On Thu, 22 Jul 2004, Oliver Neukum wrote:
>
> > Here's another possibility. For each usb_device we include a
> > pre-allocated URB, together with a usb_ctrlrequest and a 64-byte buffer.
> > This URB could be used while holding the device lock, for things like
> > get_descriptor, suspend, or reset.
>
> Good idea.
>
> > > An URB with its buffer preallocated is bound to a device, respectively
> > > an endpoint, so it cannot complete without triggering the same irq the
> > > completion handler is running in.
> >
> > Ah yes. Although I would prefer to make each URB & buffer bound to a
> > HC/bus, not to a device or endpoint -- but that doesn't affect your point.
>
> Why? Drivers cannot share URBs among interfaces, unless you export
> the concept of busses, which would violate layering.
> Furthermore to a driver the interface is the natural unit of operation.
> And the more specific an URB is the less usb_submit_urb() has to do.
The reason is simple enough; it's connected with the per-device URB
mentioned above. When used for get_descriptor or related requests it must
be sent to the device's ep0. But when used for suspend, resume, or reset
it must be sent to the parent hub's ep0.
> > This would allow us to replace an atomic_t with an unsigned long and use
> > bit-operations rather than atomic_inc/atomic_dec. It would improve
> > performance on the Sparc platform and wouldn't make much difference
> > anywhere else.
>
> Indeed one bit would be enough.
No it wouldn't. The minimum is two bits, since we need to be able to
distinguish three separate states:
(1) Idle;
(2) In progress or in completion handler (no need to keep those
two substates separate);
(3) Resubmitted with completion handler still running.
The difference between (2) and (3) is that when the completion handler
returns, (2) moves to (1) whereas (3) moves to (2).
Alan Stern
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