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

David Laight
2025-12-13T22:14:24Z

On Sat, 13 Dec 2025 10:01:36 +0000
David Laight <david.laight.linux@gmail.com> wrote:

> 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.

I've been trying to look up how _Generic() should behave for bitfields.
Basically it doesn't seem to be specified in the C standard.
Pretty much all you can do is force an integer promotion - and that only
works if the size is <= 32.
(I really hope there aren't any bigger bitfields lurking...)
But FIELD_GET() wants to check that 'reg' is 'big enough' to hold 'mask'.
An integer promotion breaks that for char/short.

I did try erroring on statically_true(mask > reg), but that throws up
a lot of false positives (even for non-constant 'reg').
It really is amazing that you get FIELD_GET(GENMASK(7, 0), val) instead
of 'val & 0xff' (and I don't mean code that want the low 8 bits of some
device register that is made up of lots of fields).

FIELD_PREP() does support bitfields (for 'val'), they get an integer
promotion applied.

There seems to be exactly one instance of 'reg' being a bitfield,
bloating all the other expansions for a trivially changeable line of
code is just stupid.

David

>
> 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
>