[kevinoid]

I was curious about this as well. The issue appears to be lack of scatter-gather support. See: https://github.com/raspberrypi/linux/issues/875#issuecomment...

[dmitrygr]

which is not required to implement any usb protocol. one can easily use normal DMA and still get a speed gain. since the bottleneck is not CPU, making it work a bit harder putting together those DMA descriptors is worth it.