On Tue, 3 Jul 2007, jidong xiao wrote:
> What's the difference between event_bits and change_bits?
> I just cannot understand why we need two variables.
>
>
> 188 struct usb_hub {
> ...
> 205 unsigned long event_bits[1]; /* status
> change bitmask */
> 206 unsigned long change_bits[1]; /* ports with
> logical connect
> 207 status change
> */
>
> Take following section for example,inside hub_events() sampled from
> drivers/usb/core/hub.c:
>
> 2594 /* deal with port status changes */
> 2595 for (i = 0; i < hub->descriptor->bNbrPorts; i++) {
> 2596 connect_change = test_bit(i,
> hub->change_bits);
> 2597 if (!test_and_clear_bit(i+1, hub->event_bits)
> &&
> 2598 !connect_change)
> 2599 continue;
>
> Why do we need to test changes_bits and then test event_bits?
event_bits records which ports have their bits set in the hub's
interrupt URB. change_bits records the ports for which we want to
force a disable and re-enumeration.
Sometimes the core needs to act as though a device has disconnected
even when it really hasn't. That's what change_bits is for.
event_bits, on the other hand, tells us about real connect and
disconnect events. So the driver has to check both bit-vectors.
Alan Stern
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