Subject: Re: [PATCH v2] add equivalent of BIT(x) for bitfields
To: Linus Torvalds <torvalds@linux-foundation.org>
References: <93523b80-6033-bafa-c1ec-4bc2c3f9c53b@laposte.net>
 <CA+55aFx2WSredqeGRJpjwhJme8zopNa54MaA3R30AGGHU+sY2Q@mail.gmail.com>
Cc: zijun_hu <zijun_hu@htc.com>, Andrew Morton <akpm@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Mason <slash.tmp@free.fr>, Harvey Harrison <harvey.harrison@gmail.com>,
        Borislav Petkov <bp@alien8.de>
From: Sebastian Frias <sf84@laposte.net>
Message-ID: <9d74aaf8-a328-a3a6-dcaa-9ade95538241@laposte.net>
Date: Tue, 6 Dec 2016 11:10:02 +0100
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101
 Thunderbird/45.4.0
MIME-Version: 1.0
In-Reply-To: <CA+55aFx2WSredqeGRJpjwhJme8zopNa54MaA3R30AGGHU+sY2Q@mail.gmail.com>
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 785
Lines: 27

On 05/12/16 19:23, Linus Torvalds wrote:
> On Mon, Dec 5, 2016 at 9:49 AM, Sebastian Frias <sf84@laposte.net> wrote:
>> Introduce GENVALUE(msb, lsb, value) macro to ease dealing with
>> continuous bitfields, just as BIT(x) does for single bits.
> 
> Oh, and looking at the implementation, this is wrong. You use "lsb"
> twice, so it mustn't have side effects.
> 
> That's fine for all expected users (since you'd expect that the only
> real use of this is with constant values for msb/lsb), 

Yes, that's what I thought.

>but please add
> actual checking.

Sure! Thanks for the advice.

> 
> You can use BUILD_BUG_ON_ZERO(!__builtin_constant_p(x)) or something.
> That returns zero, so it's easy to use in expressions.

Done. v3 of the patch is submitted.

> 
>              Linus
>