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

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Date:	Wed, 28 May 2008 08:17:16 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Miller <davem@...emloft.net>, linux-arch@...r.kernel.org,
	scottwood@...escale.com, 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


On Tue, 2008-05-27 at 15:42 -0600, Matthew Wilcox wrote:
> On Wed, May 28, 2008 at 07:38:55AM +1000, Benjamin Herrenschmidt wrote:
> > A problem with __raw_ though is that they -also- don't do byteswap,
> > which is a pain in the neck as people use them for either one reason
> > (relaxed ordering) or the other (no byteswap) without always knowing the
> > consequences of doing so...
> 
> That's why there's __readl() which does byteswap, but doesn't do
> ordering ...

Ah, that one is news to me. I don't think we ever had it on powerpc :-)

> > I'm happy to say that __raw is purely about ordering and make them
> > byteswap on powerpc tho (ie, make them little endian like the non-raw
> > counterpart).
> 
> That would break a lot of drivers.

How many actually use __raw_ * ?

> > Some archs started providing writel_be etc... I added those to powerpc a
> > little while ago, and I tend to prefer that approach for the byteswap
> > issue.

> Those are for people who use big endian chips on little endian
> architectures.

Why limit them to LE architecture ? There is nothing fundamentally
speicifc to LE architectures here, and it's wrong to provide accessors
on some archs and not others. The endianness is a property of the device
registers. Current writel/readl are basically writel_le/readl_le. It
thus makes sense to have the opposite, ie, readl_be/writel_be, which
thus byteswaps on LE platforms and not on BE platforms, which is what I
provided on powerpc a while ago.

Ben.


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