Re: DMA API issues

[Jamey Hicks]

Matt Porter wrote:

> On Fri, Jun 18, 2004 at 05:59:02PM +0100, Ian Molton wrote:
>  
>
> > I have a System On Chip device which, among other functions, contains an
> > OHCI controller and 32K of SRAM.
> >
> > heres the catch:- The OHCI controller has a different address space than
> > the host bus, and worse, can *only* DMA data from its internal SRAM.
> >
> > The architecture is not broken, merely unusual.
> >
> > This causes the following problems:
> >
> > 1) The DMA API provides no methods to set up a mapping between the host
> >   memory map and the devices view of the space
> >        example:
> >           the OHCI controller above would see its 32K of SRAM as
> >           mapped from 0x10000 - 0x1ffff and not 0xXXX10000 - 0xXXX1ffff
> >           which is the address the CPU sees.
> > 2) The DMA API assumes the device can access SDRAM
> >        example:
> >           the OHCI controller base is mapped at 0x10000000 on my platform.
> >           this is NOT is SDRAM, its in IO space.
> >    
>
> Can't you just implement an arch-specific allocator for your 32KB
> SRAM, then implement the DMA API streaming and dma_alloc/free APIs
> on top of that?  Since this architecture is obviously not designed
> for performance, it doesn't seem to be a big deal to have the streaming
> APIs copy to/from the kmalloced (or whatever) buffer to/from the SRAM
> allocated memory and then have those APIs return the proper dma_addr_t
> for the embedded OHCI's address space view of the SRAM. Same thing
> goes for implementing dma_alloc/free (used by dma_pool*). I don't
> have the knowledge of USB subsystem buffer usage to know how quickly
> that little 32KB of SRAM is going to run out. But at a DMA API level,
> this seems doable, albeit with the greater possibility of negative
> retvals from these calls.
>
>  
Your analysis of what is needed is correct.  However, we would prefer to 
fit this into the DMA API in a generic way, rather than having a 
specialized API that is not acceptable upstream.  I'm more concerned 
with finding the best way to address this problem than with whether this 
is a 2.6 or 2.7 issue.

We do need a memory allocator for the pool of SRAM.  If we're not going 
to have to build customized versions of the OHCI driver, we need that 
pool hooked into the dma_pool and dma_alloc_coherent interfaces.  We 
could do that with a platform specific implementation of 
dma_alloc_coherent, but a pointer to dma_{alloc,free} from struct device 
seems like a cleaner solution.  We also will need bounce buffers, as you 
described, but there is already a good start towards that support in 
2.6.  The other thing that might be needed is passing device to 
page_to_dma so that device specific dma addresses can be constructed.

Deepak Saxena wrote a pretty good summary as part if the discussion 
about this issue on the linux-arm-kernel mailing list:

 
http://lists.arm.linux.org.uk/pipermail/linux-arm-kernel/2004-June/022796.html

I think I'm looking for something like the PARISC hppa_dma_ops but more 
generic:
 
http://lists.arm.linux.org.uk/pipermail/linux-arm-kernel/2004-June/022813.html


Jamey Hicks



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