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Date:   Wed, 22 Aug 2018 15:59:24 +0100
From:   Eric Engestrom <eric.engestrom@intel.com>
To:     Brian Starkey <brian.starkey@arm.com>
Cc:     Matthew Wilcox <willy@infradead.org>,
        alexandru-cosmin.gheorghe@arm.com, corbet@lwn.net,
        airlied@linux.ie, daniel.vetter@ffwll.ch,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        dri-devel@lists.freedesktop.org, seanpaul@chromium.org,
        liviu.dudau@arm.com, ayan.halder@arm.com
Subject: Re: [PATCH] drm/fourcc: Add DOC: overview comment
Message-ID: <20180822145924.GA13763@intel.com>
References: <20180821161611.10424-1-brian.starkey@arm.com>
 <20180821162639.GA21697@bombadil.infradead.org>
 <20180821164416.GA11553@e107564-lin.cambridge.arm.com>
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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)

> 
> Cheers,
> -Brian
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