Date: Fri, 21 Jan 2000 00:03:59 +0000
From: Keith James Robertson <kjrobert@abio.com>
I am developing a device driver for a serial device. However, I am
confused by stacking of the lower level serial drivers and how to
interface with the appropriate one and have been unable to find much
documentation on serial drivers. A solution to all of my questions
would likely be solved by someone pointing me to a kernel space serial
device driver that requires looking at and changing values in the serial
UART control registers (ie device supports multiple speeds, pnp).
If you're writing a device driver for a serial device that plugs into an
RS-232 driver, the first question is should it be a kernel-level driver
at all? In general, if you can do it in user space, you probably
should do it there. If you just need to control the baud rate, parity,
start/stop bits, etc., then a user space program which talks to
/dev/ttySx using the POSIX termios interface is the right choice.
If for some reasons you do need to be in the kernel, but you're still
using standard termios controls, the next level of abstraction down is
to make a new line discpline. Examples of line discplines are PPP,
SLIP, etc. This has the advantage that it will work with any serial
board, including intelligent serial boards such as the Comtrol
Rocketport, Cyclades, boards, etc.
The only time you should really write your own low-level device driver
is if you have a specific board which doesn't use an existing standard
UART. I'll note that since you mentioned PNP, there will shortly be a
new version of the serial driver released into the 2.3 mainline (and
available as a stand-alone driver for Linux 2.2) that will support
standard PNP boards which use 8250/16550 compatible uarts.
So, you need to give us a lot more details before we can really give you
advice about what's the best way to about things.
- Ted
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