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Linux serial ports from Kernel space

Posted on 2001-11-14
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Last Modified: 2010-08-05
Hi

How can I access the serial port from kernel space ? (from a driver)

From user space it's just like fd=open("/dev/ttyS0", ... )

But from kernel space it seems more difficult. I had a look at the Axis bluetooth implementation and they use
a whole complex thingy called ttys (via termios.h) ?
Can someone explain why tty's are used ?

What I want to do is not far from the Axis solution i.e.
ppp over my_driver out on the serial port for further processing. The problem is that I do not understand the while tty concept, neighter can I find any doc about it.

Thanx
BTW this is my very first driver :)
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Question by:CrypToniC
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bryanh earned 800 total points
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That whole tty thing is rather complex, and steeped in history.  I don't know anywhere you can read about it.
I've accumulated a few thousand words of notes on it over years of experimenting and reading Linux source code, and some day I plan to publish the information somehow.  For now, I can give you an overview and also answer the larger question of how to do I/O from within your driver.

Ttys are all about terminals -- where people log in and interact with the system.  Not many people do that via a serial port anymore, but it used to be quite normal.  In fact, it used to be the only way.  If you're not talking to a person over this serial line, the tty layer will just get in your way.

A tty is a device defined a layer above the serial device.  The tty device driver takes read and write calls from the user program and implements them with its own read and write calls to the serial device driver.
Whereas with a serial device, you just read and write characters, a tty device has a much more intelligent read/write interface.  The tty device buffers, for one thing.  Your program can read and write whole lines at a time, thus saving the hassle and execution time of processing individual characters.  And there's line editing.  The person at the terminal can backspace and retype without the program he's talking to even knowing about it.

Though you'll want to disable most of that added function for your own serial port accesses, you can't just skip the tty layer altogether -- the serial device driver is designed to be accessed via a tty device.

The 'stty' program controls all the features of tty devices (Stty uses that termios thing to do it).  Check out the man page for it to get a better understanding.  And do a 'stty raw' on the serial port in question.

Now to the question of how to do the equivalent of open(/dev/ttyS0) inside the kernel.  This is a common question, and I still have an example program of this (actually, the more general question of how do you access files from inside the kernel) from the last time some asked on EE.  <ftp://giraffe-data.com/pub/filewrite.c>.

The key is that you can't do a system call (such as open()) from inside the kernel, but you can call the same kernel subroutines that the system call handler would (in this case, filp_open()).  Read and write are a little trickier because every file has its own read and write routines, so you have to call them indirectly.

The hardest thing about reading and writing is that the kernel subroutines that do this expect to be reading into and writing from memory in the user's address space, whereas your code probably wants to read to and write from kernel memory.  Therefore, you have to make the kernel address space look like the user address space, do the read or write, then put the user address space back where it belongs.

This is all in the example.

Because the example is not specific to ttys or serial ports, it doesn't show you any of that termios stuff.  You may want to do such things as put the tty in raw mode, and I can tell you how to do that if you can't tell from the Bluetooth example you already have.  Until you get simple read and write working, just use Stty.
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by:Anthony2000
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just listening...
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Dear Moderators ---

all my points disappeared!!
i had > 8000 expert points - i expect them back!
also the number of answered q's reset to 0 :(

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