Re: [PATCH v2] hwmon: (ntc_thermistor): try reading processed

[Chris Lesiak <chris.lesiak@licor.com>]

Linus Walleij <linus.walleij@linaro.org> wrote:
> Aha you mean that iio_read_channel_processed() loses
> precision when converting raw to scaled?

Yes.  For example, take a 16-bit ADC with 4.096 V reference.
The native resolution is 62.5 microVolts.
Yet iio_read_channel_processed() can only give integer milliVolts.
iio_read_channel_raw() followed by iio_convert_raw_to_processed()
with a scale of 1000 will preserve more of the native resolution.
User space can do this by using floating point numbers when
converting to processed.

You are likely to lose precision for all ADCs greater than about 12-bit.

Chris Lesiak <chris.leisak@licor.com> wrote:
>> I'd prefer a solution similar to the existing implementation of
>> iio_read_channel_processed.

> That seems like the wrong way to work around a problem in
> the core.

> If iio_read_channel_processed() loses precision we should
> fix iio_read_channel_processed() and not try to work around
> the problem in the consumers.

> It's fine to fix all the consumers in the kernel.

> What about changing the signature of:

> int iio_read_channel_processed(struct iio_channel *chan, int *val)

> to:

> int iio_read_channel_processed(struct iio_channel *chan, int *val,
> unsigned int scale)

> And just augment all calls to pass 1 except the ntc driver
> which then passes 1000 in the last argument?

> If Jonathan agrees I can fix a patch to alter all the ~50
> call sites like this and include the change to this NTC
> driver.

That would meet my needs and does address what I think is a
shortcoming in the existing iio_read_channel_processed interface.