Embedded.com: Linux-based USB Devices

“Linux-based USB devices use one of three different strategies
for communicating with USB hosts.

“The most ambitious and complicated devices use custom-written
kernel modules to implement sophisticated, high-level protocols
that run over the standard USB bus. A corresponding custom driver
and applications for the USB host complete the connection.

“Other Linux-based USB devices use the bus as a simple,
point-to-point serial connection to an application running on the
host. The host application uses the USB programming interface
provided with the host operating system, but otherwise behaves as
though it were communicating through a typical serial port…”

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