The tty Layer, Part II
Vern Hoxie has an excellent set of documentation and example programs on how to access serial ports from user space available at scicom.alphacdc.com/pub/linux. Most of this information will not be too useful for a kernel programmer, but some of the descriptions of the different ioctl(2) commands and the history behind the wide variety of different ways to get and set tty information are quite good. I highly recommend that anyone implementing a tty kernel driver read over these, if for no other reason than to determine how users will be trying to use your driver.
Hopefully these two articles have helped to demystify the tty layer. If you are stuck on how to implement a specific callback, quite a few drivers in the kernel interact with the tty layer as complete examples. Search for “tty_register_driver” in the drivers/char and drivers/usb directories for these files.
I would like to thank Al Borchers, who helped to determine how the write() callback really works and all of the nuances involved in it. Together with Peter Berger, they wrote drivers/usb/serial/digi_acceleport.c, a USB to serial driver for the Digi AccelePort devices. It is an excellent example of a well-working tty driver.
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 email@example.com.