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| -rw-r--r-- | sensor-architecture.md | 54 |
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diff --git a/sensor-architecture.md b/sensor-architecture.md index 50d919b..4435a24 100644 --- a/sensor-architecture.md +++ b/sensor-architecture.md @@ -304,6 +304,60 @@ Will write a value to a pwmN\_enable file on startup if present. ENABLE_fan1 = 2 #Write a 2 to pwm1_enable ``` +### Defining sensors in an IPMI YAML configuration file +For an example of how sensors entries are defined, consult the [example +YAML](https://github.com/openbmc/phosphor-host-ipmid/blob/master/scripts/sensor-example.yaml) + +#### How to best choose coefficients +Sensor reading, according to IPMI spec, is calculated as: + +```none +y = L[(Mx + B * 10^(bExp)) * 10^(rExp)] +``` + +- y: the 'final value' as reported by IPMItool +- x: 8 bits, unsigned, reading data encoded in IPMI response packet +- M: 10 bits, signed integer multiplier, `multiplierM` in YAML +- B: 10 bits, signed additive offset, `offsetB` in YAML +- bExp: 4 bits, signed, `bExp` in YAML +- rExp: 4 bits, signed, `rExp` in YAML + +In addition, phosphor-ipmi-host configuration also supports `scale` property, +which applies for analog sensors, meaning the value read over DBus should be +scaled by 10^S. + +As you can tell, one should choose the coefficients based on possible sensor +reading range and desired resolution. Commonly, B=0, we would have + + Supported range: [0, 255 * M * 10^(scale - rExp)] + Resolution: M * 10^(scale - rExp) + +For a concrete example, let's say a voltage sensor reports between 0 to 5.0V. +hwmon sysfs scales the value by 1000, so the sensor value read over DBus is +between 0 and 5000. A possible configuration for this is: + +```none +multiplierM: 20 +offsetB: 0 +bExp: 0 +rExp: -3 +scale: -3 +``` + +so for a DBus sensor value of 4986 meaning 4.986V, phosphor-ipmi-host would +encode it as + + x: 4986 / 20 = 249 + M: 20 + rExp: -3 + +When ipmitool sensor list is called, the tool fetches sensor factors and +computes value as: + +```none +y = 20 * 249 * 10^-3 = 4.98 (V) +``` + ### My sensors are defined in an MRW Setting up sensor support with an MRW is done by adding a unit-hwmon-feature |
