Hi Jakub,

On Tue, Nov 23, 2021 at 2:17 AM Jakub Kicinski [off-list ref] wrote:

On Mon, 22 Nov 2021 17:32:43 +0100 Johannes Berg wrote:

quoted

On Mon, 2021-11-22 at 16:53 +0100, Geert Uytterhoeven wrote:

quoted

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.

quoted

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}().

quoted

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