Re: [PATCH v12 05/11] iio: core: add decimal value formatting into 64-bit value

[Andy Shevchenko]

On Tue, May 12, 2026 at 05:09:32PM +0100, Rodrigo Alencar wrote:
> On 26/05/12 05:35PM, Andy Shevchenko wrote:
> > On Sun, May 10, 2026 at 01:42:23PM +0100, Rodrigo Alencar via B4 Relay wrote:
> > 
> > > Create new format types for iio values (IIO_VAL_DECIMAL64_*), which
> > > defines the representation of fixed decimal point values into a single
> > > 64-bit number. This new format increases the range of represented values,
> > > allowing for integer parts greater than 2^32, as bits are not "wasted"
> > > in the fractional part, which can be seen in IIO_VAL_INT_PLUS_MICRO and
> > > IIO_VAL_INT_PLUS_NANO. Helpers are created to compose and decompose 64-bit
> > > decimals into integer values used in IIO formatting interfaces, which
> > > creates consistency and avoid error-prone manual assignments when using
> > > wordpart macros. When doing the parsing, kstrtodec64() is used with the
> > > scale defined by the specific decimal format type.

...

> > > +		tmp2 = div64_s64_rem(iio_val_s64_from_array(vals),
> > > +				     int_pow(10, scale), &frac);
> > > +		if (tmp2 == 0 && frac < 0)
> > > +			return sysfs_emit_at(buf, offset, "-0.%0*lld", scale,
> > > +					     abs(frac));
> > > +		else
> > > +			return sysfs_emit_at(buf, offset, "%lld.%0*lld", tmp2,
> > > +					     scale, abs(frac));
> > > +	}
> > 
> > What about
> > 
> > 		/* Print a leading '-' for negative fractions */
> > 		if (tmp2 == 0 && frac < 0)
> > 			offset += sysfs_emit_at(buf, offset, "-");
> > 
> > 		return sysfs_emit_at(buf, offset, "%lld.%0*lld", tmp2, scale, abs(frac));
> > 
> > Also note this won't work with the frac that are == S64_MIN. It's UB (undefined
> > behaviour), see the comment at abs() implementation. Maybe a time to add abs()
> > corner case tests...
> 
> frac cannot be S64_MIN, it is always and remainder of a power of 10 modulus.

Okay, but what about input of -0.9999999999999999999 ? Will it fit the signed
frac type?

-- 
With Best Regards,
Andy Shevchenko