Hi Jakub,
On Tue, Nov 23, 2021 at 2:17 AM Jakub Kicinski kuba@kernel.org wrote:
On Mon, 22 Nov 2021 17:32:43 +0100 Johannes Berg 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?
+1
Yeah, a smaller API is better.
We have the upper-case (constant) versions, and already {u32,...}_get_bits()/etc.
TBH, I don't like the *_get_bits() API: in general, u32_get_bits() does the same as FIELD_GET(), but the order of the parameters is different? (*_replace_bits() seems to be useful, though)
That's why I picked field_{get,prep}().
Also, you're using __ffs(), which doesn't work for 64-bit on 32-bit architectures (afaict), so that seems a bit awkward.
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()?
Either that or add decomposition macros. Are compilers still really bad at passing small structs by value?
Sorry, I don't get what you mean by adding decomposition macros. Can you please elaborate? Thanks!
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