diff options
Diffstat (limited to 'Documentation/hwmon/lm90')
| -rw-r--r-- | Documentation/hwmon/lm90 | 39 |
1 files changed, 37 insertions, 2 deletions
diff --git a/Documentation/hwmon/lm90 b/Documentation/hwmon/lm90 index 2c4cf39471f..70abf93ea37 100644 --- a/Documentation/hwmon/lm90 +++ b/Documentation/hwmon/lm90 @@ -71,8 +71,8 @@ increased resolution of the remote temperature measurement. The different chipsets of the family are not strictly identical, although very similar. This driver doesn't handle any specific feature for now, -but could if there ever was a need for it. For reference, here comes a -non-exhaustive list of specific features: +with the exception of SMBus PEC. For reference, here comes a non-exhaustive +list of specific features: LM90: * Filter and alert configuration register at 0xBF. @@ -91,6 +91,7 @@ ADM1032: * 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) @@ -119,3 +120,37 @@ 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 explicitely 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. |