Re: [Qemu-devel] [Qemu-ppc] [PATCH 02/31] dt: add helpers for 2, 3 and 4 cell adds

[Alexander Graf]

On 07.06.2012, at 14:13, David Gibson wrote:

> On Thu, Jun 07, 2012 at 01:27:56PM +0200, Alexander Graf wrote:
>> 
>> On 07.06.2012, at 01:45, David Gibson wrote:
>> 
>>> [snip]
>>>>> You mean internally? Yeah, probably. Externally? The point of these
>>>>> helpers is to make the code look less cluttered. We can already pass in
>>>>> an array just fine, but C is quite annoying about generating those on
>>>>> the fly, while it's easy to pass in ints as parameters :)
>>>> 
>>>> Varargs?
>>> 
>>> Ugly and risky with standard C varargs (because an explicit length
>>> would be needed).  Could be done neatly with gcc macro varargs.
>> 
>> Could a combination of both like this work?
>> 
>> #include <stdio.h>
>> #include <stdarg.h>
>> 
>> #define __VA_NARG__(...) \
>>        (__VA_NARG_(_0, ## __VA_ARGS__, __RSEQ_N()) - 1)
>> #define __VA_NARG_(...) \
>>        __VA_ARG_N(__VA_ARGS__)
>> #define __VA_ARG_N( \
>>         _1, _2, _3, _4, _5, _6, _7, _8, _9,_10, \
>>        _11,_12,_13,_14,_15,_16,_17,_18,_19,_20, \
>>        _21,_22,_23,_24,_25,_26,_27,_28,_29,_30, \
>>        _31,_32,_33,_34,_35,_36,_37,_38,_39,_40, \
>>        _41,_42,_43,_44,_45,_46,_47,_48,_49,_50, \
>>        _51,_52,_53,_54,_55,_56,_57,_58,_59,_60, \
>>        _61,_62,_63,N,...) N
>> #define __RSEQ_N() \
>>        63, 62, 61, 60,                         \
>>        59, 58, 57, 56, 55, 54, 53, 52, 51, 50, \
>>        49, 48, 47, 46, 45, 44, 43, 42, 41, 40, \
>>        39, 38, 37, 36, 35, 34, 33, 32, 31, 30, \
>>        29, 28, 27, 26, 25, 24, 23, 22, 21, 20, \
>>        19, 18, 17, 16, 15, 14, 13, 12, 11, 10, \
>>         9,  8,  7,  6,  5,  4,  3,  2,  1,  0
>> 
>> #define PRINT_ELEMS(fdt, ...) print_elems(fdt, __VA_NARG__(__VA_ARGS__), 
>> __VA_ARGS__)
> 
> Um.. that might work, but it's ludicrously complicated.  If we're
> prepared to use the gcc statement expression extension and we're just
> going to abort on errors like findnode_nofail, it can be done much
> more easily using c99 variadic macros:
> 
>       #define setprop_ints(fdt, path, prop, ...) \
>               do { \
>                       uint32_t tmp[] = {__VA_ARGS__}; \
> 
>                       if (fdt_setprop(findnode_nofail(fdt, path), prop, \
>                                  tmp, sizeof(tmp)) != 0) { \
>                                 /* error message */ \
>                                 abort(); \
>                       } \
>               } while (0)

Hrm. But here we'd be overloading the name space, no? If anyone passes in 
tmp[3] as parameter to setprop_ints, it would conflict with the internal 
variable tmp, right?


Alex