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