linux-kernel - Re: [PATCH v7 1/3] lib: Add strongly typed 64bit int

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Date:   Tue, 19 Dec 2017 08:09:53 -0800
From:   Joe Perches <joe@...ches.com>
To:     Crt Mori <cmo@...exis.com>, Jonathan Cameron <jic23@...nel.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Kees Cook <keescook@...omium.org>,
        Rusty Russell <rusty@...tcorp.com.au>,
        Ian Abbott <abbotti@....co.uk>,
        Larry Finger <Larry.Finger@...inger.net>,
        Niklas Soderlund <niklas.soderlund+renesas@...natech.se>,
        Thomas Gleixner <tglx@...utronix.de>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        linux-kernel@...r.kernel.org, linux-iio@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v7 1/3] lib: Add strongly typed 64bit int_sqrt

On Tue, 2017-12-19 at 16:59 +0100, Crt Mori wrote:
> There is no option to perform 64bit integer sqrt on 32bit platform.
> Added stronger typed int_sqrt64 enables the 64bit calculations to
> be performed on 32bit platforms. Although int_sqrt() is a rough
> approximation, the same algorithm is used in int_sqrt64() as good
> enough on 32bit platform.
> 
> Signed-off-by: Crt Mori <cmo@...exis.com>
> ---
>  include/linux/kernel.h |  6 ++++++
>  lib/int_sqrt.c         | 32 ++++++++++++++++++++++++++++++++
>  2 files changed, 38 insertions(+)
> 
> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> index 0ad4c3044cf9..f3bebd3a9213 100644
> --- a/include/linux/kernel.h
> +++ b/include/linux/kernel.h
> @@ -459,6 +459,12 @@ extern int kernel_text_address(unsigned long addr);
>  extern int func_ptr_is_kernel_text(void *ptr);
>  
>  unsigned long int_sqrt(unsigned long);
> +#if BITS_PER_LONG != 64
> +#define int_sqrt64 _int_sqrt64
> +u32 _int_sqrt64(u64 x);
> +#else
> +#define int_sqrt64 int_sqrt
> +#endif

As you've changed the return type for the 64-bit function to u32,
there is a unsigned long / u32 return mismatch on 64 bit platforms.

I suspect you need to define this as:

#define int_sqrt64(x) ((u32)int_sqrt(x))
 

> diff --git a/lib/int_sqrt.c b/lib/int_sqrt.c
[]
> @@ -36,3 +36,35 @@ unsigned long int_sqrt(unsigned long x)
>  	return y;
>  }
>  EXPORT_SYMBOL(int_sqrt);
> +
> +#if BITS_PER_LONG != 64
> +/**
> + *
> + * int_sqrt64 - strongly typed int_sqrt function when minimum 64 bit input
> + * is expected.
> + * @x: 64bit integer of which to calculate the sqrt
> + */
> +u32 _int_sqrt64(u64 x)
> +{
> +	u64 b, m, y = 0;
> +
> +	if (x <= 1)
> +		return x;
> +
> +	m = 1ULL << (64 - 2);

Should probably use fls64 here.

> +	while (m != 0) {
> +		b = y + m;
> +		y >>= 1;
> +
> +		if (x >= b) {
> +			x -= b;
> +			y += m;
> +		}
> +		m >>= 2;
> +	}
> +
> +	return y;
> +}
> +EXPORT_SYMBOL(_int_sqrt64);
> +#endif
> +