Hi, I have some questions regarding latency for USB packets. After having read the USB 2.0 specification and the Intel EHCI specification (v1.0), I have come to the following conclusions: Assuming a device with 1 IN and 1 OUT endpoint (bulk), directly connected to the USB root hub, no other devices on the same port. Fixed package size of 512 bytes (in and out). IN packets are generated at a constant rate of about 4 packets per microframe, OUT packets are generated as bursts of variable size. URBs for IN and OUT are 512 bytes each (single USB packet). I would be very grateful if anyone confirmed or turned down the following statements: 1. transactions for IN and OUT will be scheduled round robin, given there are any OUT packets in the queue 2. the device will be polled for available IN data by the host every AsyncSchedSleepTime interval (10us, see EHCI spec 4.8.4) 3. OUT packets will be transmitted immediately when they are available (given no other ongoing transaction, device has available space (PING->ACK)) 4. an interrupt will only be generated by the EHCI controller once every microframe (when interrupt on completion is set), introducing a latency of up to 125 us. Assuming the device would send back any received packet immediately, this would result in the following round trip latency, as seen from user space: sequence of tokens/length in bytes including sync, turnaround ... (see USB 2.0; table 5-10): PING, ACK ~ 34 bytes OUT, DATA, ACK ~ 55 + 512 bytes IN, DATA, ACK ~ 55 + 512 bytes ignoring bit stuffing, this sums up to (144+1024)*8/(480e6/s) = 19us, + interrupt delay. If I have a userspace application which submits one 512byte OUT and one 512byte IN URB, the OUT URB can be reaped immediately, and the IN URB can be reaped (completed) at least 19us, at most 134us later (assuming no latencies when switching between kernel and userspace, and no processing overhead in kernel - how much will this be?). I hope I made any statements clear enough to answer if my conclusions are correct. Thanks in advance for answering. Stefan -- Stefan Brüns / Bergstraße 21 / 52062 Aachen phone: +49 241 53809034 mobile: +49 151 50412019 -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
