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author | Jean Delvare <khali@linux-fr.org> | 2005-10-26 21:39:40 +0200 |
---|---|---|
committer | Greg Kroah-Hartman <gregkh@suse.de> | 2005-10-28 14:02:14 -0700 |
commit | c3df5806cdae6fac678c662b527cb974bef4b60c (patch) | |
tree | a510da8447852247907dd6896cdf51c7e3d60844 /Documentation/hwmon | |
parent | 8256fe0f40f1cd72f80f2c46fe0ab1638f03a98d (diff) | |
download | kernel_samsung_tuna-c3df5806cdae6fac678c662b527cb974bef4b60c.zip kernel_samsung_tuna-c3df5806cdae6fac678c662b527cb974bef4b60c.tar.gz kernel_samsung_tuna-c3df5806cdae6fac678c662b527cb974bef4b60c.tar.bz2 |
[PATCH] hwmon: Add PEC support to the lm90 driver
Add PEC support to the lm90 driver. Only the ADM1032 chip supports it,
and in a rather tricky way, which is why this patch comes with
documentation reinforcements. At least, this demonstrates that the new
PEC support logic in i2c-core can properly deal with chips with partial
PEC support.
As enabling PEC causes a significant performance drop, it can be
disabled through a sysfs file (unsurprisingly named "pec").
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Diffstat (limited to 'Documentation/hwmon')
-rw-r--r-- | Documentation/hwmon/lm90 | 39 | ||||
-rw-r--r-- | Documentation/hwmon/sysfs-interface | 3 |
2 files changed, 40 insertions, 2 deletions
diff --git a/Documentation/hwmon/lm90 b/Documentation/hwmon/lm90 index 2c4cf39..70abf93e 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. diff --git a/Documentation/hwmon/sysfs-interface b/Documentation/hwmon/sysfs-interface index 3464005..764cdc5 100644 --- a/Documentation/hwmon/sysfs-interface +++ b/Documentation/hwmon/sysfs-interface @@ -272,3 +272,6 @@ beep_mask Bitmask for beep. eeprom Raw EEPROM data in binary form. Read only. + +pec Enable or disable PEC (SMBus only) + Read/Write |