linux-kernel - Re: [PATCH] [POWERPC] Improve (in|out)

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Date:	Tue, 20 May 2008 15:39:47 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	scottwood@...escale.com
Cc:	alan@...rguk.ukuu.org.uk, tpiepho@...escale.com,
	linux-kernel@...r.kernel.org, linuxppc-dev@...abs.org
Subject: Re: [PATCH] [POWERPC] Improve (in|out)_beXX() asm code

From: Scott Wood <scottwood@...escale.com>
Date: Tue, 20 May 2008 17:35:56 -0500

> Alan Cox wrote:
> >> It looks like we rely on -fno-strict-aliasing to prevent reordering 
> >> ordinary memory accesses (such as to DMA descriptors) past the I/O 
> > 
> > DMA descriptors in main memory are dependant on cache behaviour anyway
> > and the dma_* operators should be the ones enforcing the needed behaviour.
> 
> What about memory obtained from dma_alloc_coherent()?  We still need a 
> sync and a compiler barrier.  The current I/O accessors have the former, 
> but not the latter.

The __volatile__ in the asm construct disallows movement of the
inline asm relative to statements surrounding it.

The only reason barrier() in kernel.h needs a memory clobber is
because of a bug in ancient versions of gcc.  In fact, I think
that memory clobber might even be removable.

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