Re: iomapping a big endian area

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

On Sat, Apr 02, 2005 at 09:40:39PM -0600, James Bottomley wrote:
> On Sun, 2005-04-03 at 04:10 +0100, Matthew Wilcox wrote:
> > > SPARC64 can do it in the PTEs, but we just use raw physical
> > > addresses in our I/O accessors, and in those load/store instructions
> > > we can specify the endianness.
> > 
> > Ah right.  So you'd prefer an ioread8be() interface?
> 
> Actually, ioread8be is unnecessary, but I was planning to add
> ioread16/ioread32 and iowritexx be on be variants (equivalent to
> _raw_readw et al.)

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

Depending on this and how your devices are wired up to such a bus, you
may need to swap bytes in a word access, or munge the byte/half word
address itself.

And guess which architecture implements *both* of these...  Grumble.

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