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From:   David Laight <David.Laight@ACULAB.COM>
To:     'Sebastian Reichel' <sebastian.reichel@collabora.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Michael Turquette <mturquette@baylibre.com>,
        Stephen Boyd <sboyd@kernel.org>,
        "linux-clk@vger.kernel.org" <linux-clk@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
CC:     Vasily Gorbik <gor@linux.ibm.com>,
        Niklas Schnelle <schnelle@linux.ibm.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "kernel@collabora.com" <kernel@collabora.com>
Subject: RE: [PATCH v4 1/3] math.h: add DIV_ROUND_UP_NO_OVERFLOW
Thread-Topic: [PATCH v4 1/3] math.h: add DIV_ROUND_UP_NO_OVERFLOW
Thread-Index: AQHaBpXnM+Cp9N+qfkK9yv140Cm+RbBZHxkA
Date:   Tue, 24 Oct 2023 16:26:55 +0000
Message-ID: <488098703d6947ee8d7b19312c7097a6@AcuMS.aculab.com>
References: <20231024161931.78567-1-sebastian.reichel@collabora.com>
 <20231024161931.78567-2-sebastian.reichel@collabora.com>
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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@collabora.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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