linux-kernel - Re: [PATCH v4 next 0/9] Implement mul_u64_u64_div_u64

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Message-Id: <20251030212924.35d15008efe63256762a294b@linux-foundation.org>
Date: Thu, 30 Oct 2025 21:29:24 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: David Laight <david.laight.linux@...il.com>
Cc: linux-kernel@...r.kernel.org, u.kleine-koenig@...libre.com, Nicolas
 Pitre <npitre@...libre.com>, Oleg Nesterov <oleg@...hat.com>, Peter
 Zijlstra <peterz@...radead.org>, Biju Das <biju.das.jz@...renesas.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>, Thomas
 Gleixner <tglx@...utronix.de>, Li RongQing <lirongqing@...du.com>, Yu Kuai
 <yukuai3@...wei.com>, Khazhismel Kumykov <khazhy@...omium.org>, Jens Axboe
 <axboe@...nel.dk>, x86@...nel.org
Subject: Re: [PATCH v4 next 0/9] Implement mul_u64_u64_div_u64_roundup()

On Wed, 29 Oct 2025 17:38:19 +0000 David Laight <david.laight.linux@...il.com> 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.
> 
> The test module has been updated to test mul_u64_u64_div_u64_roundup() and
> also enhanced it to verify the C division code on x86-64 and the 32bit
> division code on 64bit.

Thanks, I added this to mm.git's mm-nonmm-unstable branch for some
linux-next exposure.  I have a note that [3/9] may be updated in
response to Nicolas's comment.