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Date: Tue, 20 May 2008 17:43:58 -0500
From: Scott Wood <scottwood@freescale.com>
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To: David Miller <davem@davemloft.net>
Subject: Re: [PATCH] [POWERPC] Improve (in|out)_beXX() asm code
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David Miller wrote:
> From: Scott Wood <scottwood@freescale.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.

Current versions of GCC seem quite happy to move non-asm memory accesses 
around a volatile asm without a memory clobber; see the test Trent posted.

-Scott