linux-kernel - Re: [PATCH V2 2/3] powerpc: Add support for swiotlb on 32-bit

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Date:	Wed, 27 May 2009 14:11:05 -0500
From:	Becky Bruce <beckyb@...nel.crashing.org>
To:	Ian Campbell <Ian.Campbell@...citrix.com>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	"linuxppc-dev@...abs.org" <linuxppc-dev@...abs.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2 2/3] powerpc: Add support for swiotlb on 32-bit


On May 26, 2009, at 7:51 AM, Ian Campbell wrote:

> On Fri, 2009-05-22 at 19:55 -0400, Jeremy Fitzhardinge wrote:
>> Ian Campbell wrote:
>>> On Thu, 2009-05-21 at 14:27 -0400, Becky Bruce wrote:
>>>
>>>> I can work with that, but it's going to be a bit inefficient, as I
>>>> actually need the dma_addr_t, not the phys_addr_t, so I'll have to
>>>> convert.  In every case, this is a conversion I've already done and
>>>> that I need in the calling code as well.
>>>>
>>>
>>> Does
>>>
>>>    dma_addr_t dma_map_range(struct device *hwdev, phys_addr_t addr,
>>>    size_t size);
>>>
>>> work for you?
>>>
>>> If the range does not need mapping then it returns the dma  
>>> address, if
>>> you needed to calculate the dma address anyway to figure out if  
>>> mapping
>>> is required then this is fine. If the range does need mapping then  
>>> it
>>> returns NULL.
>>>
>>
>> My only concern is whether dma_addr_t == 0 is actually equivalent to
>> NULL.  That is, can we be sure that address 0 will never be used?
>
> It seems not, ~0UL might have been an option, but...
>
>> Taking dma_alloc_coherent as a model, we could have something like:
>>
>>    int dma_map_range(struct device *hwdev, phys_addr_t addr, size_t  
>> size, dma_addr_t *dma_addrp);
>>
>>
>> where *dma_addrp is set if the function returns success (bool return
>> type might be clearer).
>
> ... this sounds like a good idea to me.

I agree.  This will work for me as well.

-becky

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