Re: [PATCH v10 02/11] lib: kstrtox: add kstrtoudec64() and kstrtodec64()

David Laight
2026-04-25T22:33:20Z

On Sat, 25 Apr 2026 16:40:06 +0100
Jonathan Cameron <jic23@kernel.org> wrote:

> On Fri, 17 Apr 2026 09:36:20 +0100
> Rodrigo Alencar <455.rodrigo.alencar@gmail.com> wrote:
>
> > On 26/04/15 10:51AM, Rodrigo Alencar wrote:
> > > Add helpers that parses decimal numbers into 64-bit number, i.e., decimal
> > > point numbers with pre-defined scale are parsed into a 64-bit value (fixed
> > > precision). After the decimal point, digits beyond the specified scale
> > > are ignored.
> >
> > ...
> >
> > > +static int _kstrtoudec64(const char *s, unsigned int scale, u64 *res)
> > > +{
> > > + u64 _res = 0, _frac = 0;
> > > + unsigned int rv;
> > > +
> > > + if (scale > 19) /* log10(2^64) = 19.26 */
> > > + return -EINVAL;
> > > +
> > > + if (*s != '.') {
> > > + rv = _parse_integer(s, 10, &_res);
> > > + if (rv & KSTRTOX_OVERFLOW)
> > > + return -ERANGE;
> > > + if (rv == 0)
> > > + return -EINVAL;
> > > + s += rv;
> > > + }
> > > +
> > > + if (*s == '.' && scale) {
> > > + s++; /* skip decimal point */
> > > + rv = _parse_integer_limit(s, 10, &_frac, scale);
> > > + if (rv & KSTRTOX_OVERFLOW)
> > > + return -ERANGE;
> > > + if (rv == 0)
> > > + return -EINVAL;
> > > + s += rv;
> > > + if (rv < scale)
> > > + _frac *= int_pow(10, scale - rv);
> > > + while (isdigit(*s)) /* truncate */
> > > + s++;
> > > + }
> > > +
> > > + if (*s == '\n')
> > > + s++;
> > > + if (*s)
> > > + return -EINVAL;
> > > +
> > > + if (check_mul_overflow(_res, int_pow(10, scale), &_res) ||
> > > + check_add_overflow(_res, _frac, &_res))
> > > + return -ERANGE;
> > > +
> > > + *res = _res;
> > > + return 0;
> > > +}
> >
> > I have an alternative (slightly more complex) implementation of this function
> > that handles E notation. I find this particularly handy when writting big
> > values like 25 GHz when the ABI is defined in Hz, so instead of writing
> > 25000000000, one can just use 25e9, or 2.5e10. I found that my python code
> > was printing big floating point values or really small ones using E notation
> > and that was giving me -EINVAL, so I had to adjust formatting when generating
> > the string input to the file. No big deal, and we would not need this here,
> > but if maintainers find this useful I could add it into a v11 of this series.
> >
>
> I'd rather we didn't slow this one down. However I'm waiting on some tags
> on this patch from folk who are more familiar with these parsers than
> I am. Given discussion, Andy or David Laight perhaps?
> +CC David - please make sure to include folk who have been active
> in discussion of earlier versions to decrease chance they miss the new
> one.

I can't help feeling this code would be smaller if it didn't try to use
the existing conversion functions.
Something like:
u64 r = 0;
unsigned int n = ~0;
while (*s == ' ' || *s == '\n')
s++;
for (;;) {
unsigned int dig = *s++ - '0';
if (dig <= 9) {
if (!n)
continue;
n--;
r = r * 10 + dig;
continue;
}
switch (s[-1]) {
case '.':
if (n <= scale)
return -EINVAL;
n = scale;
continue;
case '\n':
if (*s)
return -EINVAL;
break;
case 0:
break;
default:
return -EIVAL;
}
break;
}
if (n > scale)
n = scale;
while (n--)
r *= 10;
*res = r;
return 0;
}

That is missing the overflow detect for the multiply and add.
While check_add_overflow() hopefully looks at the carry flag (on non-mips
style cpu), I don't know how the 'mul' variant works - it might be horrid.
A bound check against ~0ull/10 might generate better code.

But I really prefer functions that return the terminating character to
the caller - they are more useful for parsing compound parameters.

David

>
> Maybe start a discussion about whether adding e notation as a separate
> thread after this has merged?
>
> Jonathan
>
>