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Re: Question on FIELD_PREP() for static array




On Wed, 2018-10-10 at 18:16 +0100, John Garry wrote:
> (to: get_maintainers -f include/linux/bitfield.h)
> 
> Hi,
> 
> I would like to use FIELD_PREP() macro for assigning a static array, 
> like this:
> function()
> {
> 	static u32 val[2] = {FIELD_PREP(GENMASK_ULL(10, 0), 5), 0};
> 
> }
> 
> However the compiler complains of non-const expression:
> ./include/linux/bitfield.h:88:2: error: initializer element is not constant
>     ({        \
> 
> Specifically it doesn't like the __BF_FIELD_CHECK() in FIELD_PREP().
> 
> Any ideas on compiler trickery we could do with the FIELD_PREP() 
> definition to avoid this issue (i.e. enforce the check but only use the 
> constant value)?

Perhaps __bf_shf should not use __builtin_ffsll.

As mask is known constant, maybe expand the test
in-place with some switch/case.