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| -rw-r--r-- | Documentation/hwmon/sysfs-interface | 58 |
1 files changed, 58 insertions, 0 deletions
diff --git a/Documentation/hwmon/sysfs-interface b/Documentation/hwmon/sysfs-interface index db7bb4a659c4..a2153c4616ad 100644 --- a/Documentation/hwmon/sysfs-interface +++ b/Documentation/hwmon/sysfs-interface @@ -67,6 +67,10 @@ between readings to be caught and alarmed. The exact definition of an alarm (for example, whether a threshold must be met or must be exceeded to cause an alarm) is chip-dependent. +When setting values of hwmon sysfs attributes, the string representation of +the desired value must be written, note that strings which are not a number +are interpreted as 0! For more on how written strings are interpreted see the +"sysfs attribute writes interpretation" section at the end of this file. ------------------------------------------------------------------------- @@ -411,3 +415,57 @@ beep_mask Bitmask for beep. use discouraged for the same reason. Use individual *_beep files instead. RW + + +sysfs attribute writes interpretation +------------------------------------- + +hwmon sysfs attributes always contain numbers, so the first thing to do is to +convert the input to a number, there are 2 ways todo this depending whether +the number can be negative or not: +unsigned long u = simple_strtoul(buf, NULL, 10); +long s = simple_strtol(buf, NULL, 10); + +With buf being the buffer with the user input being passed by the kernel. +Notice that we do not use the second argument of strto[u]l, and thus cannot +tell when 0 is returned, if this was really 0 or is caused by invalid input. +This is done deliberately as checking this everywhere would add a lot of +code to the kernel. + +Notice that it is important to always store the converted value in an +unsigned long or long, so that no wrap around can happen before any further +checking. + +After the input string is converted to an (unsigned) long, the value should be +checked if its acceptable. Be careful with further conversions on the value +before checking it for validity, as these conversions could still cause a wrap +around before the check. For example do not multiply the result, and only +add/subtract if it has been divided before the add/subtract. + +What to do if a value is found to be invalid, depends on the type of the +sysfs attribute that is being set. If it is a continuous setting like a +tempX_max or inX_max attribute, then the value should be clamped to its +limits using SENSORS_LIMIT(value, min_limit, max_limit). If it is not +continuous like for example a tempX_type, then when an invalid value is +written, -EINVAL should be returned. + +Example1, temp1_max, register is a signed 8 bit value (-128 - 127 degrees): +--- begin code --- +long v = simple_strtol(buf, NULL, 10) / 1000; +SENSORS_LIMIT(v, -128, 127); +/* write v to register */ +--- end code --- + +Example2, fan divider setting, valid values 2, 4 and 8: +--- begin code --- +unsigned long v = simple_strtoul(buf, NULL, 10); + +switch (v) { + case 2: v = 1; break; + case 4: v = 2; break; + case 8: v = 3; break; + default: + return -EINVAL; +} +/* write v to register */ +--- end code --- |