Re: [PATCH treewide v2 1/3] bitfield: Add non-constant field_{prep,get}() helpers

[Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>]

Hi David,

On Fri, 31 Jan 2025 at 20:03, David Laight <david.laight.linux@xxxxxxxxx> wrote:
> On Fri, 31 Jan 2025 14:46:51 +0100
> Geert Uytterhoeven <geert+renesas@xxxxxxxxx> 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.
> ...
> > diff --git a/include/linux/bitfield.h b/include/linux/bitfield.h
> > index 63928f1732230700..c62324a9fcc81241 100644
> > --- a/include/linux/bitfield.h
> > +++ b/include/linux/bitfield.h
> > @@ -203,4 +203,38 @@ __MAKE_OP(64)
> >  #undef __MAKE_OP
> >  #undef ____MAKE_OP
> >
> > +/**
> > + * field_prep() - prepare a bitfield element
> > + * @_mask: shifted mask defining the field's length and position
> > + * @_val:  value to put in the field
> > + *
> > + * field_prep() masks and shifts up the value.  The result should be
> > + * combined with other fields of the bitfield using logical OR.
> > + * Unlike FIELD_PREP(), @_mask is not limited to a compile-time constant.
> > + */
> > +#define field_prep(_mask, _val)                                              \
>
> You don't need an _ prefix on the 'parameters' - it doesn't gain anything.

I just followed the style of all other macros in this file.
I can add a new patch converting the existing macros, though...

>
> > +     ({                                                              \
> > +             typeof(_mask) __mask = (_mask);                         \
>
> Use: __auto_type __mask = (_mask);

Likewise ;-)

> > +             unsigned int __shift = sizeof(_mask) <= 4 ?             \
> > +                                    __ffs(__mask) : __ffs64(__mask); \
> > +             (((typeof(_mask))(_val) << __shift) & (__mask));        \
>
> There are a lot of () in that line, perhaps:
>
>                 __auto_type(__mask) = (_mask);
>                 typeof (__mask) __val = (_val);
>                 unsigned int __shift = ...;
>
>                 (__val << __shift) & __mask;
>
> Note the typeof (__mask) - avoids line-length 'bloat' when the arguments are non-trivial.

OK, thanks!

Gr{oetje,eeting}s,

                        Geert

--
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