Re: [PATCH v2 0/3] Inline helpers into Rust without full LTO

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Arnd,

On Mon, 30 Mar 2026 at 14:14, Arnd Bergmann <arnd@arndb.de> wrote:
> On Mon, Mar 30, 2026, at 14:03, Geert Uytterhoeven wrote:
> > On Fri, 27 Mar 2026 at 10:02, Arnd Bergmann <arnd@arndb.de> wrote:
> >>
> >> echo 'struct { short a : 3; short b :15; short c :14; } x; int y = sizeof(x);' | m68k-linux-gcc -xc - -S -o-
> >>
> >> this produces '4' on m68k-linux-gcc, but '6' everywhere else. I originally
> >> thought this was related to this 2009 change in both compilers
> >
> > Oh, now I remember.  AFAIK (holding wood and a rabbit leg) we don't
> > have any bitfield members spanning multiple base type instances in
> > the kernel.
>
> There are certainly very few of those, but two example I found in
> UAPI are
>
> struct dvd_layer {
>         __u8 book_version       : 4;
>         __u8 book_type          : 4;
>
>         __u8 min_rate           : 4;
>         __u8 disc_size          : 4;
>
>         __u8 layer_type         : 4;
>         __u8 track_path         : 1;
>         __u8 nlayers            : 2;
>
>         __u8 track_density      : 4; // crosses u8 boundary
>         __u8 linear_density     : 4;
>         __u8 bca                : 1;
>         __u32 start_sector;
>         __u32 end_sector;
>         __u32 end_sector_l0;
> };
>
> struct usb_raw_ep_caps {
>         __u32   type_control    : 1;
>         __u32   type_iso        : 1;
>         __u32   type_bulk       : 1;
>         __u32   type_int        : 1;
>         __u32   dir_in          : 1;
>         __u32   dir_out         : 1;
>        // 2 bit padding on m68k, 26 bits elsewhere
> };

So we just need to make padding explicit? Sounds familiar ;-)

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

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