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Date: Mon, 29 Jun 2009 22:54:51 +0800
Message-ID: <d82e647a0906290754t13bc1b37s6ea6fd582a7fdbe@mail.gmail.com>
Subject: Re: [PATCH][RFC] asm-generic:remove calling flush_write_buffers() in 
	dma_sync_*_for_cpu
From: Ming Lei <tom.leiming@gmail.com>
To: Joerg Roedel <joerg.roedel@amd.com>
Cc: Arnd Bergmann <arnd@arndb.de>, fujita.tomonori@lab.ntt.co.jp,
       linux-kernel@vger.kernel.org, akpm@linux-foundation.org
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2009/6/29 Joerg Roedel <joerg.roedel@amd.com>:
>>
>> 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?

> 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).


-- 
Lei Ming
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