linux-kernel - Re: [PATCH] drm/fourcc: Add DOC: overview comment

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Message-ID: <20180823143445.GA26109@bombadil.infradead.org>
Date:   Thu, 23 Aug 2018 07:34:45 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Brian Starkey <brian.starkey@....com>
Cc:     Daniel Vetter <daniel.vetter@...ll.ch>,
        Eric Engestrom <eric.engestrom@...el.com>,
        Alexandru-Cosmin Gheorghe <alexandru-cosmin.gheorghe@....com>,
        Jonathan Corbet <corbet@....net>,
        Dave Airlie <airlied@...ux.ie>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        Sean Paul <seanpaul@...omium.org>,
        Liviu Dudau <liviu.dudau@....com>,
        Ayan Kumar Halder <ayan.halder@....com>
Subject: Re: [PATCH] drm/fourcc: Add DOC: overview comment

On Wed, Aug 22, 2018 at 04:57:33PM +0100, Brian Starkey wrote:
> On Wed, Aug 22, 2018 at 05:11:55PM +0200, Daniel Vetter wrote:
> > On Wed, Aug 22, 2018 at 4:59 PM, Eric Engestrom
> > <eric.engestrom@...el.com> wrote:
> > > On Tuesday, 2018-08-21 17:44:17 +0100, Brian Starkey wrote:
> > > > On Tue, Aug 21, 2018 at 09:26:39AM -0700, Matthew Wilcox wrote:
> > > > > Can you turn them into enums?  This seems to work ok:
> 
> I'm not sure that swapping out explicit 32-bit unsigned integers for
> enums (unspecified width, signed integers) is necessarily a good idea,
> it seems like Bad Things could happen.
> 
> The C spec says:
> 
>   "the value of an enumeration constant shall be an integer constant
>   expression that has a value representable as an int"
> 
> Which likely gives us 4 bytes to play with on all machines
> that run Linux, but if drm_fourcc.h is ever going to be some kind of
> standard reference, making it non-portable seems like a fail.
>
> And even if you do have 4 bytes in an enum, signed integers act
> differently from unsigned ones, and compilers do love to invoke the UB
> clause...

I think you're exaggerating how much latitude C compilers have here.
Further down in 6.7.2.2, it says:

  Each enumerated type shall be compatible with char, a signed
  integer type, or an unsigned integer type.  The choice of type is
  implementation-defined, but shall be capable of representing the values
  of all the members of the enumeration.

So if we include an integer which isn't representable in a plain int,
then the compiler _must_ choose a larger type.  It could choose a
signed-64-bit type rather than an unsigned-32-bit type, but I can't
imagine any compiler being quite so insane.