Re: [PATCH v4 3/8] bits: introduce fixed-type genmasks

Andy Shevchenko
2025-03-06T09:28:28Z

On Thu, Mar 06, 2025 at 06:22:33PM +0900, Vincent Mailhol wrote:
> On 06/03/2025 at 04:45, Andy Shevchenko wrote:
> >>> But GENMASK_U128() becomes a special case now.
> >>> The 128-bit GENMASK is unsued, but it's exported in uapi. Is there any
> >>> simple way to end up with a common implementation for all fixed-type
> >>> GENMASKs?
> >>
> >> What bothers me is that the 128 bit types are not something available on
> >> all architectures, c.f. the CONFIG_ARCH_SUPPORTS_INT128. So, I would
> >> need a U128() equivalent to the ULL() but which does not break on
> >> architectures which do not support 128 bits integers.
> >>
> >> This is where I am stuck. If someone can guide me on how to write a
> >> robust U128() macro, then I think the common implementation could be
> >> feasible.
> >
> > I think we may leave that U128 stuff alone for now.
>
> I found the solution! The trick is to use type_max() from overflow.h.
>
> With this, GENMASK_TYPE() becomes:
>
> #define GENMASK_TYPE(t, h, l) \
> ((t)(GENMASK_INPUT_CHECK(h, l) + \
> (type_max(t) << (l) & \
> type_max(t) >> (BITS_PER_TYPE(t) - 1 - (h)))))
>
> and works with all the GENMASK variants, including the U128 one! The
> unit tests under lib/test_bits.c are all green.
>
> Of course, this does *not* work in assembly. But as explained before,
> GENMASK_TYPE() is guarded by a #if !defined(__ASSEMBLY__), so all good!
>
> The question raised by Yury on whether or not we should keep
> __GENMASK_U128() in the uapi still remains. And in full honesty, I will
> not touch that one. This is not in the scope of this series.

I vote for not touching it right now independently on its destiny.

--
With Best Regards,
Andy Shevchenko