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

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Message-ID: <20090629154406.GQ5139@amd.com>
Date:	Mon, 29 Jun 2009 17:44:06 +0200
From:	Joerg Roedel <joerg.roedel@....com>
To:	Ming Lei <tom.leiming@...il.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

On Mon, Jun 29, 2009 at 10:54:51PM +0800, Ming Lei wrote:
> 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?

CONFIG_X86_OOSTORE (when set flush_write_buffers is a memory barrier and not a
nop on x86) is defined for WINCHIP. So it seems necessary on some chips.

	Joerg

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