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

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Message-ID: <aXeRQ4AXmn1SxoNa@smile.fi.intel.com>
Date: Mon, 26 Jan 2026 18:07:31 +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 03:30:44PM +0000, Rodrigo Alencar wrote:
> On 26/01/26 03:20PM, Rodrigo Alencar wrote:
> > On 26/01/26 04:53PM, Andy Shevchenko wrote:
> > > On Mon, Jan 26, 2026 at 02:26:20PM +0000, Rodrigo Alencar wrote:

...

> > > Why? Can you elaborate how checking amount of digits is different to
> > > check_mul_overflow()?
> > 
> > consider U64_MAX = 18_446_744_073_709_551_615 as the limit:
> > - 19_000_000_000_000_000_000 contains the same amount of digits but overflows.
> > - 18_446_744_073_710_000_000 contains the same amount of digits but overflows.
> > 
> > to catch those cases, we need to check for the overflow, everytime we read a
> > character and accumulate:
> > 
> > u64 acc;
> > 
> > while(isdigit(*str))
> > 	if (check_mul_overflow(acc, 10, &acc) ||
> > 	    check_add_overflow(acc, *str - '0', &acc))
> > 		return -EOVERFLOW;
> > 
> > *res = acc;
> > 
> > acc can get weird results if not checked. 
> 
> Thinking about it again, that check could be done only in the last step
> (20th for u64)

Does kstrto*() also perform only last check? I think they do for each
iteration.

-- 
With Best Regards,
Andy Shevchenko