On Sun, Feb 02, 2025 at 05:26:04PM +0900, Vincent Mailhol wrote:
> On 31/01/2025 at 22:46, Geert Uytterhoeven wrote:
> > The existing FIELD_{GET,PREP}() macros are limited to compile-time
> > constants. However, it is very common to prepare or extract bitfield
> > elements where the bitfield mask is not a compile-time constant.
>
> Why is it that the existing FIELD_{GET,PREP}() macros must be limited to
> compile time constants?
I guess, for historical reasons?
> Instead of creating another variant for
> non-constant bitfields, wouldn't it be better to make the existing macro
> accept both?
Yes, it would definitely be better IMO.
> As far as I can see, only __BUILD_BUG_ON_NOT_POWER_OF_2() and
> __BF_FIELD_CHECK() need to be adjusted. I am thinking of this:
>
> diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
> index 63928f173223..c6bedab862d1 100644
> --- a/include/linux/bitfield.h
> +++ b/include/linux/bitfield.h
> @@ -8,6 +8,7 @@
> #define _LINUX_BITFIELD_H
>
> #include <linux/build_bug.h>
> +#include <linux/compiler.h>
> #include <asm/byteorder.h>
>
> /*
> @@ -62,15 +63,13 @@
>
> #define __BF_FIELD_CHECK(_mask, _reg, _val, _pfx) \
> ({ \
> - BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask), \
> - _pfx "mask is not constant"); \
> - BUILD_BUG_ON_MSG((_mask) == 0, _pfx "mask is zero"); \
> - BUILD_BUG_ON_MSG(__builtin_constant_p(_val) ? \
> - ~((_mask) >> __bf_shf(_mask)) & \
> - (0 + (_val)) : 0, \
> + BUILD_BUG_ON_MSG(statically_true((_mask) == 0), \
> + _pfx "mask is zero"); \
> + BUILD_BUG_ON_MSG(statically_true(~((_mask) >>
This should be a const_true(), because statically_true() may be OK
with something like:
((runtime_var << 1) & 1 == 0)
I think it's your own patch that adds const_true(): 4f3d1be4c2f8a :)
Thanks,
Yury
