Re: [PATCH v3 03/11] iio: Add channel for Position Relative

[Eugen Hristev <eugen.hristev@microchip.com>]

On 15.04.2018 22:29, Jonathan Cameron wrote:
> On Tue, 10 Apr 2018 11:57:49 +0300
> Eugen Hristev <eugen.hristev@microchip.com> wrote:
> 
>> Add new channel type for relative position on a pad.
>>
>> These type of analog sensor offers the position of a pen
>> on a touchpad, and is represented as a voltage, which can be
>> converted to a position on X and Y axis on the pad.
>> The channel will hand the relative position on the pad in both directions.
>>
>> The channel can then be consumed by a touchscreen driver or
>> read as-is for a raw indication of the touchpen on a touchpad.
>>
>> Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
>> ---
>> Changes in v2:
>>   - modified channel name to relative position as suggested.
>>   - modified kernel version to 4.18 (presumable)
>>
>>   Documentation/ABI/testing/sysfs-bus-iio | 12 ++++++++++++
>>   drivers/iio/industrialio-core.c         |  1 +
>>   include/uapi/linux/iio/types.h          |  1 +
>>   tools/iio/iio_event_monitor.c           |  2 ++
>>   4 files changed, 16 insertions(+)
>>
>> diff --git a/Documentation/ABI/testing/sysfs-bus-iio b/Documentation/ABI/testing/sysfs-bus-iio
>> index 6a5f34b..42a9287 100644
>> --- a/Documentation/ABI/testing/sysfs-bus-iio
>> +++ b/Documentation/ABI/testing/sysfs-bus-iio
>> @@ -190,6 +190,18 @@ Description:
>>   		but should match other such assignments on device).
>>   		Units after application of scale and offset are m/s^2.
>>   
>> +What:		/sys/bus/iio/devices/iio:deviceX/in_positionrelative_x_raw
>> +What:		/sys/bus/iio/devices/iio:deviceX/in_positionrelative_y_raw
>> +KernelVersion:	4.18
>> +Contact:	linux-iio@vger.kernel.org
>> +Description:
>> +		Relative position in direction x or y on a pad (may be
>> +		arbitrarily assigned but should match other such assignments on
>> +		device).
>> +		Units after application of scale and offset are milli percents
>> +		from the pad's size in both directions. Should be calibrated by
>> +		the consumer.
> I know the milli percent comes form the humidity equivalent, but I wonder
> if we are right to follow that.  10^-5 is a pretty random base unit (though
> I got argued into it being a standard choice for humidity sensors IIRC...
> 
> What do people think?  We could go with 1 for full range or just percent perhaps?
> 
> I'm not that fussed about staying consistent with humidity - we are unlikely
> to end up with sensors doing both anytime soon so there shouldn't be
> any confusion...
> 
> Jonathan
Hello Jonathan,

For my specific use case, percents are not enough, as this would mean 
the resolution will be pretty low. If we have a touchpanel of a higher 
resolution, having the position only ranged 0 to 100 is pretty bad.
Having millipercents means we can have a resolution up to 100,000 points 
in range, which is fine for the hardware as of this moment.
Of course, specific drivers reporting these channels can define a 
specific range if they desire (report just in the first percent if you 
wish (0-1000) or so, which would mean a lower resolution.
Centi-percent would work fine as well I believe (0-10,000 range), but to 
keep consistent with your suggestion regarding relative humidity, I 
picked this larger scale.
In my specific case, since we have a 12 bit ADC, range is 0-4096 for 
this value.
Calibrating these values to the actual size of the touchpanel is left 
for the consumer to do (map 4096x4096 to the actual resolution)

If you have a better idea than milli-percents I can change it, no problem.

Eugen
>> +
>>   [...]