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
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