Re: [PATCH v2 02/16] thunderbolt: Don't pass a bitfield to FIELD_GET

David Laight
2025-12-13T10:01:39Z

On Fri, 12 Dec 2025 21:28:31 -0500
Yury Norov <yury.norov@gmail.com> wrote:

> On Fri, Dec 12, 2025 at 07:37:07PM +0000, david.laight.linux@gmail.com wrote:
> > From: David Laight <david.laight.linux@gmail.com>
> >
> > None of sizeof(), typeof() or __auto_type can be used with bitfields
> > which makes it difficult to assign a #define parameter to a local
> > without promoting char and short to int.
> >
> > Change:
> > u32 thunderbolt_version:8;
> > to the equivalent:
> > u8 thunderbolt_version;
> > (and the other three bytes of 'DWORD 4' to match).
> >
> > This is necessary so that FIELD_GET can use sizeof() to verify 'reg'.
> >
> > Signed-off-by: David Laight <david.laight.linux@gmail.com>
> > ---
> >
> > Changes for v2:
> > - Change structure definition instead of call to FIELD_GET().
> >
> > FIELD_GET currently uses _Generic() which behaves differently for
> > gcc and clang (I suspect both are wrong!).
> > gcc treats 'u32 foo:8' as 'u8', but will take the 'default' for other
> > widths (which will generate an error in FIED_GET().
> > clang treats 'u32 foo:n' as 'u32'.
>
> FIELD_GET() works just well with bitfields, and whatever you do breaks
> it. I pointed that in v1, but instead of fixing it, you do really well
> hiding the problem.

It doesn't, pass 'u32 foo:6' when using gcc.

David

>
> I see no reasons to hack a random victim because of your rework. So
> NAK for this.
>
> In v3, please add an explicit test to make sure that bitfields are not
> broken with new implementation.
>
> Thanks,
> Yury