On Mon, 24 May 2004, David Brownell wrote:
Guennadi Liakhovetski wrote:
So, how can the initialisation of the USB chip affect the rest of the PCI also without USB-devices attached and without USB-traffic? Does the OHCI driver poll?
It turns on the periodic schedule processing, which means once every msec it does PCI DMA even if there are no devices connected.
Hm, that could be the reason. But, upon briefly looking through the code, it looked like the driver sets up the timer with a period of min(30ms, urb->interval), but well may be that I misunderstood what was going on there.
Look at the OHCI spec, you'll see that what I mean. The periodic schedule is a tree, 32 slots, each one scanned near the beginning of each USB frame (== 1 msec). That chunk of code in 2.4 was relatively opaque, and didn't even handle ISO transfers with period != 1msec.
When you schedule a periodic URB (iso/interrupt), its period gets rounded down to the nearest power'o'two and spliced into one or more slots. So period 30msec would morph to 16 msec, and that same ED would be accessible from two slots (like #3 and #19, if that was a good way to balance the load).
Ok, it, probably, wouldn't be fixed for 2.4 any more (actually, looks like a worthy fix, or not?), is it at least possible to increase the period, and, if yes - how (if it is really 1ms)? And would it help significantly?...
The best 2.4 fix would be someone taking my 2.4 backport of ohci-hcd and getting real about it ... OHCI is probably the most robust HCD on 2.4, but that doesn't mean it's as good as on 2.6 ! :)
That 1msec period is part of the USB spec, so the only control knob in the driver is whether the periodic schedule is on or off. If you just counted the number of periodic transfers that are active, the fix is simple: turn off periodic schedule when count == 0, and turn it on otherwise.
- Dave
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