On Tue, 4 Sep 2012, Jassi Brar wrote: > >> How about effectively increasing the queue length from 10ms to 50ms > >> (max anticipated latency) ? > > > > There are two problems with that approach. First, 50 ms isn't really > > the max anticipated latency; it's merely the largest that I've seen so > > far. (In fact, the max anticipated latency is probably < 10 ms; these > > 50-ms delays were definitely exceptional.) > > > It doesn't have to be hardcoded - maybe USB audio/video code could get > the hint via some module parameter? That's even worse. Fortunately it isn't necessary; ehci-hcd uses an I/O watchdog timer so the latency can never be much longer than 100 ms. > > Second, people involved in real-time programming (such as audio or > > video) generally want to keep latency to a minimum. > > > If we progress the h/w pointer of ALSA ring buffer at URB completion > (and not at URB submission), this shouldn't affect the latency. And I don't understand this comment. As far as I can tell, the relevant pointer already is being advanced during URB completion. > IIRC, USB isn't anyway recommended for real-time usage. People still want to minimize latency, though. I doubt that anybody would be happy with 100 ms latency. Imagine trying to talk over a loudspeaker system if the sounds coming out of the speaker were 100 ms behind your voice. Alan Stern -- 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
