linux-kernel - Re: MMIO and gcc re-ordering issue

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Message-ID: <20080603094646.0dbf2911@hskinnemo-gx745.norway.atmel.com>
Date:	Tue, 3 Jun 2008 09:46:46 +0200
From:	Haavard Skinnemoen <haavard.skinnemoen@...el.com>
To:	Scott Wood <scottwood@...escale.com>
Cc:	Geert Uytterhoeven <geert@...ux-m68k.org>,
	benh@...nel.crashing.org, Matthew Wilcox <matthew@....cx>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Miller <davem@...emloft.net>, linux-arch@...r.kernel.org,
	linuxppc-dev@...abs.org, alan@...rguk.ukuu.org.uk,
	linux-kernel@...r.kernel.org, tpiepho@...escale.com
Subject: Re: MMIO and gcc re-ordering issue

Scott Wood <scottwood@...escale.com> wrote:
> On Mon, Jun 02, 2008 at 10:11:02AM +0200, Haavard Skinnemoen wrote:
> > Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
> > > On Fri, 30 May 2008, Haavard Skinnemoen wrote:
> > > > Maybe we need another interface that does not do byteswapping but
> > > > provides stronger ordering guarantees?
> > > 
> > > The byte swapping depends on the device/bus.
> > 
> > Of course. But isn't it reasonable to assume that a device integrated
> > on the same silicon as the CPU is connected to a somewhat sane bus
> > which doesn't require any byte swapping?
> 
> No, unfortunately. :-(

Ok, I guess I was being naive.

> See the end of drivers/dma/fsldma.h.  Likewise with Freescale's PCI host
> bridges; for some reason the bus itself being little endian led to the host
> bridge control registers also being little endian.

Right. But still, isn't it better to handle it on a case-by-case basis
in the drivers? In some cases, it's best to explicitly use a certain
byte order, in others it's best to use whatever is native to the CPU.

Haavard
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