Sensor readings as 1-byte

[guhan balasubramanian]

Hi James,

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
correctly?

2. Any reference document/spec for this algorithm, if available, would be
really helpful.

Thanks,
Guhan

On Fri, Apr 13, 2018 at 8:41 AM, James Feist <james.feist at linux.intel.com>
wrote:

> 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.
>
> Thanks,
>
> 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
>>     sensor.
>>
>>     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.
>>
>>
>>     Regards,
>>     Tom
>>
>>
>>     On Saturday 10 March 2018 08:01 AM, guhan balasubramanian wrote:
>>
>>>     Hi,
>>>
>>>     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.
>>>     https://computercheese.blogspot.com/2013/10/ipmi-sensor-
>>> reading-conversion-formula.html?q=sensor
>>>     <https://computercheese.blogspot.com/2013/10/ipmi-sensor-
>>> reading-conversion-formula.html?q=sensor>
>>>
>>>     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).
>>>
>>>     0xF1:
>>>       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
>>>       interfaces:
>>>         xyz.openbmc_project.Sensor.Value:
>>>           Value:
>>>             Offsets:
>>>               0x0:
>>>                 type: int64_t
>>>
>>>
>>>     Thanks,
>>>     Guhan
>>>
>>> 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?
>>
>> Thanks,
>> Guhan
>>
>>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.ozlabs.org/pipermail/openbmc/attachments/20180419/3564a032/attachment.html>