linux-kernel - Re: __LITTLE_ENDIAN vs. __LITTLE_ENDIAN

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Date:	Wed, 10 Oct 2007 00:11:59 +0200
From:	Krzysztof Halasa <khc@...waw.pl>
To:	Timur Tabi <timur@...escale.com>
Cc:	Lennart Sorensen <lsorense@...lub.uwaterloo.ca>,
	Anton Altaparmakov <aia21@....ac.uk>,
	Jan Engelhardt <jengelh@...putergmbh.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: __LITTLE_ENDIAN vs. __LITTLE_ENDIAN_BITFIELD

Timur Tabi <timur@...escale.com> writes:

> I think when the PowerPC is running in little-endian mode, that might
> be the case.  It needs to be able to write a byte in big-endian mode,
> and then read that byte back in little-endian mode and have it be the
> same byte.

But this is exactly what excludes changing bit orders :-)

There are other CPU types which can work both LE and BE.
If you write 0x12345678 and switch endianness you get 0x78563412,
but the individual bits are still in the same (natural) order.

In the kernel, to convert from BE to LE or vice versa you need
something like swab32 (assuming 32-bit ints). It doesn't change
the order of the individual bits, it only swaps the (4 in this case)
bytes ("lanes" for hw swapping, PCI etc).

If you, for example, write an 8-bit integer in BE mode and want
it back in LE mode, you have to XOR the address with 0x3, but
the value already comes in the same natural bit order. Of course
swab32 will do as well.
-- 
Krzysztof Halasa
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