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.
On systems where the PCI is marginal, that regular DMA can make for a lot of trouble ... even though it's just fetching one word per OHCI controller. One system kept getting "Unrecoverable Error" exceptions in OHCI, which usually means DMA transfers failing.
I fixed that a while back on the 2.6 kernels, where right now OHCI does a reasonable job of turning everything off until it's really needed ... even suspending the root hub itself, which reduces power consumption a bunch. The next step would involve suspending the controller (vs just its root hub), at least on systems where remote wakeup works properly. The transceivers can detect when devices connect, raising a signal which leads to resuming the controller (then its root hub).
- Dave
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