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    SubjectRe: DMA API issues
    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.

    -Matt
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