 239
 240
 241 *******
 242 * PWM *
 243 *******
 244
 245 pwm[1-*]        Pulse width modulation fan control.
 246                 Integer value in the range 0 to 255
 247                 RW
 248                 255 is max or 100%.
 249
 250 pwm[1-*]_enable
 251                 Fan speed control method:
 252                 0: no fan speed control (i.e. fan at full speed)
 253                 1: manual fan speed control enabled (using pwm[1-*])
 254                 2+: automatic fan speed control enabled
 255                 Check individual chip documentation files for automatic mode
 256                 details.
 257                 RW
 258
 259 pwm[1-*]_mode   0: DC mode (direct current)
 260                 1: PWM mode (pulse-width modulation)
 261                 RW
 262
 263 pwm[1-*]_freq   Base PWM frequency in Hz.
 264                 Only possibly available when pwmN_mode is PWM, but not always
 265                 present even then.
 266                 RW
 267
 268 pwm[1-*]_auto_channels_temp
 269                 Select which temperature channels affect this PWM output in
 270                 auto mode. Bitfield, 1 is temp1, 2 is temp2, 4 is temp3 etc...
 271                 Which values are possible depend on the chip used.
 272                 RW
 273
 274 pwm[1-*]_auto_point[1-*]_pwm
 275 pwm[1-*]_auto_point[1-*]_temp
 276 pwm[1-*]_auto_point[1-*]_temp_hyst
 277                 Define the PWM vs temperature curve. Number of trip points is
 278                 chip-dependent. Use this for chips which associate trip points
 279                 to PWM output channels.
 280                 RW
 281
 282 temp[1-*]_auto_point[1-*]_pwm
 283 temp[1-*]_auto_point[1-*]_temp
 284 temp[1-*]_auto_point[1-*]_temp_hyst
 285                 Define the PWM vs temperature curve. Number of trip points is
 286                 chip-dependent. Use this for chips which associate trip points
 287                 to temperature channels.
 288                 RW
 289
 290 There is a third case where trip points are associated to both PWM output
 291 channels and temperature channels: the PWM values are associated to PWM
 292 output channels while the temperature values are associated to temperature
 293 channels. In that case, the result is determined by the mapping between
 294 temperature inputs and PWM outputs. When several temperature inputs are
 295 mapped to a given PWM output, this leads to several candidate PWM values.
 296 The actual result is up to the chip, but in general the highest candidate
 297 value (fastest fan speed) wins.
 298
 299
 300 ****************
 301 * Temperatures *
 302 ****************
 303
 304 temp[1-*]_type  Sensor type selection.
 305                 Integers 1 to 6
 306                 RW
 307                 1: CPU embedded diode
 308                 2: 3904 transistor
 309                 3: thermal diode
 310                 4: thermistor
 311                 5: AMD AMDSI
 312                 6: Intel PECI
 313                 Not all types are supported by all chips
 314
 315 temp[1-*]_max   Temperature max value.
 316                 Unit: millidegree Celsius (or millivolt, see below)
 317                 RW
 318
 319 temp[1-*]_min   Temperature min value.
 320                 Unit: millidegree Celsius
 321                 RW
 322
 323 temp[1-*]_max_hyst
 324                 Temperature hysteresis value for max limit.
 325                 Unit: millidegree Celsius
 326                 Must be reported as an absolute temperature, NOT a delta
 327                 from the max value.
 328                 RW
 329
 330 temp[1-*]_input Temperature input value.
 331                 Unit: millidegree Celsius
 332                 RO
 333
 334 temp[1-*]_crit  Temperature critical max value, typically greater than
 335                 corresponding temp_max values.
 336                 Unit: millidegree Celsius
 337                 RW
 338
 339 temp[1-*]_crit_hyst
 340                 Temperature hysteresis value for critical limit.
 341                 Unit: millidegree Celsius
 342                 Must be reported as an absolute temperature, NOT a delta
 343                 from the critical value.
 344                 RW
 345
 346 temp[1-*]_emergency
 347                 Temperature emergency max value, for chips supporting more than
 348                 two upper temperature limits. Must be equal or greater than
 349                 corresponding temp_crit values.
 350                 Unit: millidegree Celsius
 351                 RW
 352
 353 temp[1-*]_emergency_hyst
 354                 Temperature hysteresis value for emergency limit.
 355                 Unit: millidegree Celsius
 356                 Must be reported as an absolute temperature, NOT a delta
 357                 from the emergency value.
 358                 RW
 359
 360 temp[1-*]_lcrit Temperature critical min value, typically lower than
 361                 corresponding temp_min values.
 362                 Unit: millidegree Celsius
 363                 RW
 364
 365 temp[1-*]_offset
 366                 Temperature offset which is added to the temperature reading
 367                 by the chip.
 368                 Unit: millidegree Celsius
 369                 Read/Write value.
 370
 371 temp[1-*]_label Suggested temperature channel label.
 372                 Text string
 373                 Should only be created if the driver has hints about what
 374                 this temperature channel is being used for, and user-space
 375                 doesn't. In all other cases, the label is provided by
 376                 user-space.
 377                 RO
 378
 379 temp[1-*]_lowest
 380                 Historical minimum temperature
 381                 Unit: millidegree Celsius
 382                 RO
 383
 384 temp[1-*]_highest
 385                 Historical maximum temperature
 386                 Unit: millidegree Celsius
 387                 RO
 388
 389 temp[1-*]_reset_history
 390                 Reset temp_lowest and temp_highest
 391                 WO
 392
 393 temp_reset_history
 394                 Reset temp_lowest and temp_highest for all sensors
 395                 WO
 396
 397 Some chips measure temperature using external thermistors and an ADC, and
 398 report the temperature measurement as a voltage. Converting this voltage
 399 back to a temperature (or the other way around for limits) requires
 400 mathematical functions not available in the kernel, so the conversion
 401 must occur in user space. For these chips, all temp* files described
 402 above should contain values expressed in millivolt instead of millidegree
 403 Celsius. In other words, such temperature channels are handled as voltage
 404 channels by the driver.
 405
 406 Also see the Alarms section for status flags associated with temperatures.
 407
 408
 409 ************
 410 * Currents *
 411 ************
 412
 413 curr[1-*]_max   Current max value
 414                 Unit: milliampere
 415                 RW
 416
 417 curr[1-*]_min   Current min value.
 418                 Unit: milliampere
 419                 RW
 420
 421 curr[1-*]_lcrit Current critical low value
 422                 Unit: milliampere
 423                 RW
 424
 425 curr[1-*]_crit  Current critical high value.
 426                 Unit: milliampere
 427                 RW
 428
 429 curr[1-*]_input Current input value
 430                 Unit: milliampere
 431                 RO
 432
 433 curr[1-*]_average
 434                 Average current use
 435                 Unit: milliampere
 436                 RO
 437
 438 curr[1-*]_lowest
 439                 Historical minimum current
 440                 Unit: milliampere
 441                 RO
 442
 443 curr[1-*]_highest
 444                 Historical maximum current
 445                 Unit: milliampere
 446                 RO
 447
 448 curr[1-*]_reset_history
 449                 Reset currX_lowest and currX_highest
 450                 WO
 451
 452 curr_reset_history
 453                 Reset currX_lowest and currX_highest for all sensors
 454                 WO
 455
 456 Also see the Alarms section for status flags associated with currents.
 457
 458 *********
 459 * Power *
 460 *********
 461
 462 power[1-*]_average              Average power use
 463                                 Unit: microWatt
 464                                 RO
 465
 466 power[1-*]_average_interval     Power use averaging interval.  A poll
 467                                 notification is sent to this file if the
 468                                 hardware changes the averaging interval.
 469                                 Unit: milliseconds
 470                                 RW
 471
 472 power[1-*]_average_interval_max Maximum power use averaging interval
 473                                 Unit: milliseconds
 474                                 RO
 475
 476 power[1-*]_average_interval_min Minimum power use averaging interval
 477                                 Unit: milliseconds
 478                                 RO
 479
 480 power[1-*]_average_highest      Historical average maximum power use
 481                                 Unit: microWatt
 482                                 RO
 483
 484 power[1-*]_average_lowest       Historical average minimum power use
 485                                 Unit: microWatt
 486                                 RO
 487
 488 power[1-*]_average_max          A poll notification is sent to
 489                                 power[1-*]_average when power use
 490                                 rises above this value.
 491                                 Unit: microWatt
 492                                 RW
 493
 494 power[1-*]_average_min          A poll notification is sent to
 495                                 power[1-*]_average when power use
 496                                 sinks below this value.
 497                                 Unit: microWatt
 498                                 RW
 499
 500 power[1-*]_input                Instantaneous power use
 501                                 Unit: microWatt
 502                                 RO
 503
 504 power[1-*]_input_highest        Historical maximum power use
 505                                 Unit: microWatt
 506                                 RO
 507
 508 power[1-*]_input_lowest         Historical minimum power use
 509                                 Unit: microWatt
 510                                 RO
 511
 512 power[1-*]_reset_history        Reset input_highest, input_lowest,
 513                                 average_highest and average_lowest.
 514                                 WO
 515
 516 power[1-*]_accuracy             Accuracy of the power meter.
 517                                 Unit: Percent
 518                                 RO
 519
 520 power[1-*]_cap                  If power use rises above this limit, the
 521                                 system should take action to reduce power use.
 522                                 A poll notification is sent to this file if the
 523                                 cap is changed by the hardware.  The *_cap
 524                                 files only appear if the cap is known to be
 525                                 enforced by hardware.
 526                                 Unit: microWatt
 527                                 RW
 528
 529 power[1-*]_cap_hyst             Margin of hysteresis built around capping and
 530                                 notification.
 531                                 Unit: microWatt
 532                                 RW
 533
 534 power[1-*]_cap_max              Maximum cap that can be set.
 535                                 Unit: microWatt
 536                                 RO
 537
 538 power[1-*]_cap_min              Minimum cap that can be set.
 539                                 Unit: microWatt
 540                                 RO
 541
 542 power[1-*]_max                  Maximum power.
 543                                 Unit: microWatt
 544                                 RW
 545
 546 power[1-*]_crit                 Critical maximum power.
 547                                 If power rises to or above this limit, the
 548                                 system is expected take drastic action to reduce
 549                                 power consumption, such as a system shutdown or
 550                                 a forced powerdown of some devices.
 551                                 Unit: microWatt
 552                                 RW
 553
 554 Also see the Alarms section for status flags associated with power readings.
 555
 556 **********
 557 * Energy *
 558 **********
 559
 560 energy[1-*]_input               Cumulative energy use
 561                                 Unit: microJoule
 562                                 RO
 563
 564
 565 ************
 566 * Humidity *
 567 ************
 568
 569 humidity[1-*]_input             Humidity
 570                                 Unit: milli-percent (per cent mille, pcm)
 571                                 RO
 572
 573
 574 **********
 575 * Alarms *
 576 **********
 577
 578 Each channel or limit may have an associated alarm file, containing a
 579 boolean value. 1 means than an alarm condition exists, 0 means no alarm.
 580
 581 Usually a given chip will either use channel-related alarms, or
 582 limit-related alarms, not both. The driver should just reflect the hardware
 583 implementation.
 584
 585 in[0-*]_alarm
 586 curr[1-*]_alarm
 587 power[1-*]_alarm
 588 fan[1-*]_alarm
 589 temp[1-*]_alarm
 590                 Channel alarm
 591                 0: no alarm
 592                 1: alarm
 593                 RO
 594
 595 OR
 596
 597 in[0-*]_min_alarm
 598 in[0-*]_max_alarm
 599 in[0-*]_lcrit_alarm
 600 in[0-*]_crit_alarm
 601 curr[1-*]_min_alarm
 602 curr[1-*]_max_alarm
 603 curr[1-*]_lcrit_alarm
 604 curr[1-*]_crit_alarm
 605 power[1-*]_cap_alarm
 606 power[1-*]_max_alarm
 607 power[1-*]_crit_alarm
 608 fan[1-*]_min_alarm
 609 fan[1-*]_max_alarm
 610 temp[1-*]_min_alarm
 611 temp[1-*]_max_alarm
 612 temp[1-*]_lcrit_alarm
 613 temp[1-*]_crit_alarm
 614 temp[1-*]_emergency_alarm
 615                 Limit alarm
 616                 0: no alarm
 617                 1: alarm
 618                 RO
 619
 620 Each input channel may have an associated fault file. This can be used
 621 to notify open diodes, unconnected fans etc. where the hardware
 622 supports it. When this boolean has value 1, the measurement for that
 623 channel should not be trusted.
 624
 625 fan[1-*]_fault
 626 temp[1-*]_fault
 627                 Input fault condition
 628                 0: no fault occurred
 629                 1: fault condition
 630                 RO
 631
 632 Some chips also offer the possibility to get beeped when an alarm occurs:
 633
 634 beep_enable     Master beep enable
 635                 0: no beeps
 636                 1: beeps
 637                 RW
 638
 639 in[0-*]_beep
 640 curr[1-*]_beep
 641 fan[1-*]_beep
 642 temp[1-*]_beep
 643                 Channel beep
 644                 0: disable
 645                 1: enable
 646                 RW
 647
 648 In theory, a chip could provide per-limit beep masking, but no such chip
 649 was seen so far.
 650
 651 Old drivers provided a different, non-standard interface to alarms and
 652 beeps. These interface files are deprecated, but will be kept around
 653 for compatibility reasons:
 654
 655 alarms          Alarm bitmask.
 656                 RO
 657                 Integer representation of one to four bytes.
 658                 A '1' bit means an alarm.
 659                 Chips should be programmed for 'comparator' mode so that
 660                 the alarm will 'come back' after you read the register
 661                 if it is still valid.
 662                 Generally a direct representation of a chip's internal
 663                 alarm registers; there is no standard for the position
 664                 of individual bits. For this reason, the use of this
 665                 interface file for new drivers is discouraged. Use
 666                 individual *_alarm and *_fault files instead.
 667                 Bits are defined in kernel/include/sensors.h.
 668
 669 beep_mask       Bitmask for beep.
 670                 Same format as 'alarms' with the same bit locations,
 671                 use discouraged for the same reason. Use individual
 672                 *_beep files instead.
 673                 RW
 674
 675
 676 ***********************
 677 * Intrusion detection *
 678 ***********************
 679
 680 intrusion[0-*]_alarm
 681                 Chassis intrusion detection
 682                 0: OK
 683                 1: intrusion detected
 684                 RW
 685                 Contrary to regular alarm flags which clear themselves
 686                 automatically when read, this one sticks until cleared by
 687                 the user. This is done by writing 0 to the file. Writing
 688                 other values is unsupported.
 689
 690 intrusion[0-*]_beep
 691                 Chassis intrusion beep
 692                 0: disable
 693                 1: enable
 694                 RW
 695
 696
 697 sysfs attribute writes interpretation
 698 -------------------------------------
 699
 700 hwmon sysfs attributes always contain numbers, so the first thing to do is to
 701 convert the input to a number, there are 2 ways todo this depending whether
 702 the number can be negative or not:
 703 unsigned long u = simple_strtoul(buf, NULL, 10);
 704 long s = simple_strtol(buf, NULL, 10);
 705
 706 With buf being the buffer with the user input being passed by the kernel.
 707 Notice that we do not use the second argument of strto[u]l, and thus cannot
 708 tell when 0 is returned, if this was really 0 or is caused by invalid input.
 709 This is done deliberately as checking this everywhere would add a lot of
 710 code to the kernel.
 711
 712 Notice that it is important to always store the converted value in an
 713 unsigned long or long, so that no wrap around can happen before any further
 714 checking.
 715
 716 After the input string is converted to an (unsigned) long, the value should be
 717 checked if its acceptable. Be careful with further conversions on the value
 718 before checking it for validity, as these conversions could still cause a wrap
 719 around before the check. For example do not multiply the result, and only
 720 add/subtract if it has been divided before the add/subtract.
 721
 722 What to do if a value is found to be invalid, depends on the type of the
 723 sysfs attribute that is being set. If it is a continuous setting like a
 724 tempX_max or inX_max attribute, then the value should be clamped to its
 725 limits using clamp_val(value, min_limit, max_limit). If it is not continuous
 726 like for example a tempX_type, then when an invalid value is written,
 727 -EINVAL should be returned.
 728
 729 Example1, temp1_max, register is a signed 8 bit value (-128 - 127 degrees):
 730
 731         long v = simple_strtol(buf, NULL, 10) / 1000;
 732         v = clamp_val(v, -128, 127);
 733         /* write v to register */
 734
 735 Example2, fan divider setting, valid values 2, 4 and 8:
 736
 737         unsigned long v = simple_strtoul(buf, NULL, 10);
 738
 739         switch (v) {
 740         case 2: v = 1; break;
 741         case 4: v = 2; break;
 742         case 8: v = 3; break;
 743         default:
 744                 return -EINVAL;
 745         }
 746         /* write v to register */