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

[Ming Lei <tom.leiming@gmail.com>]

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
>
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-- 
Lei Ming