hwmon: (lm90) Don't access nonexistent registers on Maxim chips

[deliverable/linux.git] / Documentation / hwmon / lm90

Kernel driver lm90
==================

Supported chips:
  * National Semiconductor LM90
    Prefix: 'lm90'
    Addresses scanned: I2C 0x4c
    Datasheet: Publicly available at the National Semiconductor website
               http://www.national.com/pf/LM/LM90.html
  * National Semiconductor LM89
    Prefix: 'lm99'
    Addresses scanned: I2C 0x4c and 0x4d
    Datasheet: Publicly available at the National Semiconductor website
               http://www.national.com/pf/LM/LM89.html
  * National Semiconductor LM99
    Prefix: 'lm99'
    Addresses scanned: I2C 0x4c and 0x4d
    Datasheet: Publicly available at the National Semiconductor website
               http://www.national.com/pf/LM/LM99.html
  * National Semiconductor LM86
    Prefix: 'lm86'
    Addresses scanned: I2C 0x4c
    Datasheet: Publicly available at the National Semiconductor website
               http://www.national.com/pf/LM/LM86.html
  * Analog Devices ADM1032
    Prefix: 'adm1032'
    Addresses scanned: I2C 0x4c and 0x4d
    Datasheet: Publicly available at the Analog Devices website
               http://www.analog.com/en/prod/0,2877,ADM1032,00.html
  * Analog Devices ADT7461
    Prefix: 'adt7461'
    Addresses scanned: I2C 0x4c and 0x4d
    Datasheet: Publicly available at the Analog Devices website
               http://www.analog.com/en/prod/0,2877,ADT7461,00.html
    Note: Only if in ADM1032 compatibility mode
  * Maxim MAX6657
    Prefix: 'max6657'
    Addresses scanned: I2C 0x4c
    Datasheet: Publicly available at the Maxim website
               http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2578
  * Maxim MAX6658
    Prefix: 'max6657'
    Addresses scanned: I2C 0x4c
    Datasheet: Publicly available at the Maxim website
               http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2578
  * Maxim MAX6659
    Prefix: 'max6657'
    Addresses scanned: I2C 0x4c, 0x4d (unsupported 0x4e)
    Datasheet: Publicly available at the Maxim website
               http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2578
  * Maxim MAX6680
    Prefix: 'max6680'
    Addresses scanned: I2C 0x18, 0x19, 0x1a, 0x29, 0x2a, 0x2b,
                           0x4c, 0x4d and 0x4e
    Datasheet: Publicly available at the Maxim website
               http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3370
  * Maxim MAX6681
    Prefix: 'max6680'
    Addresses scanned: I2C 0x18, 0x19, 0x1a, 0x29, 0x2a, 0x2b,
                           0x4c, 0x4d and 0x4e
    Datasheet: Publicly available at the Maxim website
               http://www.maxim-ic.com/quick_view2.cfm/qv_pk/3370


Author: Jean Delvare <khali@linux-fr.org>


Description
-----------

The LM90 is a digital temperature sensor. It senses its own temperature as
well as the temperature of up to one external diode. It is compatible
with many other devices such as the LM86, the LM89, the LM99, the ADM1032,
the MAX6657, MAX6658, MAX6659, MAX6680 and the MAX6681 all of which are
supported by this driver.

Note that there is no easy way to differentiate between the MAX6657,
MAX6658 and MAX6659 variants. The extra address and features of the
MAX6659 are not supported by this driver. The MAX6680 and MAX6681 only
differ in their pinout, therefore they obviously can't (and don't need to)
be distinguished. Additionally, the ADT7461 is supported if found in
ADM1032 compatibility mode.

The specificity of this family of chipsets over the ADM1021/LM84
family is that it features critical limits with hysteresis, and an
increased resolution of the remote temperature measurement.

The different chipsets of the family are not strictly identical, although
very similar. For reference, here comes a non-exhaustive list of specific
features:

LM90:
  * Filter and alert configuration register at 0xBF.
  * ALERT is triggered by temperatures over critical limits.

LM86 and LM89:
  * Same as LM90
  * Better external channel accuracy

LM99:
  * Same as LM89
  * External temperature shifted by 16 degrees down

ADM1032:
  * Consecutive alert register at 0x22.
  * Conversion averaging.
  * Up to 64 conversions/s.
  * ALERT is triggered by open remote sensor.
  * SMBus PEC support for Write Byte and Receive Byte transactions.

ADT7461:
  * Extended temperature range (breaks compatibility)
  * Lower resolution for remote temperature

MAX6657 and MAX6658:
  * Better local resolution
  * Remote sensor type selection

MAX6659:
  * Better local resolution
  * Selectable address
  * Second critical temperature limit
  * Remote sensor type selection

MAX6680 and MAX6681:
  * Selectable address
  * Remote sensor type selection

All temperature values are given in degrees Celsius. Resolution
is 1.0 degree for the local temperature, 0.125 degree for the remote
temperature, except for the MAX6657, MAX6658 and MAX6659 which have a
resolution of 0.125 degree for both temperatures.

Each sensor has its own high and low limits, plus a critical limit.
Additionally, there is a relative hysteresis value common to both critical
values. To make life easier to user-space applications, two absolute values
are exported, one for each channel, but these values are of course linked.
Only the local hysteresis can be set from user-space, and the same delta
applies to the remote hysteresis.

The lm90 driver will not update its values more frequently than every
other second; reading them more often will do no harm, but will return
'old' values.

PEC Support
-----------

The ADM1032 is the only chip of the family which supports PEC. It does
not support PEC on all transactions though, so some care must be taken.

When reading a register value, the PEC byte is computed and sent by the
ADM1032 chip. However, in the case of a combined transaction (SMBus Read
Byte), the ADM1032 computes the CRC value over only the second half of
the message rather than its entirety, because it thinks the first half
of the message belongs to a different transaction. As a result, the CRC
value differs from what the SMBus master expects, and all reads fail.

For this reason, the lm90 driver will enable PEC for the ADM1032 only if
the bus supports the SMBus Send Byte and Receive Byte transaction types.
These transactions will be used to read register values, instead of
SMBus Read Byte, and PEC will work properly.

Additionally, the ADM1032 doesn't support SMBus Send Byte with PEC.
Instead, it will try to write the PEC value to the register (because the
SMBus Send Byte transaction with PEC is similar to a Write Byte transaction
without PEC), which is not what we want. Thus, PEC is explicitly disabled
on SMBus Send Byte transactions in the lm90 driver.

PEC on byte data transactions represents a significant increase in bandwidth
usage (+33% for writes, +25% for reads) in normal conditions. With the need
to use two SMBus transaction for reads, this overhead jumps to +50%. Worse,
two transactions will typically mean twice as much delay waiting for
transaction completion, effectively doubling the register cache refresh time.
I guess reliability comes at a price, but it's quite expensive this time.

So, as not everyone might enjoy the slowdown, PEC can be disabled through
sysfs. Just write 0 to the "pec" file and PEC will be disabled. Write 1
to that file to enable PEC again.