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

[Joerg Roedel]

On Mon, Jun 29, 2009 at 10:54:51PM +0800, Ming Lei wrote:
> 2009/6/29 Joerg Roedel <joerg.roedel@xxxxxxx>:
> >>
> >> IMHO, even we do not call flush_write_buffer(), CPU can read correct
> >> data from the
> >> dma buffer since write buffer can't affect cache, right?
> >
> > flush_write_buffer is not about cache flushing. It is about read/write
> > reordering in the CPU. Think of it as a memory barrier. On most x86
> 
> You mean we may need a memory barrier between writing data from bouncing buffer
> to dma buffer and reading data from dma buffer, do you?

CONFIG_X86_OOSTORE (when set flush_write_buffers is a memory barrier and not a
nop on x86) is defined for WINCHIP. So it seems necessary on some chips.

	Joerg

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