On Oct 21, 2005, at 9:17 AM, Hamilton, Ian wrote:
Is there a full description of the timing problem somewhere on the web? In particular, how quickly does the interrupt need to be serviced?
There are two major challenges (and many minor ones). The major challenges are timing related. First, the USB status is not cumulative, if you can't service the interrupt within the latency of the next status change, you lose. The second challenge is the management of the data flow. You need to turn around DMA buffers very quickly, as the FIFO is small. If you happen to DMA a power of 2 size that matches what you just told the FIFOs, you then have to also do a zero length DMA to properly terminate the transfer on the USB. To reduce the latency of DMA processing, I was considering not using the general DMA functions, but rather implementing custom versions of the DMA functions in the USB driver. This will reduce the latency window, but not eliminate it. Oh yeah, there is also an "old" and "new" version of this peripheral in the Au1100. The new one tried to address some of the problems, and it helped a little. If you code to the "old" interface, I believe it will work on all silicon (with challenges), but if you detect the new silicon it's a little easier to meet the latency requirements. However, I could always find too many cases where the interrupt latency of Linux just caused us to miss interrupts and lose the USB state. The Gadget interface is really nice, but the additional indirection of the software layers makes the problems even more challenging. Good Luck. I've been there and don't care to visit again. -- Dan
