Re: [PATCH][RFC] asm-generic:remove calling flush_write_buffers()in dma_sync_*_for_cpu

[Joerg Roedel]

On Mon, Jun 29, 2009 at 06:45:15PM +0200, Arnd Bergmann wrote:
> On Monday 29 June 2009, Alan Cox wrote:
> > > Wouldn't it be better to put the flush_write_buffer in the specific
> > > operation (swiotlb_sync_*_for_*) rather than the multiplexer?
> > > 
> > > Maybe in that case, smp_wmb() would be more appropriate because
> > > it is defined on all architectures.
> > 
> > smp_wmb() is stronger and it would slow down x86 if we did that (we'd go
> > from no-op on a coherent platform to using mfence/lfence etc)
> > 
> Really? In my copy of system.h, I read
> 
> #ifdef CONFIG_SMP
> # ifdef CONFIG_X86_OOSTORE
> #  define smp_wmb()      wmb()
> # else
> #  define smp_wmb()      barrier()
> # endif
> #else
> # define smp_wmb()       barrier()
> #endif

With that definition an smp_wmb() would do the right job on x86. If thats also
true for other architectures using this generic header we can remove
flush_write_buffer().

> 
> That actually looks weaker than flush_write_buffer, as it would turn into
> a barrier() in case of !SMP or !X86_OOSTORE, and into an sfence instead of
> lock addl on all modern CPUs in case of SMP && X86_OOSTORE.

X86_OOSTORE is defined for (MWINCHIP3D || MWINCHIPC6) && MTRR. I am not sure if
these chips have sfence.
It would also help to know what OOSTORE exactly means in the context of that
chip.

	Joerg

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