Messages in this thread | | | Date | Mon, 29 Jun 2009 16:45:36 +0200 | From | Joerg Roedel <> | Subject | Re: [PATCH][RFC] asm-generic:remove calling flush_write_buffers() in dma_sync_*_for_cpu |
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On Mon, Jun 29, 2009 at 09:51:34PM +0800, Ming Lei wrote: > 2009/6/29 Joerg Roedel <joerg.roedel@amd.com>: > > On Sun, Jun 28, 2009 at 03:34:35PM +0000, Arnd Bergmann wrote: > >> On Sunday 28 June 2009 14:39:19 tom.leiming@gmail.com wrote: > >> > From: Ming Lei <tom.leiming@gmail.com> > >> > > >> > dma_sync_*_for_cpu() is introduced to make cpu access dma buffers safely when > >> > dma transfer is over, it seems there is nothing to do with cpu write buffer, > >> > so remove it. > >> > > >> > Signed-off-by: Ming Lei <tom.leiming@gmail.com> > >> > >> Right, this looks correct. On a related note, flush_write_buffers is > >> architecture specific right now: only x86 and frv implement it at all, > >> though and with slightly different semantics. > > > > This doen't look correct to me. The sync functions may do bounce buffering > > which is all about copying data from one place in main memory to another. So we > > need these flush_write_buffer() calls in the _for_cpu path too. > > 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 systems this function is therefore a nop. Cache flushing for architectures without cache-coherent DMA is typically handled in their low-level dma-api code (I have seen that at least in parisc32).
Joerg
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