linux-kernel - Re: [PATCH v5 2/8] iio: core: add fixed point parsing with 64-bit parts

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Message-ID: <aXdtpkL5QUhhB_hh@smile.fi.intel.com>
Date: Mon, 26 Jan 2026 15:35:34 +0200
From: Andy Shevchenko <andriy.shevchenko@...el.com>
To: Rodrigo Alencar <455.rodrigo.alencar@...il.com>
Cc: rodrigo.alencar@...log.com, linux-kernel@...r.kernel.org,
	linux-iio@...r.kernel.org, devicetree@...r.kernel.org,
	linux-doc@...r.kernel.org, Jonathan Cameron <jic23@...nel.org>,
	David Lechner <dlechner@...libre.com>,
	Andy Shevchenko <andy@...nel.org>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Michael Hennerich <Michael.Hennerich@...log.com>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH v5 2/8] iio: core: add fixed point parsing with 64-bit
 parts

On Mon, Jan 26, 2026 at 12:42:53PM +0000, Rodrigo Alencar wrote:
> On 26/01/26 01:49PM, Andy Shevchenko wrote:
> > On Fri, Jan 23, 2026 at 03:53:07PM +0000, Rodrigo Alencar via B4 Relay wrote:

...

> > > +static int __iio_str_to_fixpoint64(const char *str, u64 fract_mult,
> > > +				   s64 *integer, s64 *fract, bool scale_db)
> > > +{
> > > +	u64 i = 0, f = 0;
> > > +	char *end;
> > > +	int digit_count, precision = ffs(fract_mult);
> > > +	bool negative = false;
> > > +
> > > +	if (str[0] == '-') {
> > > +		negative = true;
> > > +		str++;
> > > +	} else if (str[0] == '+') {
> > > +		str++;
> > > +	}
> > > +
> > > +	i = simple_strtoull(str, &end, 10);
> > > +	digit_count = end - str;
> > > +	if (digit_count > 20)
> > > +		return -EINVAL;
> > 
> > Not really. If we are talking about decimal (only) cases we need to also count
> > leading 0:s.
> > 
> > 0000000000000000000000000000000025 is still 25, no overflow.
> > 
> > That's why I recommend to have a helper, maybe for now locally here, like
> > 
> > int safe_strtoull(..., unsigned long long *res)
> > {
> > 	...
> > }
> 
> Are you suggesting to not use simple_strtoull then?

Nope, I suggest to do an additional step before checking for the range.

> Understood, leading zeros can be ignored only when parsing the integer 
> part. Also, would be nice to have truncation of the fractional part
> while doing the parsing. How about:
> 
> static int iio_safe_strtoull(const char *str, const char **end,
> 			     size_t max_chars, u64 *res)

> - max_chars = 0: ignores leading 0's and process all digits
> - max_chars > 0: process only initial max_chars digits and ignores the rest

I'm not sure why we would need that. It should parse the whole line until the
first invalid character or overflow.

> on overflow of u64, the function would return -EOVERFLOW
> 
> > that will do all necessary checks and returns -EINVAL, -ERANGE, et cetera.
> > In the below we would need check for the error codes respectively.
> > 
> > > +	if (precision && *end == '.') {
> > > +		str = end + 1;
> > > +		f = simple_strtoull(str, &end, 10);
> > > +		digit_count = end - str;
> > > +		if (!digit_count || digit_count > 20)
> > > +			return -EINVAL;
> > > +
> > > +		if (digit_count > precision) {
> > > +			digit_count -= precision;
> > > +			f = div64_u64(f, int_pow(10, digit_count));
> > > +		} else {
> > > +			digit_count = precision - digit_count;
> > > +			f *= int_pow(10, digit_count);
> > > +		}
> > > +	} else if (!digit_count) {
> > > +		return -EINVAL;
> > > +	}
> > > +
> > > +	if (scale_db) {
> > 
> > > +		/* Ignore the dB suffix */
> > > +		if (!strncmp(end, " dB", sizeof(" dB") - 1))
> > > +			end += sizeof(" dB") - 1;
> > > +		else if (!strncmp(end, "dB", sizeof("dB") - 1))
> > > +			end += sizeof("dB") - 1;
> > 
> > Now we have strends()
> 
> strends() would not account for the acceptable '\n' before the end.

Good point.

> I don't think we would need to test for " dB", " dB\n", "dB" and "dB\n"

Then you can try sysfs_eq() which does that check. But I think it requires
the (end of the) string to be exact, and not something like 'dB   \n'.

-- 
With Best Regards,
Andy Shevchenko