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Subject: Re: Looking for clarification on sysfs IIO devices, do _raw devices
 require both _offset and _scale?
To: Lars-Peter Clausen <lars@metafoo.de>, linux-iio@vger.kernel.org
References: <cc74f30f-9ab8-45ce-1f44-8e55c3f9c5ce@linux.vnet.ibm.com>
 <5c79425f-6e88-36b6-cdfe-4080738d039f@metafoo.de>
From: Bruce Mitchell <bruce.mitchell@linux.vnet.ibm.com>
Message-ID: <b9f78f37-fbb6-ea68-48c1-917cf42ca57f@linux.vnet.ibm.com>
Date: Thu, 9 Sep 2021 06:49:19 -0700
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On 9/9/2021 00:12, Lars-Peter Clausen wrote:
> On 9/9/21 12:10 AM, Bruce Mitchell wrote:
>> In reference to:
>> https://www.kernel.org/doc/Documentation/ABI/testing/sysfs-bus-iio
>>
>> I have Temperature, Pressure, and Humidity IIO sensors.
>> IIO _raw devices look like this on sysfs:
>>   this happens to be a SI7020 type device with 2 sensors
>>     /sys/bus/iio/devices/iio:device0/in_humidityrelative_offset
>>     /sys/bus/iio/devices/iio:device0/in_humidityrelative_raw
>>     /sys/bus/iio/devices/iio:device0/in_humidityrelative_scale
>>     /sys/bus/iio/devices/iio:device0/in_temp_offset
>>     /sys/bus/iio/devices/iio:device0/in_temp_raw
>>     /sys/bus/iio/devices/iio:device0/in_temp_scale
>>
>> Other IIO _input devices look like this on sysfs:
>>   this happens to be a DPS310 device with 2 sensors
>>      /sys/bus/iio/devices/iio:device1/in_temp_input
>>      /sys/bus/iio/devices/iio:device1/in_pressure_input
>>
>> As I read it if the IIO device was an _input type on sysfs,
>> just read it (and possibly scale it for units).
>>
>> But if the IIO device was a _raw type on sysfs my understanding
>> is that it must be accompanied by a _offset and a _scale for
>> at least temperature, pressure, humidity, voltage, and current
>> sensors.
>> Is that correct?
>>
>> Further for any IIO device that is a _raw type on sysfs is it
>> required to be accompanied by a _offset and a _scale as well?
> 
> Hi,
> 
> That sounds about right.
> 
> The _input name is historically and comes from hwmon framework. It means 
> that the data has been processed by the kernel driver and converted to 
> the right SI units for the channel type. This is usually used for sensor 
> that have a non-linear transfer function. `raw` on the other hand means 
> the data is just as it is reported by the hardware. The reason for this 
> is that conversion to SI units is often not lossless, since we have 
> finite precision. So it is up to the application to decide whether it 
> wants to work on the raw data or how it wants to round the converted data.
> 
> `input` attributes never have scale and offset since they are already in 
> the right unit. For raw scale and offset are optional. If scale does not 
> exist assume it is 1, if offset does not exist assume it is 0. You'll 
> rarely see a device with raw attributes without scale, but there are 
> quite a few without offset.
> 
> - Lars
> 
> 

Thank you Lars!

-- 
Bruce