linux-kernel - Re: __LITTLE_ENDIAN vs. __LITTLE_ENDIAN

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Message-ID: <jed4vtnxa5.fsf@sykes.suse.de>
Date:	Fri, 05 Oct 2007 22:04:18 +0200
From:	Andreas Schwab <schwab@...e.de>
To:	Timur Tabi <timur@...escale.com>
Cc:	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'm writing a driver that talks to hardware that has a shift register.
> The register can be shifted either left or right, so all the bits
> obviously have to be in order, but it can be either order.

Bit addressing is strictly internal to the cpu, the smallest unit that
the cpu can address externally is a byte.  The only place where bit
order matters on the C level is in a bitfield that is overlayed over a
block of memory, but this is not visible outside the cpu.

> What I want to do is to have the driver detect when byte-endianness
> doesn't match bit-endianness when it writes the the word to a
> memory-mapped device.

The bit mapping on your device is strictly internal to the device and
has nothing to do with bit order on the C level.

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@...e.de
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
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