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

[Brian Starkey]

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@xxxxxxxxx> 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