On Wed, 7 Apr 2010, Robert Hancock wrote:
The problem is appearantly the way the transfer buffer is allocated in the drivers. In the snd-usb-caiaq driver, I used kzalloc() to get memory which works fine on 32bit systems. On x86_64, however, it seems that kzalloc() hands out memory beyond the 32bit addressable boundary, which the DMA controller of the 32bit PCI-connected EHCI controller is unable to write to or read from. Am I correct on this conclusion?
That seems like the right answer. You are correct that an EHCI controller capable only of 32-bit memory accesses would not be able to use a buffer above the 4 GB line.
AFAIK, the driver shouldn't have to worry about this at all. When the buffer gets DMA-mapped for the controller, the DMA mapping code should see that the device has a 32-bit DMA mask and either bounce or IOMMU-map the memory so that it appears below 4GB.
That's true. It would of course be more efficient for the buffer to be allocated below 4 GB, but it should work okay either way. Daniel, do you have any idea why it fails?
Alan Stern