Messages in this thread | | | Date | Mon, 29 Jun 2009 21:51:34 +0800 | Subject | Re: [PATCH][RFC] asm-generic:remove calling flush_write_buffers() in dma_sync_*_for_cpu | From | Ming Lei <> |
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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?
Thanks.
> >> 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? > > Joerg > > -- > | Advanced Micro Devices GmbH > Operating | Karl-Hammerschmidt-Str. 34, 85609 Dornach bei München > System | > Research | Geschäftsführer: Thomas M. McCoy, Giuliano Meroni > Center | Sitz: Dornach, Gemeinde Aschheim, Landkreis München > | Registergericht München, HRB Nr. 43632 > >
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