The per-port LEDs on the most USB 2.0 hubs are programmable.
And the USB spec describes some ways to use them, blinking
to alert users about hardware (amber) or software (green)
problems.

This patch is the infrastructure for that blinking.  And
if you should happen to "modprobe usbcore blinkenlights",
the LEDs will cycle through all the ports ... which is
not a USB-standard mode, but it can certainly handy be
handy as a system heartbeat visible across the room.