Messages in this thread | | | From | Arnd Bergmann <> | Subject | Re: [PATCH][RFC] asm-generic:remove calling flush_write_buffers() in dma_sync_*_for_cpu | Date | Mon, 29 Jun 2009 18:22:17 +0200 |
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On Monday 29 June 2009, Joerg Roedel wrote: > 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.
Right, I didn't consider that.
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.
> > Maybe it would be more consistent to change the dma_map_* and > > dma_sync_*_for_device stuff there to wmb() to make it portable > > to other architectures. > > If we change it to wmb() it would be executed every time there even if the > processor doesn't require it. Other architectures could simply add a > flush_write_buffers() implemention if they want to adapt the common dma-mapping > implementation, no?
As mentioned, the definition of flush_write_buffers() seems a little dodgy, I would feel much more comfortable with putting it into the architecture specific code or using one of the existing common barriers, since we already have so many of them.
Arnd <><
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