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

[Yury Norov <yury.norov@xxxxxxxxx>]

On Fri, Feb 14, 2025 at 12:03:16PM +0100, Geert Uytterhoeven wrote:
> 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.

Hi Geert,

That was my suggestion. Now that we're going to have many flavors
of the same macro, can we subordinate them to each other?