Request for feedback on common data backstore in the kernel

[John Baldwin]

On Wednesday 26 September 2007 12:44:55 pm Hans Petter Selasky wrote:
> What I meant was that USB dma directly into main memory. But then another 
PCI 
> device like an Ethernet device might want to forward that data by dma'ing it 
> out of main memory. The dma address as seen by the two different PCI devices 
> might not be the same.
> 
> I admit that I'm not an expert on how DMA is done on the Sparc, but could 
you 
> explain a little bit more how a mbuf is loaded into DMA for a network card 
on 
> the Sparc ?
> 
> What I'm looking for is a function that transforms a virtual memory address 
> and a bus-DMA tag into a physical address without blocking. If a mapping is 
> not possible I want that an error be returned so that I can bounce the data 
> using a pre-allocate buffer, and not a buffer allocated by bus_dma on the 
> fly.

bus_dma already preallocates bounce pages when you create a map.  I would just 
try using the existing bus_dma first w/o trying to use private buffers, etc.  

That said, there is one case where this could be useful: for disk dumps it 
might be nice to pre-allocate a known "safe" set of pages to use for bouncing 
pages (such as on i386 with PAE with a controller that does 32-bit 
addressing) that can't be mapped directly.

-- 
John Baldwin