linux-kernel - Re: [PATCH] math64: Provide an uprounding variant of mul_u64_u64_div

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Message-ID: <epuk3zijp2jt6jh72z3xi2wxneeunf5xx2h77kvim6xmzminwj@4saalgxu3enu>
Date: Mon, 31 Mar 2025 18:14:29 +0200
From: Uwe Kleine-König <u.kleine-koenig@...libre.com>
To: David Laight <david.laight.linux@...il.com>, 
	Andrew Morton <akpm@...ux-foundation.org>
Cc: Nicolas Pitre <npitre@...libre.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] math64: Provide an uprounding variant of
 mul_u64_u64_div_u64()

Hello,

On Fri, Mar 21, 2025 at 01:18:13PM +0000, David Laight wrote:
> On Wed, 19 Mar 2025 18:14:25 +0100
> Uwe Kleine-König <u.kleine-koenig@...libre.com> wrote:
> 
> > This is needed (at least) in the pwm-stm32 driver. Currently the
> > pwm-stm32 driver implements this function itself. This private
> > implementation can be dropped as a followup of this patch.
> > 
> > Signed-off-by: Uwe Kleine-König <u.kleine-koenig@...libre.com>
> > ---
> >  include/linux/math64.h |  1 +
> >  lib/math/div64.c       | 15 +++++++++++++++
> >  2 files changed, 16 insertions(+)
> > 
> > diff --git a/include/linux/math64.h b/include/linux/math64.h
> > index 6aaccc1626ab..0c545b3ddaa5 100644
> > --- a/include/linux/math64.h
> > +++ b/include/linux/math64.h
> > @@ -283,6 +283,7 @@ static inline u64 mul_u64_u32_div(u64 a, u32 mul, u32 divisor)
> >  #endif /* mul_u64_u32_div */
> >  
> >  u64 mul_u64_u64_div_u64(u64 a, u64 mul, u64 div);
> > +u64 mul_u64_u64_div_u64_roundup(u64 a, u64 mul, u64 div);
> >  
> >  /**
> >   * DIV64_U64_ROUND_UP - unsigned 64bit divide with 64bit divisor rounded up
> > diff --git a/lib/math/div64.c b/lib/math/div64.c
> > index 5faa29208bdb..66beb669992d 100644
> > --- a/lib/math/div64.c
> > +++ b/lib/math/div64.c
> > @@ -267,3 +267,18 @@ u64 mul_u64_u64_div_u64(u64 a, u64 b, u64 c)
> >  }
> >  EXPORT_SYMBOL(mul_u64_u64_div_u64);
> >  #endif
> > +
> > +#ifndef mul_u64_u64_div_u64_roundup
> > +u64 mul_u64_u64_div_u64_roundup(u64 a, u64 b, u64 c)
> > +{
> > +	u64 res = mul_u64_u64_div_u64(a, b, c);
> > +	/* Those multiplications might overflow but it doesn't matter */
> > +	u64 rem = a * b - c * res;
> > +
> > +	if (rem)
> > +		res += 1;
> 
> Ugg...
> 	return (((unsigned __int128_t)a * b) + (c - 1)) / c;
> nearly works (on 64bit) but needs a u64 div_128_64()

Both mul_u64_u64_div_u64_roundup() and mul_u64_u64_div_u64() would not
be needed if we had a 128 bit type and a corresponding division on all
supported architectures.

So given we're not in that situation, I wonder if Andrew is still
considering this patch or if someone else would pick up this patch?

Best regards
Uwe

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