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

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Date:	Wed, 28 May 2008 08:19:39 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	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 14:55 -0700, Linus Torvalds wrote:
> 
> On Wed, 28 May 2008, Benjamin Herrenschmidt wrote:
> > 
> > A problem with __raw_ though is that they -also- don't do byteswap,
> 
> Well, that's why there is __readl() and __raw_readl(), no?

As I replied to somebody else, __readl() is news to me :-) we dont' have
those on powerpc.

> Neither does ordering, and __raw_readl() doesn't do byte-swap.

But I can add them :-)

> Of course, I'm not going to guarantee every architecture even has all 
> those versions, nor am I going to guarantee they all work as advertised :)
> 
> For x86, they have historially all been 100% identical. With the inline 
> asm patch I posted, the "__" version (whether "raw" or not) lack the 
> "memory" barrier, so they allow a *little* bit more re-ordering.
> 
> (They won't be re-ordered wrt spinlocks etc, unless gcc starts reordering 
> volatile asm's against each other, which would be a bug).
> 
> In practice, I doubt it matters. Whatever small compiler re-ordering it 
> might affect won't have any real performance impack one way or the other, 
> I think.

I prefer explicit endian. Always. Thus I prefer introducing _be variants
(we already have those on powerpc and iomap has it's own _be versions
too) so we should probably generalize _be.

Ben.


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