On Wed, 4 Jul 2007, jidong xiao wrote:
> Still consider following piece of code, I see there is a change in
> this part of code,here showed the same part of code but came from
> different kernel version,one is sampled from kernel 2.6.10,while
> another is sampled from kernel 2.6.21.
>
> inside hub_events() sampled from drivers/usb/core/hub.c, 2.6.10:
>
> 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;
>
> This is the same part of code but just picked up from 2.6.21.
> 2682 /* deal with port status changes */
> 2683 for (i = 1; i <= hub->descriptor->bNbrPorts; i++) {
> 2684 if (test_bit(i, hub->busy_bits))
> 2685 continue;
> 2686 connect_change = test_bit(i,
> hub->change_bits);
> 2687 if (!test_and_clear_bit(i, hub->event_bits) &&
> 2688 !connect_change &&
> !hub->activating)
> 2689 continue;
>
> an interesting thing is, in the former one,we call
> test_and_clear_bit(i+1, **), while in the latter one, we call
> test_and_clear_bit(i, **),this change is somehow strange,is one of
> them wrong?
Look again. In the first snippet i ranges from 0 to bNbrPorts-1, and
in the second snippet i ranges from 1 to bNbrPorts. So the
test_and_clear_bit calls do the same thing.
> additionally,I am wondering the reason, for the change_bits map, we
> just test, for the event_bits,besides test,we also need to clear the
> bit, why?
Both bits get cleared. Look in hub_port_connect_change() to see where
change_bits is reset.
Alan Stern
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