Re: iomapping a big endian area

["David S. Miller" <davem@davemloft.net>]

On Sun, 3 Apr 2005 02:37:57 +0100
Matthew Wilcox <matthew@wil.cx> wrote:

> My thought on this is that we should encode the endianness of the
> registers in the ioremap cookie.  Some architectures (sparc, I think?) can
> do this in their PTEs.  The rest of us can do it in our ioread/writeN
> methods.  I've planned for this in the parisc iomap implementation but
> not actually implemented it.

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.

Be careful though.  Endianness can be dealt with on a hardware
level.  Consider a byte access to a 32-bit word sized config space
datum, the PCI controller on a big-endian system will likely byte-twist
the data lanes in order for this to work properly.

It's a subtle issue, and it's explained pretty well in some of the
UltraSPARC PCI controller docs at:

	http://www.sun.com/processors/documentation.html

In particular, "U2P UPA to PCI User's Manual", chapter 10
"Little-Endian Support", has some informative diagrams.

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