MIME-Version: 1.0
In-Reply-To: <1513614245.31581.98.camel@perches.com>
References: <20171218150544.8000-1-cmo@melexis.com> <1513614245.31581.98.camel@perches.com>
From: Crt Mori <cmo@melexis.com>
Date: Tue, 19 Dec 2017 10:32:56 +0100
Message-ID: <CAKv63usZnz9N3TOv7OC-tjGd+ro4=1yADjY0dwsFpQwnnQqOfQ@mail.gmail.com>
Subject: Re: [PATCH v6 1/3] lib: Add strongly typed 64bit int_sqrt
To: Joe Perches <joe@perches.com>
Cc: Jonathan Cameron <jic23@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Kees Cook <keescook@chromium.org>,
        Rusty Russell <rusty@rustcorp.com.au>, Ian Abbott <abbotti@mev.co.uk>,
        Larry Finger <Larry.Finger@lwfinger.net>,
        Niklas Soderlund <niklas.soderlund+renesas@ragnatech.se>,
        Thomas Gleixner <tglx@linutronix.de>,
        Krzysztof Kozlowski <krzk@kernel.org>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        linux-kernel@vger.kernel.org, linux-iio <linux-iio@vger.kernel.org>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2094
Lines: 74

On 18 December 2017 at 17:24, Joe Perches <joe@perches.com> wrote:
> On Mon, 2017-12-18 at 16:05 +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.
> []
>> diff --git a/include/linux/kernel.h b/include/linux/kernel.h
> []
>> @@ -459,6 +459,7 @@ extern int kernel_text_address(unsigned long addr);
>>  extern int func_ptr_is_kernel_text(void *ptr);
>>
>>  unsigned long int_sqrt(unsigned long);
>> +u64 int_sqrt64(u64 x);
>>
>>  extern void bust_spinlocks(int yes);
>>  extern int oops_in_progress;         /* If set, an oops, panic(), BUG() or die() is in progress */
>> diff --git a/lib/int_sqrt.c b/lib/int_sqrt.c
> []
>> @@ -36,3 +36,29 @@ unsigned long int_sqrt(unsigned long x)
>>       return y;
>>  }
>>  EXPORT_SYMBOL(int_sqrt);
>> +
>> +/**
>> + * int_sqrt64 - strongly typed int_sqrt function
>> + * @x: 64bit integer of which to calculate the sqrt
>> + */
>> +u64 int_sqrt64(u64 x)
>> +{
>> +     u64 b, m, y = 0;
>> +
>> +     if (x <= 1)
>> +             return x;
>> +
>> +     m = 1ULL << (64 - 2);
>> +     while (m != 0) {
>> +             b = y + m;
>> +             y >>= 1;
>> +
>> +             if (x >= b) {
>> +                     x -= b;
>> +                     y += m;
>> +             }
>> +             m >>= 2;
>> +     }
>> +     return y;
>> +}
>> +EXPORT_SYMBOL(int_sqrt64);
>
> int_sqrt is now optimized with __fls
>
> Because u64 is unsigned long on 64 bit arches, this
> could probably be optimized for 64 bit distributions
> with something like:
>
> #if BITS_PER_LONG != 64
> #define int_sqrt64 _int_sqrt64
> u64 _int_sqrt64(u64 x);
> #else
> #define int_sqrt64(x) int_sqrt
> #endif
>
> ...
>
> #if BITS_PER_LONG != 64
> u64 _int_sqrt64(u64 x)
> { etc.. }
> #endif
>

Thanks for the tip. I will respin with this change so that 64 bit
arches use the optimized int_sqrt from above.