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 14:31:42 +0200
From:	Joerg Roedel <joerg.roedel@....com>
To:	Arnd Bergmann <arnd@...db.de>
CC:	tom.leiming@...il.com, 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 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.

> 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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