Messages in this thread | | | Date | Wed, 22 Aug 2018 16:57:33 +0100 | From | Brian Starkey <> | Subject | Re: [PATCH] drm/fourcc: Add DOC: overview comment |
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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@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
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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