linux-kernel - Re: [PATCH 01/17] bitfield: Add non-constant field

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Message-ID: <CAMuHMdUnBgFpqhgjf5AA0LH9MZOFALeC=YinZ4Tv_V+Y9hkRSg@mail.gmail.com>
Date:   Tue, 23 Nov 2021 09:30:14 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     Tony Lindgren <tony@...mide.com>,
        Russell King <linux@...linux.org.uk>,
        Rajendra Nayak <rnayak@...eaurora.org>,
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Subject: Re: [PATCH 01/17] bitfield: Add non-constant field_{prep,get}() helpers

Hi Johannes,

On Mon, Nov 22, 2021 at 5:33 PM Johannes Berg <johannes@...solutions.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@...ux-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