On Mon, 28 Sep 2009, Devin Heitmueller wrote: > Hello, > > I have been doing some testing in an attempt to support two USB video > capture devices on the same bus. > > For a single device running with maxpacket of 3072 per microframe, the > EHCI driver is indicating 57% utilization of the bus (the sysfs stats > already takes into account the fact that only 80% of the bus is > available for isoc traffic). > > Is this expected behavior? By my math, an alternate of 3072 should > take exactly 50% of the available bandwidth. Have I misinterpreted > the spec? Let's go through the arithmetic. EHCI runs at 480 million bits per second. That's 480000 bits per ms, or 60000 bytes per ms, or 7500 bytes per uframe. Only 80% of those bytes are available for periodic transfers, leaving 6000 bytes per uframe. You want to transfer 3072 bytes per uframe; allowance for bit-stuffing raises that to 3584. There's some additional protocol overhead as well, but let's ignore that. 3584/6000 is a little under 60% (closer to 59.7%). So if anything, the 57% estimate is a little low. 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
