linux-kernel - Re: [PATCH next 0/3] lib: Implement mul_u64_u64_div_u64

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Message-ID: <5e7mp3nreaadppjhxhpffyff2d2ccwcjf2suonxe43eofngddu@qfufr2wiw7yn>
Date: Fri, 16 May 2025 11:47:58 +0200
From: Uwe Kleine-König <u.kleine-koenig@...libre.com>
To: David Laight <david.laight.linux@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	linux-kernel@...r.kernel.org, Nicolas Pitre <npitre@...libre.com>, 
	Oleg Nesterov <oleg@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
	Biju Das <biju.das.jz@...renesas.com>
Subject: Re: [PATCH next 0/3] lib: Implement mul_u64_u64_div_u64_roundup()

Hello David,

On Sat, Apr 05, 2025 at 09:45:27PM +0100, David Laight wrote:
> The pwm-stm32.c code wants a 'rounding up' version of mul_u64_u64_div_u64().
> This can be done simply by adding 'divisor - 1' to the 128bit product.
> Implement mul_u64_add_u64_div_u64(a, b, c, d) = (a * b + c)/d based on the
> existing code.
> Define mul_u64_u64_div_u64(a, b, d) as mul_u64_add_u64_div_u64(a, b, 0, d) and
> mul_u64_u64_div_u64_roundup(a, b, d) as mul_u64_add_u64_div_u64(a, b, d-1, d).
> 
> Only x86-64 has an optimsed (asm) version of the function.
> That is optimised to avoid the 'add c' when c is known to be zero.
> In all other cases the extra code will be noise compared to the software
> divide code.
> 
> I've updated the test module to test mul_u64_u64_div_u64_roundup() and
> also enhanced it to verify the C division code on x86-64.
> 
> Note that the code generated by gcc (eg for 32bit x86) just for the multiply
> is rather more horrid than one would expect (clang does better).
> I dread to think how long the divide loop takes.
> And I'm not at all sure the call in kernel/sched/cputime.c isn't in a
> relatively common path (rather than just hardware initialisation).
> 
> David Laight (3):
>   lib: Add mul_u64_add_u64_div_u64() and mul_u64_u64_div_u64_roundup()
>   lib: Add tests for mul_u64_u64_div_u64_roundup()
>   lib: Update the muldiv64 tests to verify the C on x86-64

I wonder what happend to this series. I'd like to make use of
mul_u64_u64_div_u64_roundup() so I'd be interested to get this into the
mainline.

Best regards
Uwe

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