Messages in this thread | | | From | Geert Uytterhoeven <> | Date | Tue, 23 Nov 2021 09:30:14 +0100 | Subject | Re: [PATCH 01/17] bitfield: Add non-constant field_{prep,get}() helpers |
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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
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