linux-kernel - RE: [PATCH v4 1/3] math.h: add DIV_ROUND_UP_NO

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Message-ID: <488098703d6947ee8d7b19312c7097a6@AcuMS.aculab.com>
Date:   Tue, 24 Oct 2023 16:26:55 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Sebastian Reichel' <sebastian.reichel@...labora.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        "linux-clk@...r.kernel.org" <linux-clk@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     Vasily Gorbik <gor@...ux.ibm.com>,
        Niklas Schnelle <schnelle@...ux.ibm.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "kernel@...labora.com" <kernel@...labora.com>
Subject: RE: [PATCH v4 1/3] math.h: add DIV_ROUND_UP_NO_OVERFLOW

From: Sebastian Reichel
> Sent: 24 October 2023 17:18
> 
> Add a new DIV_ROUND_UP helper, which cannot overflow when
> big numbers are being used.

For non-zero you can use (n - 1)/d + 1 instead of (n + d - 1)/d
So maybe add:
#define DIV_ROUND_UP_NON_ZERO(n, d) (((n) - 1)/(d) + 1)

Saves the compiler having to get the remainder (if not
generated by a divide instruction.

	David

> 
> Signed-off-by: Sebastian Reichel <sebastian.reichel@...labora.com>
> ---
>  include/linux/math.h | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/include/linux/math.h b/include/linux/math.h
> index dd4152711de7..f80bfb375ab9 100644
> --- a/include/linux/math.h
> +++ b/include/linux/math.h
> @@ -36,6 +36,17 @@
> 
>  #define DIV_ROUND_UP __KERNEL_DIV_ROUND_UP
> 
> +/**
> + * DIV_ROUND_UP_NO_OVERFLOW - divide two numbers and always round up
> + * @n: numerator / dividend
> + * @d: denominator / divisor
> + *
> + * This functions does the same as DIV_ROUND_UP, but internally uses a
> + * division and a modulo operation instead of math tricks. This way it
> + * avoids overflowing when handling big numbers.
> + */
> +#define DIV_ROUND_UP_NO_OVERFLOW(n, d) (((n) / (d)) + !!((n) % (d)))
> +
>  #define DIV_ROUND_DOWN_ULL(ll, d) \
>  	({ unsigned long long _tmp = (ll); do_div(_tmp, d); _tmp; })
> 
> --
> 2.42.0

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