Re: [PATCH v4 4/6] iio: test: rescale: compare scales against the precision of the output

[Rodrigo Alencar]

On 26/10/26 16:40, Andy Shevchenko wrote:
> On Mon, Oct 26, 2026 at 08:54:08AM -0300, Rodrigo Alencar via B4 Relay wrote:
> > 
> > The scale tests format the rescaled value with iio_format_value(), which
> > prints at most nano precision, and require the relative error to the
> > expected value to be below 1 ppm. Both strings are parsed back with
> > iio_str_to_fixpoint() at nano precision, so for the small scale cases the
> > check only passes because the expected value, like the formatted one, is
> > truncated to nano. For example, "0.000000033594" is parsed as 33 nano,
> > equal to the formatted "0.000000033", although the actual error is about
> > 1.8%. If the expected value were rounded instead, it would be parsed as
> > 34 nano and the test would fail even though the output is as accurate as
> > the format allows.
> > 
> > Parse both values with kstrtodec64() at a scale of 10, one digit more
> > than the output precision, so the expected value keeps the digits lost
> > when formatting. Scale 10 is the largest that still fits the overflow
> > cases (about 2.1e8) in an s64. Compute the relative error with
> > mul_u64_u64_div_u64(), as the product with 10^6 no longer fits in 64 bits
> > for those cases.
> > 
> > Then allow a relative error up to the relative size of one nano of the
> > expected value, the precision of the formatted output. This is 0 ppm for
> > values of 0.01 and above, so those cases are checked as before, and it
> > accepts the formatting error for the small scale cases.
> 
> ...
> 
> > +/*
> > + * iio_format_value() prints scales with up to nano precision. Parse values
> > + * with one more digit, so the expected values keep the precision lost there.
> > + * Scale 10 is the largest that fits the overflow test cases in an s64.
> >   */
> > +#define IIO_TEST_DEC_SCALE	10
> > +#define IIO_TEST_NANO		10	/* one nano at IIO_TEST_DEC_SCALE */
> 
> I would put them as (9 + 1) to show that actually show that "with one more digit".
> I also don't follow the comment near to the last definition. What does it mean?

With decimal scale of 10, a unit is 10^-10, 10 units is 10^-9 = NANO, which is
the scale used in iio_format_value().

-- 
Kind regards,

Rodrigo Alencar