Re: [PATCH 02/14] can: rx-fifo: Increase MB size limit from 32 to 64

[Marc Kleine-Budde <mkl@pengutronix.de>]

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On 11/06/2014 05:20 PM, David Jander wrote:
>>> Btw, there is a subtle bug in the line you linked to:
>>>
>>>> #define GENMASK_ULL(h, l)       (((U64_C(1) << ((h) - (l) + 1)) - 1) <<
>>>> (l))
>>>
>>> The _ULL in the name of the macro seems to hint to "unsigned long long",
>>> while the implementation seems to assume that it means U64_C. This is
>>> obviously wrong, although for most platforms that are currently capable of
>>> running Linux, those are probably equivalent...
>>
>> #define U64_C(x) x ## ULL
> 
> This line:
> 
> http://lxr.free-electrons.com/source/include/asm-generic/int-ll64.h#L4
> 
> ...seems to indicate, that it may not always be the case :-)
> AFAIK, C99 specifies that "long long" should at _least_ have 64 bit length,
> but says nothing about when it may be larger.
> OTOH, I have yet to find a machine/compiler combination that defines "long
> long" as more than 64bit... at least not running Linux.
> A correct definition should be trivial though:
> 
> #define GENMASK_ULL(h, l)       (((1ULL << ((h) - (l) + 1)) - 1) << (l))

That's what it does:

$ cat << EOF | gcc -xc -E -
#define U64_C(x) x ## ULL
#define GENMASK_ULL(h, l)   (((U64_C(1) << ((h) - (l) + 1)) - 1) << (l))
GENMASK_ULL(foo, bar)
EOF

Results in:
(((1ULL << ((foo) - (bar) + 1)) - 1) << (bar))

Marc

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