Re: DMA API issues

[Jeff Garzik <jgarzik@pobox.com>]

Ian Molton wrote:
> Hi.
> 
> This may have come up previously but I havent seen it, so...
> 
> My colleagues and I are encountering a number of difficulties with the
> DMA API, to which a generic solution is required (or risk multiple
> architectures, busses, and devices going their own way...)
> 
> Here is an example system that illustrates these problems:
> 
> 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.

Unfortunately, I tend to think that you should not be using the DMA API 
for this, since it's not host RAM.

There are loads of devices with on-board SRAM/DRAM/... you really have 
to manage your own resources at that point.

I know that sucks, WRT driver re-use.  Maybe you can wrap it inside the 
OHCI driver, ohci_dma_alloc_xxx() etc.  For the normal case, the wrapper 
resolves directly to the DMA API.  For the embedded case, the wrapper 
does SRAM-specific stuff.

You _might_ convince the kernel DMA gurus that this could be done by 
creating a driver-specific bus, and pointing struct device to that 
internal bus, but that seems like an awful lot of work as opposed to the 
wrappers.

	Jeff