All the other HCs apply a certain degree of round-robin logic to
promote fairness in silicon. UHCI has to do that in software.
I'd leave this part in.
OHCI only does one TD per frame per transfer?
No, but it only tries one transfer before going to the
next endpoint. Normally that won't mean an entire TD,
since (for example) a bulk TD might describe 4KBytes
(more than one transfer).
OHCI has a control/bulk service ratio, and the HC will try
N control endpoints (from 1 to 4) per bulk endpoint.
EHCI is similar, but without such a ratio: one transfer,
then move to the next QH. But EHCI 1.0 controllers can
support a "park" mode where up to 3 TDs are processed before
they look at the next QH. Presumably that's useful.
In both cases the drivers try to keep the queues as
short as possible, so it's quite possible for the HC
to run through one endpoint's queue without looking at
another endpoint.
- Dave
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