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

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Message-ID: <20090629144536.GP5139@amd.com>
Date:	Mon, 29 Jun 2009 16:45:36 +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 09:51:34PM +0800, Ming Lei wrote:
> 2009/6/29 Joerg Roedel <joerg.roedel@....com>:
> > On Sun, Jun 28, 2009 at 03:34:35PM +0000, Arnd Bergmann wrote:
> >> On Sunday 28 June 2009 14:39:19 tom.leiming@...il.com wrote:
> >> > From: Ming Lei <tom.leiming@...il.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@...il.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?

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
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).

	Joerg

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