Re: [PATCH 01/17] bitfield: Add non-constant field_{prep,get}() helpers

From: Geert Uytterhoeven
Date: Tue Nov 23 2021 - 03:36:51 EST

Hi Jakub,

On Tue, Nov 23, 2021 at 2:17 AM Jakub Kicinski <kuba@xxxxxxxxxx> 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?



Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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