Universal Serial Bus (USB) is a specification for a serial bus
subsystem which offers higher speeds and more features than the
traditional PC serial port. The bus supplies power to peripherals
and allows for hot swapping. Up to 127 USB peripherals can be
connected to a single USB host in a tree structure.

The USB host is the root of the tree, the peripherals are the
leaves and the inner nodes are special USB devices called hubs.
Most PCs now have USB host ports, used to connect peripherals
such as scanners, keyboards, mice, modems, cameras, disks,
flash memory, network links, and printers to the PC.

Say Y here if your computer has a host-side USB port and you want
to use USB devices. You then need to say Y to at least one of the
Host Controller Driver (HCD) options below. Choose a USB 1.1
controller, such as "UHCI HCD support" or "OHCI HCD support",
and "EHCI HCD (USB 2.0) support" except for older systems that
do not have USB 2.0 support. It doesn't normally hurt to select
them all if you are not certain.

If your system has a device-side USB port, used in the peripheral
side of the USB protocol, see the "USB Gadget" framework instead.

After choosing your HCD, then select drivers for the USB peripherals
you'll be using. You may want to check out the information provided
in Documentation/usb/ and especially the links given in
Documentation/usb/usb-help.txt.

To compile this driver as a module, choose M here: the
module will be called usbcore.

source code:

drivers/usb/core/