Re: include/linux/build_bug.h:78:41: error: static assertion failed: "sizeof(struct rtw8814a_efuse) == 512"

[Geert Uytterhoeven]

Hi Ping-Ke,

CC Arnd

On Mon, 13 Apr 2026 at 02:48, Ping-Ke Shih <pkshih@xxxxxxxxxxx> wrote:
> > On Fri, 10 Apr 2026 at 14:15, Ping-Ke Shih <pkshih@xxxxxxxxxxx> wrote:
> > > Bitterblue Smith <rtl8821cerfe2@xxxxxxxxx> wrote:
> > > > On 10/04/2026 12:25, kernel test robot wrote:
> > > > > tree:    https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> > > > > head:   9a9c8ce300cd3859cc87b408ef552cd697cc2ab7
> > > > > commit: deb3ddeb18652118956fb581a39ac299e1ee5623 wifi: rtw88: Enable the new RTL8814AE/RTL8814AU
> > drivers
> > > > > date:   1 year, 1 month ago
> > > > > config: arm-randconfig-001-20260410
> > (https://download.01.org/0day-ci/archive/20260410/202604101721.fsv5XmCX-lkp@xxxxxxxxx/config)
> > > > > compiler: arm-linux-gnueabi-gcc (GCC) 8.5.0
> > > > > reproduce (this is a W=1 build):
> > (https://download.01.org/0day-ci/archive/20260410/202604101721.fsv5XmCX-lkp@xxxxxxxxx/reproduce)
> > > > >
> > > > > If you fix the issue in a separate patch/commit (i.e. not just a new version of
> > > > > the same patch/commit), kindly add following tags
> > > > > | Fixes: deb3ddeb1865 ("wifi: rtw88: Enable the new RTL8814AE/RTL8814AU drivers")
> > > > > | Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > > > > | Closes:  https://lore.kernel.org/oe-kbuild-all/202604101721.fsv5XmCX-lkp@xxxxxxxxx/
> > > > >
> > > > > All errors (new ones prefixed by >>):
> > > > >
> > > > >    In file included from include/linux/bitfield.h:10,
> > > > >                     from include/linux/fortify-string.h:5,
> > > > >                     from include/linux/string.h:389,
> > > > >                     from include/linux/uuid.h:11,
> > > > >                     from include/linux/mod_devicetable.h:14,
> > > > >                     from include/linux/usb.h:5,
> > > > >                     from drivers/net/wireless/realtek/rtw88/rtw8814a.c:5:
> > > > >>> include/linux/build_bug.h:78:41: error: static assertion failed: "sizeof(struct rtw8814a_efuse)
> > == 512"
> > > >
> > > > struct rtw_2g_txpwr_idx is missing __packed. Could that be the reason
> > > > for the error? I don't see what else could be causing it.
> > > >
> > >
> > > I also found that flaw, but it isn't cause. I download cross compiler
> > > to build arm on my x86, and address the cause is:
> > >
> > > diff --git a/drivers/net/wireless/realtek/rtw88/rtw8814a.h
> > b/drivers/net/wireless/realtek/rtw88/rtw8814a.h
> > > index c57c7c8f915e..7f2260117487 100644
> > > --- a/drivers/net/wireless/realtek/rtw88/rtw8814a.h
> > > +++ b/drivers/net/wireless/realtek/rtw88/rtw8814a.h
> > > @@ -51,10 +51,15 @@ struct rtw8814a_efuse {
> > >         union {
> > >                 struct rtw8814au_efuse u;
> > >                 struct rtw8814ae_efuse e;
> > > -       };
> > > +       } __packed;
> >
> > How can that make a difference?
> > Both rtw8814au_efuse and rtw8814ae_efuse contain just (the same number
> > of) bytes, so __packed on the union should not make any difference?
> > Moreover, rtw8814au_efuse and rtw8814ae_efuse are already tagged with
> > __packed, which should also not make any difference.  Someone's been
> > sprinkling too many __packed all over the place?
>
> I have similar thought as yours, but arm-linux-gnueabi-gcc compiler seems
> have special treatment with union. For this case, without __packed
> the offset of followed field res5 will become 0xe0.

Hmm, I can confirm the original issue, and that adding __packed does
have an impact (it reduces the size of rtw8814a_efuse from 522 to 520),
but the actual size is still wrong (should be 512).

drivers/net/wireless/realtek/rtw88/rtw88xxa.h has a similar issue, but
adding __packed does not make a difference there.

Removing the __packed from struct rtw8814au_efuse increases its size
from 14 to 16 bytes.  Looks like that ABI pads every structure to a
multiple of 4 bytes?

>
> When investing this error, I also found another waning [1].
>
> I don't know why the compiler treats the union so special, and I fix
> them by experimental results.
>
> [1] https://lore.kernel.org/linux-wireless/20260411072509.1556635-1-pkshih@xxxxxxxxxxx/T/#u
>
> >
> > >         u8 res5[0x122];                 /* 0xde */
> > >  } __packed;
> > >
> > > I will send a patch soon.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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