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

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Message-ID: <20180823154022.GA6535@e107564-lin.cambridge.arm.com>
Date:   Thu, 23 Aug 2018 16:40:22 +0100
From:   Brian Starkey <brian.starkey@....com>
To:     Matthew Wilcox <willy@...radead.org>
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

Hi Matthew,

On Thu, Aug 23, 2018 at 07:34:45AM -0700, Matthew Wilcox wrote:
>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.

I don't think so... the sentence I pasted says that including a value
which isn't representable in a plain int would be illegal, and so the
compiler doesn't _have_ to do anything (nasal demons, right?).

>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.

The paragraph about the implementation choosing a representation is
separate from the valid range of values - the compiler can pick
whatever storage it likes (smaller or even larger than an int), so
long as that storage can fit all the defined values. However,
providing a value in an enum definition which is not representable as
an int would still be invalid (irrespective of how large the storage
is) - it's a separate restriction.

Anyhow, I'm not dying to replace all the current definitions with
enums, so if someone else wants to pick that up, be my guest.

Cheers,
-Brian