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SubjectRe: DMA API issues
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:

I think I'm looking for something like the PARISC hppa_dma_ops but more

Jamey Hicks

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