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

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Date:	Tue, 26 May 2009 13:51:47 +0100
From:	Ian Campbell <Ian.Campbell@...citrix.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Becky Bruce <beckyb@...nel.crashing.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 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.

Ian.

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