linux-kernel - Re: [PATCH v4 04/11] swiotlb: support NOT_COHERENT

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Date:	Tue, 16 Mar 2010 07:09:43 +0100
From:	Albert Herranz <albert_herranz@...oo.es>
To:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>
CC:	linux-usb@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org, x86@...nel.org,
	linux-ia64@...r.kernel.org
Subject: Re: [PATCH v4 04/11] swiotlb: support NOT_COHERENT_CACHE PowerPC
 platforms

FUJITA Tomonori wrote:
> On Fri, 12 Mar 2010 20:12:39 +0100
> Albert Herranz <albert_herranz@...oo.es> wrote:
> 
>> The current SWIOTLB code does not support NOT_COHERENT_CACHE platforms.
>> This patch adds support for NOT_COHERENT_CACHE platforms to SWIOTLB by
>> adding two platform specific functions swiotlb_dma_sync_page() and
>> swiotlb_dma_sync() which can be used to explicitly manage cache coherency.
>>
>> On PowerPC these functions are mapped to their corresponding
>> __dma_sync_page() and __dma_sync() functions.
>> On other architectures using SWIOTLB these functions are optimized out.
>>
>> This will be used later to support SWIOTLB on the Nintendo Wii video game
>> console.
>>
>> CC: linuxppc-dev@...ts.ozlabs.org
>> CC: linux-kernel@...r.kernel.org
>> CC: x86@...nel.org
>> CC: linux-ia64@...r.kernel.org
>> Signed-off-by: Albert Herranz <albert_herranz@...oo.es>
>> ---
>>  arch/ia64/include/asm/swiotlb.h    |   10 ++++++++++
>>  arch/powerpc/include/asm/swiotlb.h |    3 +++
>>  arch/x86/include/asm/swiotlb.h     |   10 ++++++++++
>>  lib/swiotlb.c                      |   30 ++++++++++++++++++++++++------
>>  4 files changed, 47 insertions(+), 6 deletions(-)
> 
> Why can't you use dma_sync_single_* instead of inventing new
> swiotlb sync functions?
> 

At least on PowerPC, the DMA ops are per-device hooks. We attach the swiotlb DMA ops functions to those hooks when we are using swiotlb.
So calling dma_sync_single_*() would end up calling swiotlb_sync_single_*() which is not what we want.

Thanks,
Albert

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