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                 whitespace, dashes, or the wildcard character '*'.
  90                 This attribute represents the chip name. It is the only
  91                 mandatory attribute.
  92                 I2C devices get this attribute created automatically.
  93                 RO
  94
  95 update_interval The interval at which the chip will update readings.
  96                 Unit: millisecond
  97                 RW
  98                 Some devices have a variable update rate or interval.
  99                 This attribute can be used to change it to the desired value.
 100
 101
 102 ************
 103 * Voltages *
 104 ************
 105
 106 in[0-*]_min     Voltage min value.
 107                 Unit: millivolt
 108                 RW
 109
 110 in[0-*]_lcrit   Voltage critical min value.
 111                 Unit: millivolt
 112                 RW
 113                 If voltage drops to or below this limit, the system may
 114                 take drastic action such as power down or reset. At the very
 115                 least, it should report a fault.
 116
 117 in[0-*]_max     Voltage max value.
 118                 Unit: millivolt
 119                 RW
 120
 121 in[0-*]_crit    Voltage critical max value.
 122                 Unit: millivolt
 123                 RW
 124                 If voltage reaches or exceeds this limit, the system may
 125                 take drastic action such as power down or reset. At the very
 126                 least, it should report a fault.
 127
 128 in[0-*]_input   Voltage input value.
 129                 Unit: millivolt
 130                 RO
 131                 Voltage measured on the chip pin.
 132                 Actual voltage depends on the scaling resistors on the
 133                 motherboard, as recommended in the chip datasheet.
 134                 This varies by chip and by motherboard.
 135                 Because of this variation, values are generally NOT scaled
 136                 by the chip driver, and must be done by the application.
 137                 However, some drivers (notably lm87 and via686a)
 138                 do scale, because of internal resistors built into a chip.
 139                 These drivers will output the actual voltage. Rule of
 140                 thumb: drivers should report the voltage values at the
 141                 "pins" of the chip.
 142
 143 in[0-*]_average
 144                 Average voltage
 145                 Unit: millivolt
 146                 RO
 147
 148 in[0-*]_lowest
 149                 Historical minimum voltage
 150                 Unit: millivolt
 151                 RO
 152
 153 in[0-*]_highest
 154                 Historical maximum voltage
 155                 Unit: millivolt
 156                 RO
 157
 158 in[0-*]_reset_history
 159                 Reset inX_lowest and inX_highest
 160                 WO
 161
 162 in_reset_history
 163                 Reset inX_lowest and inX_highest for all sensors
 164                 WO
 165
 166 in[0-*]_label   Suggested voltage channel label.
 167                 Text string
 168                 Should only be created if the driver has hints about what
 169                 this voltage channel is being used for, and user-space
 170                 doesn't. In all other cases, the label is provided by
 171                 user-space.
 172                 RO
 173
 174 cpu[0-*]_vid    CPU core reference voltage.
 175                 Unit: millivolt
 176                 RO
 177                 Not always correct.
 178
 179 vrm             Voltage Regulator Module version number.
 180                 RW (but changing it should no more be necessary)
 181                 Originally the VRM standard version multiplied by 10, but now
 182                 an arbitrary number, as not all standards have a version
 183                 number.
 184                 Affects the way the driver calculates the CPU core reference
 185                 voltage from the vid pins.
 186
 187 Also see the Alarms section for status flags associated with voltages.
 188
 189
 190 ********
 191 * Fans *
 192 ********
 193
 194 fan[1-*]_min    Fan minimum value
 195                 Unit: revolution/min (RPM)
 196                 RW
 197
 198 fan[1-*]_max    Fan maximum value
 199                 Unit: revolution/min (RPM)
 200                 Only rarely supported by the hardware.
 201                 RW
 202
 203 fan[1-*]_input  Fan input value.
 204                 Unit: revolution/min (RPM)
 205                 RO
 206
 207 fan[1-*]_div    Fan divisor.
 208                 Integer value in powers of two (1, 2, 4, 8, 16, 32, 64, 128).
 209                 RW
 210                 Some chips only support values 1, 2, 4 and 8.
 211                 Note that this is actually an internal clock divisor, which
 212                 affects the measurable speed range, not the read value.
 213
 214 fan[1-*]_pulses Number of tachometer pulses per fan revolution.
 215                 Integer value, typically between 1 and 4.
 216                 RW
 217                 This value is a characteristic of the fan connected to the
 218                 device's input, so it has to be set in accordance with the fan
 219                 model.
 220                 Should only be created if the chip has a register to configure
 221                 the number of pulses. In the absence of such a register (and
 222                 thus attribute) the value assumed by all devices is 2 pulses
 223                 per fan revolution.
 224
 225 fan[1-*]_target
 226                 Desired fan speed
 227                 Unit: revolution/min (RPM)
 228                 RW
 229                 Only makes sense if the chip supports closed-loop fan speed
 230                 control based on the measured fan speed.
 231
 232 fan[1-*]_label  Suggested fan channel label.
 233                 Text string
 234                 Should only be created if the driver has hints about what
 235                 this fan channel is being used for, and user-space doesn't.
 236                 In all other cases, the label is provided by user-space.
 237                 RO
 238
 239 Also see the Alarms section for status flags associated with fans.
 240
 241
 242 *******
 243 * PWM *
 244 *******
 245
 246 pwm[1-*]        Pulse width modulation fan control.
 247                 Integer value in the range 0 to 255
 248                 RW
 249                 255 is max or 100%.
 250
 251 pwm[1-*]_enable
 252                 Fan speed control method:
 253                 0: no fan speed control (i.e. fan at full speed)
 254                 1: manual fan speed control enabled (using pwm[1-*])
 255                 2+: automatic fan speed control enabled
 256                 Check individual chip documentation files for automatic mode
 257                 details.
 258                 RW
 259
 260 pwm[1-*]_mode   0: DC mode (direct current)
 261                 1: PWM mode (pulse-width modulation)
 262                 RW
 263
 264 pwm[1-*]_freq   Base PWM frequency in Hz.
 265                 Only possibly available when pwmN_mode is PWM, but not always
 266                 present even then.
 267                 RW
 268
 269 pwm[1-*]_auto_channels_temp
 270                 Select which temperature channels affect this PWM output in
 271                 auto mode. Bitfield, 1 is temp1, 2 is temp2, 4 is temp3 etc...
 272                 Which values are possible depend on the chip used.
 273                 RW
 274
 275 pwm[1-*]_auto_point[1-*]_pwm
 276 pwm[1-*]_auto_point[1-*]_temp
 277 pwm[1-*]_auto_point[1-*]_temp_hyst
 278                 Define the PWM vs temperature curve. Number of trip points is
 279                 chip-dependent. Use this for chips which associate trip points
 280                 to PWM output channels.
 281                 RW
 282
 283 temp[1-*]_auto_point[1-*]_pwm
 284 temp[1-*]_auto_point[1-*]_temp
 285 temp[1-*]_auto_point[1-*]_temp_hyst
 286                 Define the PWM vs temperature curve. Number of trip points is
 287                 chip-dependent. Use this for chips which associate trip points
 288                 to temperature channels.
 289                 RW
 290
 291 There is a third case where trip points are associated to both PWM output
 292 channels and temperature channels: the PWM values are associated to PWM
 293 output channels while the temperature values are associated to temperature
 294 channels. In that case, the result is determined by the mapping between
 295 temperature inputs and PWM outputs. When several temperature inputs are
 296 mapped to a given PWM output, this leads to several candidate PWM values.
 297 The actual result is up to the chip, but in general the highest candidate
 298 value (fastest fan speed) wins.
 299
 300
 301 ****************
 302 * Temperatures *
 303 ****************
 304
 305 temp[1-*]_type  Sensor type selection.
 306                 Integers 1 to 6
 307                 RW
 308                 1: CPU embedded diode
 309                 2: 3904 transistor
 310                 3: thermal diode
 311                 4: thermistor
 312                 5: AMD AMDSI
 313                 6: Intel PECI
 314                 Not all types are supported by all chips
 315
 316 temp[1-*]_max   Temperature max value.
 317                 Unit: millidegree Celsius (or millivolt, see below)
 318                 RW
 319
 320 temp[1-*]_min   Temperature min value.
 321                 Unit: millidegree Celsius
 322                 RW
 323
 324 temp[1-*]_max_hyst
 325                 Temperature hysteresis value for max limit.
 326                 Unit: millidegree Celsius
 327                 Must be reported as an absolute temperature, NOT a delta
 328                 from the max value.
 329                 RW
 330
 331 temp[1-*]_min_hyst
 332                 Temperature hysteresis value for min limit.
 333                 Unit: millidegree Celsius
 334                 Must be reported as an absolute temperature, NOT a delta
 335                 from the min value.
 336                 RW
 337
 338 temp[1-*]_input Temperature input value.
 339                 Unit: millidegree Celsius
 340                 RO
 341
 342 temp[1-*]_crit  Temperature critical max value, typically greater than
 343                 corresponding temp_max values.
 344                 Unit: millidegree Celsius
 345                 RW
 346
 347 temp[1-*]_crit_hyst
 348                 Temperature hysteresis value for critical limit.
 349                 Unit: millidegree Celsius
 350                 Must be reported as an absolute temperature, NOT a delta
 351                 from the critical value.
 352                 RW
 353
 354 temp[1-*]_emergency
 355                 Temperature emergency max value, for chips supporting more than
 356                 two upper temperature limits. Must be equal or greater than
 357                 corresponding temp_crit values.
 358                 Unit: millidegree Celsius
 359                 RW
 360
 361 temp[1-*]_emergency_hyst
 362                 Temperature hysteresis value for emergency limit.
 363                 Unit: millidegree Celsius
 364                 Must be reported as an absolute temperature, NOT a delta
 365                 from the emergency value.
 366                 RW
 367
 368 temp[1-*]_lcrit Temperature critical min value, typically lower than
 369                 corresponding temp_min values.
 370                 Unit: millidegree Celsius
 371                 RW
 372
 373 temp[1-*]_lcrit_hyst
 374                 Temperature hysteresis value for critical min limit.
 375                 Unit: millidegree Celsius
 376                 Must be reported as an absolute temperature, NOT a delta
 377                 from the critical min value.
 378                 RW
 379
 380 temp[1-*]_offset
 381                 Temperature offset which is added to the temperature reading
 382                 by the chip.
 383                 Unit: millidegree Celsius
 384                 Read/Write value.
 385
 386 temp[1-*]_label Suggested temperature channel label.
 387                 Text string
 388                 Should only be created if the driver has hints about what
 389                 this temperature channel is being used for, and user-space
 390                 doesn't. In all other cases, the label is provided by
 391                 user-space.
 392                 RO
 393
 394 temp[1-*]_lowest
 395                 Historical minimum temperature
 396                 Unit: millidegree Celsius
 397                 RO
 398
 399 temp[1-*]_highest
 400                 Historical maximum temperature
 401                 Unit: millidegree Celsius
 402                 RO
 403
 404 temp[1-*]_reset_history
 405                 Reset temp_lowest and temp_highest
 406                 WO
 407
 408 temp_reset_history
 409                 Reset temp_lowest and temp_highest for all sensors
 410                 WO
 411
 412 Some chips measure temperature using external thermistors and an ADC, and
 413 report the temperature measurement as a voltage. Converting this voltage
 414 back to a temperature (or the other way around for limits) requires
 415 mathematical functions not available in the kernel, so the conversion
 416 must occur in user space. For these chips, all temp* files described
 417 above should contain values expressed in millivolt instead of millidegree
 418 Celsius. In other words, such temperature channels are handled as voltage
 419 channels by the driver.
 420
 421 Also see the Alarms section for status flags associated with temperatures.
 422
 423
 424 ************
 425 * Currents *
 426 ************
 427
 428 curr[1-*]_max   Current max value
 429                 Unit: milliampere
 430                 RW
 431
 432 curr[1-*]_min   Current min value.
 433                 Unit: milliampere
 434                 RW
 435
 436 curr[1-*]_lcrit Current critical low value
 437                 Unit: milliampere
 438                 RW
 439
 440 curr[1-*]_crit  Current critical high value.
 441                 Unit: milliampere
 442                 RW
 443
 444 curr[1-*]_input Current input value
 445                 Unit: milliampere
 446                 RO
 447
 448 curr[1-*]_average
 449                 Average current use
 450                 Unit: milliampere
 451                 RO
 452
 453 curr[1-*]_lowest
 454                 Historical minimum current
 455                 Unit: milliampere
 456                 RO
 457
 458 curr[1-*]_highest
 459                 Historical maximum current
 460                 Unit: milliampere
 461                 RO
 462
 463 curr[1-*]_reset_history
 464                 Reset currX_lowest and currX_highest
 465                 WO
 466
 467 curr_reset_history
 468                 Reset currX_lowest and currX_highest for all sensors
 469                 WO
 470
 471 Also see the Alarms section for status flags associated with currents.
 472
 473 *********
 474 * Power *
 475 *********
 476
 477 power[1-*]_average              Average power use
 478                                 Unit: microWatt
 479                                 RO
 480
 481 power[1-*]_average_interval     Power use averaging interval.  A poll
 482                                 notification is sent to this file if the
 483                                 hardware changes the averaging interval.
 484                                 Unit: milliseconds
 485                                 RW
 486
 487 power[1-*]_average_interval_max Maximum power use averaging interval
 488                                 Unit: milliseconds
 489                                 RO
 490
 491 power[1-*]_average_interval_min Minimum power use averaging interval
 492                                 Unit: milliseconds
 493                                 RO
 494
 495 power[1-*]_average_highest      Historical average maximum power use
 496                                 Unit: microWatt
 497                                 RO
 498
 499 power[1-*]_average_lowest       Historical average minimum power use
 500                                 Unit: microWatt
 501                                 RO
 502
 503 power[1-*]_average_max          A poll notification is sent to
 504                                 power[1-*]_average when power use
 505                                 rises above this value.
 506                                 Unit: microWatt
 507                                 RW
 508
 509 power[1-*]_average_min          A poll notification is sent to
 510                                 power[1-*]_average when power use
 511                                 sinks below this value.
 512                                 Unit: microWatt
 513                                 RW
 514
 515 power[1-*]_input                Instantaneous power use
 516                                 Unit: microWatt
 517                                 RO
 518
 519 power[1-*]_input_highest        Historical maximum power use
 520                                 Unit: microWatt
 521                                 RO
 522
 523 power[1-*]_input_lowest         Historical minimum power use
 524                                 Unit: microWatt
 525                                 RO
 526
 527 power[1-*]_reset_history        Reset input_highest, input_lowest,
 528                                 average_highest and average_lowest.
 529                                 WO
 530
 531 power[1-*]_accuracy             Accuracy of the power meter.
 532                                 Unit: Percent
 533                                 RO
 534
 535 power[1-*]_cap                  If power use rises above this limit, the
 536                                 system should take action to reduce power use.
 537                                 A poll notification is sent to this file if the
 538                                 cap is changed by the hardware.  The *_cap
 539                                 files only appear if the cap is known to be
 540                                 enforced by hardware.
 541                                 Unit: microWatt
 542                                 RW
 543
 544 power[1-*]_cap_hyst             Margin of hysteresis built around capping and
 545                                 notification.
 546                                 Unit: microWatt
 547                                 RW
 548
 549 power[1-*]_cap_max              Maximum cap that can be set.
 550                                 Unit: microWatt
 551                                 RO
 552
 553 power[1-*]_cap_min              Minimum cap that can be set.
 554                                 Unit: microWatt
 555                                 RO
 556
 557 power[1-*]_max                  Maximum power.
 558                                 Unit: microWatt
 559                                 RW
 560
 561 power[1-*]_crit                 Critical maximum power.
 562                                 If power rises to or above this limit, the
 563                                 system is expected take drastic action to reduce
 564                                 power consumption, such as a system shutdown or
 565                                 a forced powerdown of some devices.
 566                                 Unit: microWatt
 567                                 RW
 568
 569 Also see the Alarms section for status flags associated with power readings.
 570
 571 **********
 572 * Energy *
 573 **********
 574
 575 energy[1-*]_input               Cumulative energy use
 576                                 Unit: microJoule
 577                                 RO
 578
 579
 580 ************
 581 * Humidity *
 582 ************
 583
 584 humidity[1-*]_input             Humidity
 585                                 Unit: milli-percent (per cent mille, pcm)
 586                                 RO
 587
 588
 589 **********
 590 * Alarms *
 591 **********
 592
 593 Each channel or limit may have an associated alarm file, containing a
 594 boolean value. 1 means than an alarm condition exists, 0 means no alarm.
 595
 596 Usually a given chip will either use channel-related alarms, or
 597 limit-related alarms, not both. The driver should just reflect the hardware
 598 implementation.
 599
 600 in[0-*]_alarm
 601 curr[1-*]_alarm
 602 power[1-*]_alarm
 603 fan[1-*]_alarm
 604 temp[1-*]_alarm
 605                 Channel alarm
 606                 0: no alarm
 607                 1: alarm
 608                 RO
 609
 610 OR
 611
 612 in[0-*]_min_alarm
 613 in[0-*]_max_alarm
 614 in[0-*]_lcrit_alarm
 615 in[0-*]_crit_alarm
 616 curr[1-*]_min_alarm
 617 curr[1-*]_max_alarm
 618 curr[1-*]_lcrit_alarm
 619 curr[1-*]_crit_alarm
 620 power[1-*]_cap_alarm
 621 power[1-*]_max_alarm
 622 power[1-*]_crit_alarm
 623 fan[1-*]_min_alarm
 624 fan[1-*]_max_alarm
 625 temp[1-*]_min_alarm
 626 temp[1-*]_max_alarm
 627 temp[1-*]_lcrit_alarm
 628 temp[1-*]_crit_alarm
 629 temp[1-*]_emergency_alarm
 630                 Limit alarm
 631                 0: no alarm
 632                 1: alarm
 633                 RO
 634
 635 Each input channel may have an associated fault file. This can be used
 636 to notify open diodes, unconnected fans etc. where the hardware
 637 supports it. When this boolean has value 1, the measurement for that
 638 channel should not be trusted.
 639
 640 fan[1-*]_fault
 641 temp[1-*]_fault
 642                 Input fault condition
 643                 0: no fault occurred
 644                 1: fault condition
 645                 RO
 646
 647 Some chips also offer the possibility to get beeped when an alarm occurs:
 648
 649 beep_enable     Master beep enable
 650                 0: no beeps
 651                 1: beeps
 652                 RW
 653
 654 in[0-*]_beep
 655 curr[1-*]_beep
 656 fan[1-*]_beep
 657 temp[1-*]_beep
 658                 Channel beep
 659                 0: disable
 660                 1: enable
 661                 RW
 662
 663 In theory, a chip could provide per-limit beep masking, but no such chip
 664 was seen so far.
 665
 666 Old drivers provided a different, non-standard interface to alarms and
 667 beeps. These interface files are deprecated, but will be kept around
 668 for compatibility reasons:
 669
 670 alarms          Alarm bitmask.
 671                 RO
 672                 Integer representation of one to four bytes.
 673                 A '1' bit means an alarm.
 674                 Chips should be programmed for 'comparator' mode so that
 675                 the alarm will 'come back' after you read the register
 676                 if it is still valid.
 677                 Generally a direct representation of a chip's internal
 678                 alarm registers; there is no standard for the position
 679                 of individual bits. For this reason, the use of this
 680                 interface file for new drivers is discouraged. Use
 681                 individual *_alarm and *_fault files instead.
 682                 Bits are defined in kernel/include/sensors.h.
 683
 684 beep_mask       Bitmask for beep.
 685                 Same format as 'alarms' with the same bit locations,
 686                 use discouraged for the same reason. Use individual
 687                 *_beep files instead.
 688                 RW
 689
 690
 691 ***********************
 692 * Intrusion detection *
 693 ***********************
 694
 695 intrusion[0-*]_alarm
 696                 Chassis intrusion detection
 697                 0: OK
 698                 1: intrusion detected
 699                 RW
 700                 Contrary to regular alarm flags which clear themselves
 701                 automatically when read, this one sticks until cleared by
 702                 the user. This is done by writing 0 to the file. Writing
 703                 other values is unsupported.
 704
 705 intrusion[0-*]_beep
 706                 Chassis intrusion beep
 707                 0: disable
 708                 1: enable
 709                 RW
 710
 711
 712 sysfs attribute writes interpretation
 713 -------------------------------------
 714
 715 hwmon sysfs attributes always contain numbers, so the first thing to do is to
 716 convert the input to a number, there are 2 ways todo this depending whether
 717 the number can be negative or not:
 718 unsigned long u = simple_strtoul(buf, NULL, 10);
 719 long s = simple_strtol(buf, NULL, 10);
 720
 721 With buf being the buffer with the user input being passed by the kernel.
 722 Notice that we do not use the second argument of strto[u]l, and thus cannot
 723 tell when 0 is returned, if this was really 0 or is caused by invalid input.
 724 This is done deliberately as checking this everywhere would add a lot of
 725 code to the kernel.
 726
 727 Notice that it is important to always store the converted value in an
 728 unsigned long or long, so that no wrap around can happen before any further
 729 checking.
 730
 731 After the input string is converted to an (unsigned) long, the value should be
 732 checked if its acceptable. Be careful with further conversions on the value
 733 before checking it for validity, as these conversions could still cause a wrap
 734 around before the check. For example do not multiply the result, and only
 735 add/subtract if it has been divided before the add/subtract.
 736
 737 What to do if a value is found to be invalid, depends on the type of the
 738 sysfs attribute that is being set. If it is a continuous setting like a
 739 tempX_max or inX_max attribute, then the value should be clamped to its
 740 limits using clamp_val(value, min_limit, max_limit). If it is not continuous
 741 like for example a tempX_type, then when an invalid value is written,
 742 -EINVAL should be returned.
 743
 744 Example1, temp1_max, register is a signed 8 bit value (-128 - 127 degrees):
 745
 746         long v = simple_strtol(buf, NULL, 10) / 1000;
 747         v = clamp_val(v, -128, 127);
 748         /* write v to register */
 749
 750 Example2, fan divider setting, valid values 2, 4 and 8:
 751
 752         unsigned long v = simple_strtoul(buf, NULL, 10);
 753
 754         switch (v) {
 755         case 2: v = 1; break;
 756         case 4: v = 2; break;
 757         case 8: v = 3; break;
 758         default:
 759                 return -EINVAL;
 760         }
 761         /* write v to register */