Sensor readings as 1-byte
guhan.sac at gmail.com
Fri Apr 20 11:30:19 AEST 2018
Thank you for the reference implementation.
I see that you are using the GetAttributes function in sensorutils.cpp,
that accepts the Min and Max value and outputs mValue, bValue, rExp and
bExp. This is exactly what I was looking for.
I still had some concerns, can you please help with the following,
1. When I plug in values from Tom's example (in this thread), I don't seem
to get the expected values for B. When min and max are (-127 , 128), I am
getting M=1 and B=-1 (including bSigned) where according to Tom's
calculation, it is M=1 and B=-127. Am I interpreting the output of B
2. Any reference document/spec for this algorithm, if available, would be
On Fri, Apr 13, 2018 at 8:41 AM, James Feist <james.feist at linux.intel.com>
> 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.
> James Feist
> 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?
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the openbmc