netdev - Re: [PATCH 2/2] netdev: octeon_mgmt: Fix structure layout for little-endian.

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Message-ID: <20130627104048.GR7171@linux-mips.org>
Date:	Thu, 27 Jun 2013 12:40:48 +0200
From:	Ralf Baechle <ralf@...ux-mips.org>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	David Daney <ddaney.cavm@...il.com>, netdev@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>, linux-mips@...ux-mips.org,
	David Daney <david.daney@...ium.com>
Subject: Re: [PATCH 2/2] netdev: octeon_mgmt: Fix structure layout for
 little-endian.

On Thu, Jun 20, 2013 at 10:47:57AM +0100, David Laight wrote:

> > The C ABI reverses the bitfield fill order when compiled as
> > little-endian.
> 
> No - it is completely implementation defined.
> The general concensus is not to use bitfields if you
> care at all about the bit assignments.

FWIW, bitfields often alow things to be expressed more nicely.  Just the
endian-dependent definition suck, so I came up with this little hack
for arch/mips/include/uapi/asm/inst.h:

#ifdef __MIPSEB__
#define BITFIELD_FIELD(field, more)                                     \
        field;                                                          \
        more

#elif defined(__MIPSEL__)

#define BITFIELD_FIELD(field, more)                                     \
        more                                                            \
        field;
#endif

struct i_format {                       /* signed immediate format */
        BITFIELD_FIELD(unsigned int opcode : 6,
        BITFIELD_FIELD(unsigned int rs : 5,
        BITFIELD_FIELD(unsigned int rt : 5,
        BITFIELD_FIELD(signed int simmediate : 16,
        ;))))
};

  Ralf
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