[PATCH treewide v3 2/4] bitfield: Add non-constant field_{prep,get}() helpers

[Geert Uytterhoeven]

Hi Jakub,

On Fri, 14 Feb 2025 at 16:34, Jakub Kicinski <kuba at kernel.org> wrote:
> On Fri, 14 Feb 2025 14:55:51 +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.
> >
> > To avoid this limitation, the AT91 clock driver and several other
> > drivers already have their own non-const field_{prep,get}() macros.
> > Make them available for general use by consolidating them in
> > <linux/bitfield.h>, and improve them slightly:
> >   1. Avoid evaluating macro parameters more than once,
> >   2. Replace "ffs() - 1" by "__ffs()",
> >   3. Support 64-bit use on 32-bit architectures.
> >
> > This is deliberately not merged into the existing FIELD_{GET,PREP}()
> > macros, as people expressed the desire to keep stricter variants for
> > increased safety, or for performance critical paths.
>
> I really really think that people should just use the static inline
> helpers if the field is not constant. And we should do something like
> below so that people can actually find them.
>
> diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
> index 63928f173223..e02afcd7aeee 100644
> --- a/include/linux/bitfield.h
> +++ b/include/linux/bitfield.h
> @@ -156,6 +156,80 @@
>                 (typeof(_mask))(((_reg) & (_mask)) >> __bf_shf(_mask)); \
>         })
>
> +/**
> + * u32_encode_bits() - prepare a u32 bitfield element (non-const)
> + * @v: value to put in the field
> + * @field: shifted mask defining the field's length and position
> + *
> + * Equivalent of FIELD_PREP() for u32, field does not have to be constant.
> + *
> + * Note that the helper is available for other field widths (generated below).
> + */
> +static __always_inline __u32 u32_encode_bits(u32 v, u32 field)
> +{
> +       if (__builtin_constant_p(v) && (v & ~field_mask(field)))
> +               __field_overflow();
> +       return ((v & field_mask(field)) * field_multiplier(field));
> +}

Unfortunately gcc emits actual divisions or __*div*() calls, and
multiplications in the non-constant case.

Gr{oetje,eeting}s,

                        Geert


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