On Sun, 1 Jul 2007, David Brownell wrote:
> As I understand it the actual mechanism is more like: platforms
> always set up a chunk of memory as uncached, and always map that
> through the IOMMU. Call that the DMA-{coherent,consistent} region.
>
> Utilities like dma_alloc_coherent() -- and hence usb_buffer_alloc(),
> dma_pool_*(), and so on -- return memory from that region.
>
> So the resources are *always* tied up; fixed overhead. What's visible
> to drivers is that per-transfer buffer operations are not needed.
I wonder about the wisdom of allocating QHs, TDs, and so forth as
single contiguous entities. Since the DMA-able part is liable to end
up in uncached memory, wouldn't we be better off splitting the data
structures in two and putting the other parts in normal memory?
Alan Stern
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