The USB Serial Driver Layer

Understanding the USB-to-serial layer and how to get devices into it.
urb Callback Function Pointers

The read_int_callback, read_bulk_callback and write_bulk_callback function pointers are all used by the USB serial core to set up the initial callbacks for these kinds of USB endpoints. If the driver does not specify the read or write bulk callback functions, the generic callbacks are used. There is no generic read interrupt callback function, so if your device has an interrupt endpoint, you must provide this callback.

The operation of the generic read bulk callback adds the data received by the USB urb to the port's tty buffer, to be sent to user space when read() is called. It then resubmits the urb to the device. If your device does not need to interpret the data received in any way, I recommend using this function instead of writing a new one. The generic bulk write callback is much smaller and only wakes up the tty layer (in case it was sleeping, waiting for data to be transmitted to the device).


In this article we have explained how to register and unregister a USB serial driver, as well as the basics of the main usb_serial_driver_type structure that all USB serial drivers must provide. Next time, we will go into the specifics of how the USB serial driver layer works and provide an example driver.


I would like to thank all of the different programmers who have helped to create the USB serial layer over the years. Special thanks to Peter Berger and Al Borchers for their loadable module code offered back in July 2000, which is still in place today.

Greg Kroah-Hartman is currently the Linux USB and PCI Hot Plug kernel maintainer. He works for IBM, doing various Linux kernel-related things and can be reached at



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Interesting article, when

Anonymous's picture

Interesting article, when searching for Linux compatible USB serial adapters
this is a good source.