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Message-ID: <4706A096.4000700@freescale.com>
Date: Fri, 05 Oct 2007 15:37:42 -0500
From: Timur Tabi <timur@...escale.com>
To: "linux-os (Dick Johnson)" <linux-os@...logic.com>
CC: Andreas Schwab <schwab@...e.de>,
Jan Engelhardt <jengelh@...putergmbh.de>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: __LITTLE_ENDIAN vs. __LITTLE_ENDIAN_BITFIELD
linux-os (Dick Johnson) wrote:
> It makes no sense because a bitfield is something having to
> do with a 'C' compiler and it must NEVER be used as a template
> to address hardware! 'C' gives no guarantee of the ordering
> within machine words. The only way you can access them is
> using 'C'. They don't have addresses like other objects
> (of course they do exist --somewhere). They are put into
> "storage units," according to the standard, and these
> storage units are otherwise undefined although you can
> align them (don't go there).
Well, if it doesn't make any sense why do we have __LITTLE_ENDIAN_BITFIELD and
__BIG_ENDIAN_BITFIELD? That is, why do we do this:
#if defined(__BIG_ENDIAN_BITFIELD)
__u8 reserved1 : 2;
__u8 ili : 1;
__u8 reserved2 : 1;
__u8 sense_key : 4;
#elif defined(__LITTLE_ENDIAN_BITFIELD)
__u8 sense_key : 4;
__u8 reserved2 : 1;
__u8 ili : 1;
__u8 reserved1 : 2;
#endif
when we can just do this:
#if defined(__BIG_ENDIAN)
__u8 reserved1 : 2;
__u8 ili : 1;
__u8 reserved2 : 1;
__u8 sense_key : 4;
#elif defined(__LITTLE_ENDIAN)
__u8 sense_key : 4;
__u8 reserved2 : 1;
__u8 ili : 1;
__u8 reserved1 : 2;
#endif
> If you want to call machine-control bits by name, just
> define them as hexadecimal numbers (unsigned ints) and,
> if your hardware is for both little/big endian, use
> a macro that resolves the issue between the number
> and the hardware.
That wasn't my intention. I was hoping that __LITTLE_ENDIAN_BITFIELD could be
used to test bit-endianness, but I guess it can't.
--
Timur Tabi
Linux Kernel Developer @ Freescale
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