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

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Date:	Mon, 29 Jun 2009 22:54:51 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Joerg Roedel <joerg.roedel@....com>
Cc:	Arnd Bergmann <arnd@...db.de>, fujita.tomonori@....ntt.co.jp,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [PATCH][RFC] asm-generic:remove calling flush_write_buffers() in 
	dma_sync_*_for_cpu

2009/6/29 Joerg Roedel <joerg.roedel@....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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