On Mon, 21 May 2007, Steve Calfee wrote: > Hi Alan, > > About EHCI frame numbers. From the EHCI manual section 2.3.4: > > <quote> ... > </quote>
That's alright; I have a copy of the spec. You don't need to include a lengthy extract, just put a reference to a section number. > So FRINDEX does count microframes, but the bus only sees bits d13 to d3. In > fact what Linux can respond in 125 microseconds? On a 2-GHz system that's enough time for 250,000 cycles, which is more than a few instructions. I should think a good deal could be done. > Realistically, very few > systems can respond in 1ms to use the frame number. So worrying about giving > an exact microframe number some unknown amount of time in the past is not > very useful. But this affects more than the software running on the host; it affects the functioning of the device as well. You will agree that devices can easily implement latencies much smaller than those of Linux? So it's conceivable that a driver really might need to arrange for a particular packet to be sent during a particular microframe. > Guaranteeing taking a scheduling value 1 second is the worst possible case, > but really a system should somehow know what its worst latency is, and use > that as the minimum. I don't see what this has to do with latency -- a driver might want to schedule a transfer for 2 hours in the future! Obviously we don't need to support anything as large as that, but the principle is the same no matter what the delay value is. If we allow drivers to schedule transfers for a particular time, then we must guarantee that submissions will accepted anywhere up to X milliseconds into the future, where X is some reasonable but fixed number. > At the least, the size of the periodic table is the > (useful) worst case -(which is 256, 512 or 1024 ms). As I said to Dave, 256 ms is a perfectly reasonable minimum. Assuming we agree to support this at all. Alan Stern ------------------------------------------------------------------------- This SF.net email is sponsored by DB2 Express Download DB2 Express C - the FREE version of DB2 express and take control of your XML. No limits. Just data. Click to get it now. http://sourceforge.net/powerbar/db2/ _______________________________________________ linux-usb-devel@lists.sourceforge.net To unsubscribe, use the last form field at: https://lists.sourceforge.net/lists/listinfo/linux-usb-devel
