linux-kernel - Re: [PATCH] linux/bits.h: Add compile time sanity check of GENMASK inputs

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Date:   Sat, 3 Aug 2019 20:36:37 +0200
From:   Rikard Falkeborn <rikard.falkeborn@...il.com>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     Rikard Falkeborn <rikard.falkeborn@...il.com>,
        Joe Perches <joe@...ches.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Berg <johannes@...solutions.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] linux/bits.h: Add compile time sanity check of GENMASK
 inputs

On Sat, Aug 03, 2019 at 12:12:46PM +0900, Masahiro Yamada wrote:
> On Sat, Aug 3, 2019 at 12:03 PM Masahiro Yamada
> <yamada.masahiro@...ionext.com> wrote:
> 
> >
> > BTW, v2 is already inconsistent.
> > If you wanted GENMASK_INPUT_CHECK() to return 'unsigned long',,
> > you would have to cast (low) > (high) as well:
> >
> >                (unsigned long)((low) > (high)), UL(0))))
> >
> > This is totally redundant, and weird.
> 
> I take back this comment.
> You added (unsigned long) to the beginning of this macro.
> So, the type is consistent, but I believe all casts should be removed.

Maybe you're right. BUILD_BUG_ON_ZERO returns size_t regardless of
inputs. I was worried that on some platform, size_t would be larger than
unsigned long (as far as I could see, the standard does not give any
guarantees), and thus all of a sudden GENMASK would be 8 bytes instead
of 4, but perhaps that is not a problem?