Hi Johannes,
On Mon, Nov 22, 2021 at 5:33 PM Johannes Berg johannes@sipsolutions.net wrote:
On Mon, 2021-11-22 at 16:53 +0100, 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.
I'm not sure it's really a good idea to add a third API here?
We have the upper-case (constant) versions, and already {u32,...}_get_bits()/etc.
These don't work for non-const masks.
Also, you're using __ffs(), which doesn't work for 64-bit on 32-bit architectures (afaict), so that seems a bit awkward.
That's a valid comment. Can be fixed by using a wrapper macro that checks if typeof(mask) == u64, and uses an __ffs64() version when needed.
Maybe we can make {u32,...}_get_bits() be doing compile-time only checks if it is indeed a constant? The __field_overflow() usage is already only done if __builtin_constant_p(v), so I guess we can do the same with __bad_mask()?
Are all compilers smart enough to replace the division by field_multiplier(field) by a shift?
Gr{oetje,eeting}s,
Geert
-- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
In personal conversations with technical people, I call myself a hacker. But when I'm talking to journalists I just say "programmer" or something like that. -- Linus Torvalds