Re: Question about struct usb_hub

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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