rt_com call-back function

Hi everyone,

Recently, I've tried to run serial communications in real-time using
RTLinux. For this I use the rt_com module. Since the module doesn't
offer blocking reads I made my own blocking read with time-out. I used
the call-back function to wake up my real-time thread when the UART's
read FIFO is triggered. The time-out feature was done by a nanosleep
with an interval proportional to the number of bytes to read. This
seemed to do the trick, and the whole thing runs very efficiently.

However, when I looked I looked at the Linux system clock I noticed it
jumped around like a madman. Once of twice per minute it does jumps of
4 seconds. After some investigation it seemed the rt_com call-back
function was the cause. I disabled the call-back and made the thread
periodic. This seemed to stop the jumps from occuring.

The way it runs now is very satisfying. However, I still would like to
have some insight into the flaws of this call-back mechanism. My guess
is it disables interrupts for a long time, which causes the system
timer to malfunction? Can it be that "pthread_wakeup_np()" also
contributes to this effect? Does anyone share similar experiences?

My setup is a 350 MHz PII with kernel 2.4.20, RTLinux 3.2-pre2, rt_com 0.5.5. 

Kind regards,

Pieter van der Meer
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