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

[Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>]

Hu Yury,

On Mon, 3 Feb 2025 at 17:48, Yury Norov <yury.norov@xxxxxxxxx> wrote:
> On Tue, Feb 04, 2025 at 12:41:55AM +0900, Vincent Mailhol wrote:
> > On 03/02/2025 at 22:59, Geert Uytterhoeven wrote:
> > > On Mon, 3 Feb 2025 at 14:37, Vincent Mailhol <mailhol.vincent@xxxxxxxxxx> wrote:
> > >> On 03/02/2025 at 16:44, Johannes Berg wrote:
> > >>> On Sun, 2025-02-02 at 12:53 -0500, Yury Norov wrote:
> > >>>>> Instead of creating another variant for
> > >>>>> non-constant bitfields, wouldn't it be better to make the existing macro
> > >>>>> accept both?
> > >>>>
> > >>>> Yes, it would definitely be better IMO.
> > >>>
> > >>> On the flip side, there have been discussions in the past (though I
> > >>> think not all, if any, on the list(s)) about the argument order. Since
> > >>> the value is typically not a constant, requiring the mask to be a
> > >>> constant has ensured that the argument order isn't as easily mixed up as
> > >>> otherwise.
> > >>
> > >> If this is a concern, then it can be checked with:
> > >>
> > >>   BUILD_BUG_ON_MSG(!__builtin_constant_p(_mask) &&
> > >>                    __builtin_constant_p(_val),
> > >>                    _pfx "mask is not constant");
> > >>
> > >> It means that we forbid FIELD_PREP(non_const_mask, const_val) but allow
> > >> any other combination.
> > >
> > > Even that case looks valid to me. Actually there is already such a user
> > > in drivers/iio/temperature/mlx90614.c:
> > >
> > >     ret |= field_prep(chip_info->fir_config_mask, MLX90614_CONST_FIR);
> > >
> > > So if you want enhanced safety, having both the safer/const upper-case
> > > variants and the less-safe/non-const lower-case variants makes sense.
>
> I agree with that. I just don't want the same shift-and operation to be
> opencoded again and again.
>
> What I actually meant is that I'm OK with whatever number of field_prep()
> macro flavors, if we make sure that they don't duplicate each other. So
> for me, something like this would be the best solution:
>
>  #define field_prep(mask, val) \
>        (((typeof(_mask))(_val) << __bf_shf(_mask)) & (_mask))
>
>  #define FIELD_PREP(mask, val)                                         \
>          (                                                             \
>                  FIELD_PREP_INPUT_CHECK(_mask, _val,);                 \
>                  field_prep(mask, val);                                \
>          )
>
> #define FIELD_PREP_CONST(_mask, _val)                                  \
>         (                                                              \
>                 FIELD_PREP_CONST_INPUT_CHECK(mask, val);
>                 FIELD_PREP(mask, val); // or field_prep()
>         )
>
> We have a similar macro GENMASK() in linux/bits.h. It is implemented
> like this:
>
>  #define GENMASK_INPUT_CHECK(h, l) BUILD_BUG_ON_ZERO(const_true((l) > (h)))
>  #define GENMASK(h, l) \
>          (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l))
>
> And it works just well. Can we end up with a similar approach here?

Note that there already exists a FIELD_PREP_CONST() macro, which is
intended for struct member initialization.

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