I wondered about my system performance slowing down (-15% with an old Pentium 133) if I insert USB devices which use an interrupt endpoint (e.g. USB <-> Ethernet Adaptor, USB <-> Bluetooth Stick). This happens if I just set the interfaces to UP, no data transfer at all.
For interrupt endpoints, the host must (a) scan the periodic schedule, which is at least one extra memory access each millisecond, (b) poll the devices active in each period, which is usually several more memory accesses. You imply period = 8msec for your devices.
I took an USB analyser and found that there are a lot of NAK's on the referring USB's interrupt pipe (in the same 8 usec period). It seems to be that the interrupt pipe gets round about 117 NAK's from the device per milli second. Is this a correct behaviour?
The interrupt transfer should normally poll each interrupt endpoint just once per period ... so there should be at most one NAK per period.
Are you sure your driver is scheduling an interrupt transfer? Sounds like it's trying to use a bulk endpoint instead. A bulk transfer could easily waste USB and PCI bandwidth like that.
- Dave
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