From: Arnd Bergmann <arnd@arndb.de>
To: Ming Lei <tom.leiming@gmail.com>
Subject: Re: [PATCH][RFC] asm-generic:remove calling flush_write_buffers() in dma_sync_*_for_cpu
Date: Tue, 7 Jul 2009 16:06:48 +0200
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Cc: "Russell King - ARM Linux" <linux@arm.linux.org.uk>,
       Alan Cox <alan@lxorguk.ukuu.org.uk>,
       Joerg Roedel <joerg.roedel@amd.com>, fujita.tomonori@lab.ntt.co.jp,
       linux-kernel@vger.kernel.org, akpm@linux-foundation.org,
       "linux-arm-kernel" <linux-arm-kernel@lists.arm.linux.org.uk>
References: <1246199959-6548-1-git-send-email-tom.leiming@gmail.com> <20090707074821.GB6995@n2100.arm.linux.org.uk> <d82e647a0907070643i198b9188q80203ab0408b4df8@mail.gmail.com>
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On Tuesday 07 July 2009, Ming Lei wrote:
> > ARM has two (normal, and dma bounce), and in the long run we need to do
> 
> OK, Can we use dma-mapping-common.h on ARM?

It should work in principle. It may be a good idea to also move to
the generic swiotlb instead of the traditional dma bounce at the
same time.

Note that dma-mapping-common.h is only needed if you want to support
two or more different DMA implementations in a single kernel, which
I'm not sure is needed for ARM.

> > cache handling on unmap as well as map due to CPU speculative fetches.
> 
> IMHO, it seems we can fix this problem now.
> 
> For DMA_TO_DEVICE transfer, clean cache in dma map, but does nothing in
> dma unmap;
> 
> For  DMA_FROM_DEVICE, we may do nothing in dma map, but invaliate cache
> in dma unmap.

A number of other architectures do this already. You also need to
have dma_sync_*_for_cpu and dma_sync_*_for_device, where the *_for_device
operation needs to do the same flushing as dma_map_* and *_for_cpu
does the same as dma_unmap_*.

Note that actually you need to do writeback+invalidate in DMA_TO_DEVICE
and at least an invalidate in DMA_FROM_DEVICE during dma_map_*.
For the unmap, I don't think you ever need to invalidate the cache.
If you invalidate only at unmap time for DMA_FROM_DEVICE, a dirty
cache line might be accidentally flushed to the buffer after
the device has written to it.

	Arnd <><
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