Re: DMA API issues

[David Brownell <david-b@pacbell.net>]

Ian Molton wrote:
> On 18 Jun 2004 13:52:46 -0500
> James Bottomley <James.Bottomley@SteelEye.com> wrote:
> 
> 
>>You still haven't explained what you want to do though.  Apart from the
>>occasional brush with usbstorage, I don't have a good knowledge of the
>>layout of the USB drivers.  I assume you simply want to persuade the
>>ohci driver to use your memory area somehow, but what do you actually
>>want the ohci driver to do with it?  And how much leeway do you get to
>>customise the driver.

A fair amount, so long as drivers above it don't need to
change much at all (this is "stable").  That includes
usbcore and all the usb drivers.

> 
> In *theory* the OHCI driver is doing everything right - its asking for DMAable memory and using it. if the DMA api simply understood the device in question, and alocated accordingly, it would just work.
> 
> there are two solutions:
> 
> 1) Break up the OHCI driver and make it into a chip driver as you describe

It's heading that way already ... breaking along the lines
where standard APIs solve problems, such as this.

> 2) Make the DMA API do the right thing with these devices
> 
> 1) means everyone gets to write their own allocator - not pretty
> 2) means we get to share code and it all just works.
> 

For example, if usbaudio uses usb_buffer_alloc to stream data,
that eliminates dma bouncing.  That's dma_alloc_coherent at
its core ... it should allocate from that 32K region.

- Dave