Sensor readings as 1-byte
james.feist at linux.intel.com
Sat Apr 14 01:41:05 AEST 2018
There is an RFC I posted that calculates the M, B, and exponents from
the D-Bus sensor endpoints: https://gerrit.openbmc-project.xyz/#/c/8521/
We were hoping to roll this into host-ipmi sometime in future.
On 04/12/2018 07:41 PM, guhan balasubramanian wrote:
> Hi Tom,
> Thanks for the explanation. But I still see some concerns in this
> conversion logic.
> For input, we have 4 given parameters as
> 1. Min and Max values of the sensor (y)
> 2. the Min and Max values of the byte value (x - assuming this is
> always 0 to 255).
> For output, we have 4 paramters to calculate
> 1. multiplierM (M)
> 2. offsetB (B)
> 3. bExp (k1)
> 4. rExp (k2)
> We have two equations to compute 4 variables in this logic.
> On Sat, Mar 10, 2018 at 9:45 AM, Tom Joseph <tomjose at linux.vnet.ibm.com
> <mailto:tomjose at linux.vnet.ibm.com>> wrote:
> Hello Guhan,
> The updated sensor-example.yaml has the Rexp field as well. These
> values are calculated based on the minimum and maximum value of the
> If a temperature sensor has a range of -127 - +128, means y is -127
> to +128 and the range of x is 0-255.
> By substituting x = 0 and y = -127 (k2 is the decimal precision and
> assume in this case as whole nos) implies Bexp = -127.
> Similarly substitute x = 255 and y = 128 and M is evaluated as 1.
> In the above example, with 2 values [(0, -127) and (255, 128)] for (x,y)
> we were able to calculate M and B. We are assuming k2 as 0 since we are
> taking only whole numbers for our ranges (y values). But we are still
> assuming the values for k1 as 0.
> On Saturday 10 March 2018 08:01 AM, guhan balasubramanian wrote:
>> According to the IPMI spec, we represent all sensor readings
>> (Volt, Temp, Amp, etc.) as 1-byte.
>> Since we represent them as 1-byte, we use a linearization formula
>> to convert to actual value as follows:
>> y = (M*x + B*10^(k1))*10^(k2)
>> where x is the 1-byte value that is filled in the get sensor
>> reading command.
>> We see in the ipmitool example on how an example of 3.36 V is
>> represented as 1-byte.
>> Can any one please help on how the values for M, B, k1 and k2 are
>> populated for each sensor?
>> In the openbmc repo, I believe these values are present in the
>> config.yaml of phosphor-ipmi-sensor-inventory (based on the
>> following sample).
>> sensorType: 0x01
>> path: /xyz/openbmc_project/sensors/temperature/temp1
>> sensorReadingType: 0x01
>> *multiplierM*: 511
>> *offsetB*: 0
>> *bExp*: 0
>> mutability: Mutability::Write|Mutability::Read
>> serviceInterface: org.freedesktop.DBus.Properties
>> readingType: readingData
>> type: int64_t
> With the given information, do you think there is a generic way
> (algorithm) where we can compute all the 4 variables M, B, k1 and k2?
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