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

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Message-ID: <20250321131813.6a332944@pumpkin>
Date: Fri, 21 Mar 2025 13:18:13 +0000
From: David Laight <david.laight.linux@...il.com>
To: Uwe Kleine-König <u.kleine-koenig@...libre.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Nicolas Pitre
 <npitre@...libre.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] math64: Provide an uprounding variant of
 mul_u64_u64_div_u64()

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()

	David


> +
> +	return res;
> +}
> +EXPORT_SYMBOL(mul_u64_u64_div_u64_roundup);
> +#endif
> 
> base-commit: 2014c95afecee3e76ca4a56956a936e23283f05b