Question about struct usb_hub

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?

Regards
Jason

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