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

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Message-ID: <20180822155732.GA39066@e107564-lin.cambridge.arm.com>
Date:   Wed, 22 Aug 2018 16:57:33 +0100
From:   Brian Starkey <brian.starkey@....com>
To:     Daniel Vetter <daniel.vetter@...ll.ch>
Cc:     Eric Engestrom <eric.engestrom@...el.com>,
        Matthew Wilcox <willy@...radead.org>,
        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,

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:
>>> Hi Matthew,
>>>
>>> On Tue, Aug 21, 2018 at 09:26:39AM -0700, Matthew Wilcox wrote:
>>> > On Tue, Aug 21, 2018 at 05:16:11PM +0100, Brian Starkey wrote:
>>> > > There's a number of things which haven't previously been documented
>>> > > around the usage of format modifiers. Capture the current
>>> > > understanding in an overview comment and add it to the rst
>>> > > documentation.
>>> > >
>>> > > Ideally, the generated documentation would also include documentation
>>> > > of all of the #defines, but the kernel-doc system doesn't currently
>>> > > support kernel-doc comments on #define constants.
>>> >
>>> > Can you turn them into enums?  This seems to work ok:
>>> >
>>> > -/* color index */
>>> > -#define DRM_FORMAT_C8          fourcc_code('C', '8', ' ', ' ') /* [7:0] C */
>>> > -
>>> > -/* 8 bpp Red */
>>> > -#define DRM_FORMAT_R8          fourcc_code('R', '8', ' ', ' ') /* [7:0] R */
>>> > +enum {
>>> > +       /* color index */
>>> > +       DRM_FORMAT_C8   = fourcc_code('C', '8', ' ', ' '), /* [7:0] C */
>>> > +       /* 8 bpp Red */
>>> > +       DRM_FORMAT_R8   = fourcc_code('R', '8', ' ', ' '), /* [7:0] R */
>>> > +};
>>> >
>>> > but I appreciate this is user API and maybe there's some code out there
>>> > that does #ifndef DRM_FORMAT_C8 ...
>>>
>>> Thanks for the suggestion, Daniel did mention the same. However,
>>> unfortunately I don't think we can safely change the UAPI header in
>>> this manner.
>>
>> You could get the best of both worlds by doing both:
>>
>>   enum {
>>     foo = fourcc(...),
>>     bar = fourcc(...),
>>   }
>>   #define foo foo
>>   #define bar bar
>>
>> It would mean a bit more code though, but that way these would now be
>> enums (with all the advantages of enums vs plain literals) and still
>> pass #ifdef checks :)
>>
>> (BTW, on the "maybe there's some code that does #ifdef": I can tell you
>> there is indeed, having written this myself for an out-of-tree driver
>> for customer-modified kernels that may contain additional formats)
>
>Looks reasonable. I'd even put the #define right within each enum line
>(as a reminder so people don't forget to add them. Would happily ack a
>patch to mass-convert, if that ups the odds of good kerneldoc for all
>this.
>
>enum also should support the inline style of kerneldoc (otherwise I
>guess we'd need to fix that first, or it just makes no sense at all).
>-Daniel

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...

Cheers,
-Brian

>-- 
>Daniel Vetter
>Software Engineer, Intel Corporation
>+41 (0) 79 365 57 48 - http://blog.ffwll.ch