Re: Confuse with big endian bitwise field

["Li YanBo" <dreamfly281@gmail.com>]

On 6/28/07, Arjan van de Ven <arjan@infradead.org> wrote:
> On Tue, 2007-06-26 at 18:20 +0800, Li YanBo wrote:
> > hi all,
> >
> > I am try  to define a hardware specific struct, it is like below:
> >
> > struct xxx {
> >         __be32 pdu_cnt:6;
> >         __be32 y:3;
> >         __be32 wep_key:2;
> >         __be32 uses_wep_key:1;
> >         __be32 keep_alive:1;
> >         __be32 buff_tail_addr:19;
> >
> >         __be32 cts_11g:1;
> >         __be32 rts_11g:1;
> >         __be32 x:2;
> >         __be32 frag_size:12;
> >         __be32 payload_len:12;
> >         __be32 frag_num:4;
> > }
>
> this isn't safe if you want to mimic hardware layout; the order of the
> bits in the struct is different for little endian and big endian
> machines...
>

Yes, I realize it now, and maybe shifting bits is a more portability solution.

> Kristof wrote:
>You'll need to use cpu_to_be32, to make sure the values you assign
arebig endian too. >You'll need to do this for every field larger than
8 bits (as endianness is not an issue if >there's only one byte).

I have done many tests and it seemed by using cpu_to_be32 to the field
larger than 8 bits is not a correct solution. it will generate wrong
data too. Anywhere the switch between Be to Le or from Le to Be is a
complex thing, because it is not just the order of bytes but also the
order of bits, I found a good article to describe this problem
"http://www.linuxjournal.com/article/6788"

Thank for all your suggestion.

BR

LiYanBo