Re: [PATCH] [POWERPC] Improve (in|out)_beXX() asm code

Scott Wood
2008-05-20T22:45:40Z

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