linux-kernel - Re: [PATCH] log2: make order_base_2() behave correctly on const input value zero

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Date:   Thu, 2 Feb 2017 20:51:53 +0000
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Laura Abbott <labbott@...oraproject.org>,
        Will Deacon <will.deacon@....com>,
        Stephen Boyd <sboyd@...eaurora.org>,
        Gregory Clement <gregory.clement@...e-electrons.com>,
        james.greenhalgh@....com, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Markus Trippelsdorf <markus@...ppelsdorf.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Joe Perches <joe@...ches.com>
Subject: Re: [PATCH] log2: make order_base_2() behave correctly on const input value zero


> On 2 Feb 2017, at 20:49, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> On Thu, Feb 2, 2017 at 9:04 AM, Ard Biesheuvel
> <ard.biesheuvel@...aro.org> wrote:
>> 
>> So update order_base_2() to adhere to its own documented interface.
> 
> Ok, looks like you screwed up the types according to the build server.
> 
> Making the return type "unsigned long" is slightly excessive. If you
> need an unsigned long to describe the log2 of a number in the kernel,
> you're playing with numbers that are *way* too big.
> 
> The old code used "ilog2()", which returns an int.
> 

Yeah I cloned roundup_pow_of_2, and failed to spot that ilog2 returns an int. I sent out the v2 already

> Which is plenty big enough of a type.
> 
>                Linus