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From:   Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed, 22 Aug 2018 17:11:55 +0200
Message-ID: <CAKMK7uHxPDfxJaA+1esSW8L2HAjMD6xgF0cMH-nkTjA=6S5+CA@mail.gmail.com>
Subject: Re: [PATCH] drm/fourcc: Add DOC: overview comment
To:     Eric Engestrom <eric.engestrom@intel.com>
Cc:     Brian Starkey <brian.starkey@arm.com>,
        Matthew Wilcox <willy@infradead.org>,
        Alexandru-Cosmin Gheorghe <alexandru-cosmin.gheorghe@arm.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Dave Airlie <airlied@linux.ie>,
        Linux Doc Mailing List <linux-doc@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        dri-devel <dri-devel@lists.freedesktop.org>,
        Sean Paul <seanpaul@chromium.org>,
        Liviu Dudau <liviu.dudau@arm.com>,
        Ayan Kumar Halder <ayan.halder@arm.com>
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On Wed, Aug 22, 2018 at 4:59 PM, Eric Engestrom
<eric.engestrom@intel.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
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch