linux-kernel - Re: ioread32 endianess.

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Date:	Tue, 27 Feb 2007 17:36:22 +0300
From:	"Alexey Zaytsev" <alexey.zaytsev@...il.com>
To:	"Kyle McMartin" <kyle@...artin.ca>
Cc:	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: Re: ioread32 endianess.

On 2/27/07, Kyle McMartin <kyle@...artin.ca> wrote:
> On Tue, Feb 27, 2007 at 08:20:21AM -0500, Kyle McMartin wrote:
> > PCI is always little endian, unless it's not. In which case you're probably
PCI is always LE, but the host may be different. If you read some data
from a PCI device
on a LE host, the data would be OK, but on a BE host, you may need to
swap it. Right?

More fun comes when you are doing DMA...

> > dealing with a graphics card which likely has some kind of palindromic
> > register which you can read and write to set the endianness of the host
> > interface. Whoo. Run on sentence.
> >
>
> Perhaps we should have a Documentation/ entry for this...
>
> io(read|write){8,16,32} are the "pci iomap" functions (see
> asm-generic/iomap.h) they always byteswap so the value is little endian.
They do even if the target bus is not little endian, right?

>
> io(read|write){8,16,32}be are sister functions added to deal with big
> endian busses. They always byteswap so the value is in big endian.
>
> Both these previous functions can handle using a cookie based on an IO port
> range, or an MMIO region.
>
> (read|write){b,w,l} are the old style MMIO-mapped accessors. They also always
> byteswap so the value is in little endian. There is no big endian equivalent
> for the generic case.
>
> __raw_(read|write){b,w,l} are also old style accessors. They always operate
> in host endianness.
>
> The above are (AFAIK) the only functions guaranteed to exist for MMIO.
>
> Of course, most platforms either provide (in|out){b,w,l} or don't support
> Port IO as well, but MMIO is the really complicated case.
>
> In any event, <asm-generic/iomap.h> should shed a bit more light on using
> these.
>
> Cheers,
>         Kyle M.
>
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