USB Device Identification

In linux system when a pnp usb device is plugged in then how does the system gets a notification of it?
I mean to say in linux usb system there is usb host controller above which is host controller driver above which is usb core.
So does the host controller/usb core keeps on polling the usb bus for the device or it is interrupt based?
Any link for the explainations will be of much help.

I'm sure it's interrupt-based -- powertop on my laptop would show the USB driver if it were polling all the time, it certainly showed my network driver doing so -- but nobody except people who write device drivers for USB can tell you much more. You might have more luck looking at specification sheets, like the EHCI specification.

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