Re: ti-ads7950: selecting the adc input range

["Nuno Sá" <noname.nuno@gmail.com>]

On Fri, 2022-07-08 at 11:28 +0200, Lars-Peter Clausen wrote:
> On 7/8/22 10:02, Uwe Kleine-König wrote:
> > Hello,
> > 
> > the ADS7950 has a register bit (called TI_ADS7950_CR_RANGE_5V in
> > the
> > driver) that selects the input range. Depending on that bit the
> > input
> > range is either [0 .. V_{REF}] or [0 .. 2 * V_{REF}].
> > 
> > The driver currently defaults to setting that bit, so the range is
> > the
> > big one.
> > 
> > On a machine here however I know the input is in the smaller range
> > and
> > I'd like to benefit from the higher resolution of the small range.
> > I
> > wonder how to make this tunable. Should that be done using a
> > firmware
> > property? ("single-input-range" vs. "double-input-range"? Or input-
> > range
> > = <1> vs. input-range = <2> which better matches the data sheet
> > that
> > calls the two modes "Range 1 (0 to V_{REF})" and "Range 2 (0 to
> > 2xV_{REF})") Or should this be made tunable via sysfs? (E.g. by
> > writing
> > to the scale property? Or a separate property?)
> 
> Hi,
> 
> Its a bit of a tricky one. You can find arguments for and against 
> either. Like "devicetree is for hardware description and not
> application 
> specific configuration data", or "I know which setting I want to use,
> having the kernel apply it makes it a lot easier".
> 
> What we've done in the past in the IIO framework is to make the scale
> property writable for such devices. Together with a scale_available 
> property to list valid options. This is the most flexible option as
> it 
> allows to change the setting at runtime for applications where it is 
> required.
> 

Yes, it's a tricky one and I have the feeling that thoughts about this
are changing frequently :).

Well, this feels identical to what I had in a DAC [1] I recently
upstreamed. IIRC, on the first version of the series or during
discussion on the RFC I also had in mind to make it configurable
through sysfs but Jonathan advised me to go with a fw property.

[1]:https://elixir.bootlin.com/linux/latest/source/drivers/iio/dac/ltc2688.c#L786

- Nuno Sá