Re: iomapping a big endian area

[Russell King <rmk+lkml@arm.linux.org.uk>]

On Tue, Apr 05, 2005 at 09:05:15AM -0500, James Bottomley wrote:
> On Tue, 2005-04-05 at 08:42 +0100, Russell King wrote:
> > Not so.  There are two different styles of big endian.  (Lets just face
> > it, BE is fucked in the head anyway...)
> > 
> > physical bus:	31...24	23...16	15...8	7...0
> > 
> > BE version 1 (word invariant)
> >   byte access	byte 0	byte 1	byte 2	byte 3
> >   word access	31-24	23-16	15-8	7-0
> > 
> > BE version 2 (byte invariant)
> >   byte access	byte 3	byte 2	byte 1	byte 0
> >   word access	7-0	15-8	23-16	31-24
> 
> These are just representations of the same thing.  However, I did
> deliberately elect not to try to solve this problem in the accessors.  I
> know all about the register relayout, because 53c700 has to do that on
> parisc.

They aren't.  On some of our platforms, we have to exclusive-or the address
for byte accesses with 3 to convert to the right endian-ness.

Sure, from the point of view of which byte each byte of a word represents,
it's true that they're indentical.  But as far as the hardware is concerned,
they're definitely different.

See the Intel IXP platforms for an example.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:  2.6 Serial core