Sensor value calculation formula

[Deng Tyler]

Hi Emily
    I found it hardcoded in phosphor-hwmon for different sensor type. I
think that scale value may be customized for more flexible to fit custom
platform.

Tyler

Emily Shaffer <emilyshaffer at google.com> 於 2019年3月7日 週四 下午1:22寫道：

> Tyler,
>
> Sorry that I forgot. Yes, the scale is hardcoded for certain sensor types,
> ie voltage is always millivolts. I have asked about this before but maybe
> someone else on the list can give a better answer why that's the case.
>
> Emily
>
> On Wed, Mar 6, 2019, 9:19 PM Deng Tyler <tyler.sabdon at gmail.com> wrote:
>
>> Hi Emily, thank for your kindly reply. I understood how it wok now. But I
>> still have a question, which customized file can assign scale value to
>> Dbus? It seems always equal -3.
>>
>> Tyler
>>
>> Emily Shaffer <emilyshaffer at google.com> 於 2019年3月7日 週四 上午11:46寫道：
>>
>>> Tyler,
>>>
>>> In the example you cited, ideally that DBus entry reads Value: 4986;
>>> Unit: Volts; Scale: -3.
>>>
>>>
>>> https://github.com/openbmc/phosphor-dbus-interfaces/blob/master/xyz/openbmc_project/Sensor/Value.interface.yaml
>>>
>>> This essentially means the DBus entry is expressing 4986 millivolts,
>>> or 4986*10^-3 volts.
>>>
>>> On Wed, Mar 6, 2019 at 7:39 PM Deng Tyler <tyler.sabdon at gmail.com>
>>> wrote:
>>> >
>>> > Hi Emily, thank for your reply and you are really help me a lot. I use
>>> "GAIN_in1=5.0" in configuration file convert 2100 to 10500. Now Dbus store
>>> value is 10500 and it mean 10.5V in ipmitool end. But I still have some
>>> doubt about " DBus should be holding the value in units". There is a
>>> example in openbmc sensor document as below:
>>> > ====
>>> > 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:
>>> >
>>> > y = 20 * 249 * 10^-3 = 4.98 (V)
>>> >
>>> > ====
>>> > In above description, Dbus store value is 4986 but it is not real
>>> value in unit. If user switch to REST or redfish for getting sensor then
>>> 4986 will be display in same time. Is it correct?
>>> >
>>> > Tyler
>>> >
>>> > Emily Shaffer <emilyshaffer at google.com> 於 2019年3月7日 週四 上午10:02寫道：
>>> >>
>>> >> It sounds like you may want to convert, on your sensor end, so that
>>> >> you are expressing 12.0V in DBus. DBus should be holding the value in
>>> >> units, rather than a raw sensor value. The alternative here would be
>>> >> for you to put some context-aware conversion into host-ipmid or into
>>> >> your handler on top of ipmitool, and that's not something anybody
>>> >> wants :)  Later if you decide to switch to Redfish, you will have the
>>> >> same problem, due to your value in DBus being a raw reading instead of
>>> >> something in units.
>>> >>
>>> >> Is it possible for you to modify your code where the value is being
>>> >> pushed to DBus?
>>> >>
>>> >> On Wed, Mar 6, 2019 at 5:54 PM Deng Tyler <tyler.sabdon at gmail.com>
>>> wrote:
>>> >> >
>>> >> > Hi Emily, thank for your kindly reply. This is not typo. I want to
>>> dispaly 12.0V in ipmitool and my raw data in Dbus is 2100.
>>> >> > I know value x length is 1 byte. I concern that why raw data
>>> calculate with M, rExp before response to ipmitool.
>>> >> >
>>> >> > Tyler
>>> >> >
>>> >> > Emily Shaffer <emilyshaffer at google.com> 於 2019年3月7日 週四 上午6:47寫道：
>>> >> >>
>>> >> >> Hi Tyler,Is this a typo? Do you mean you want to display 21.0 in
>>> >> >> ipmitool? I don't know why there is scaling besides power of 10 on
>>> >> >> your raw value. That is odd.Assuming it was a typo, what you want
>>> to
>>> >> >> do is set the scale in the sensor info in order to perform /100
>>> >> >> operation. Or you could set the unit, though that might be
>>> difficult
>>> >> >> with a /100 (rather than increment of 1000).The scaling with m,
>>> rExp,
>>> >> >> etc is because the IPMI protocol has such a narrow field for sensor
>>> >> >> data (1 byte, as you saw; this is value x).So my recommendation to
>>> you
>>> >> >> is to use the scale to do your 2100->21.0 rounding when displaying
>>> >> >> output from ipmitool; and then to use your estimated maximum and
>>> >> >> minimum y to try and figure out what kind of coefficients you want
>>> to
>>> >> >> use.  This doesn't need to be perfect; you will lose some
>>> precision if
>>> >> >> you choose exponents which give you a much wider range than you
>>> need,
>>> >> >> but otherwise the operation should be invisible at the other end.If
>>> >> >> you really did mean you want 2100 -> 12.0 then that's a different
>>> >> >> question, which may require some more massaging on one end or
>>> another,
>>> >> >> so let us know.
>>> >> >>
>>> >> >> Emily
>>> >> >>
>>> >> >> On Wed, Mar 6, 2019 at 5:33 AM Deng Tyler <tyler.sabdon at gmail.com>
>>> wrote:
>>> >> >> >
>>> >> >> > Hi all:
>>> >> >> >     Sensor reading, according to IPMI spec, is calculated as:
>>> >> >> >
>>> >> >> > 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
>>> >> >> >
>>> >> >> >  ipmid calculate raw data by
>>> >> >> > double value = variant_ns::get<T>(propValue) * std::pow(10,
>>> sensorInfo.scale - sensorInfo.exponentR);
>>> >> >> > auto rawData = static_cast<uint8_t>((value -
>>> sensorInfo.scaledOffset) / sensorInfo.coefficientM);sue
>>> >> >> >
>>> >> >> > I am confused about why process raw data with M and rExp? M,
>>> offsetB, bExp and rExp shall just responded to ipmitool and calculate with
>>> raw
>>> >> >> > data. I thought raw data only need calculated by scale if raw
>>> data need to be adjust before respond to ipmitool.
>>> >> >> > Now I met a problem, I have a raw data is 2100 and I want to
>>> display 12.0 in ipmitool sdr command. I don't know how to assign M, rExp
>>> and scale value in yaml.
>>> >> >> > Could some one tell me how to translate sensor value? tks.
>>> >> >> >
>>> >> >> > Tyler
>>> >> >> >
>>> >> >> >
>>> >> >>
>>> >> >>
>>> >> >> --
>>> >> >> Emily Shaffer
>>> >>
>>> >>
>>> >>
>>> >> --
>>> >> Emily Shaffer
>>>
>>>
>>>
>>> --
>>> Emily Shaffer
>>>
>>
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