I'm running
Linux 2.6.38.8
Xenomai 2.6.0
Atom N270 CPU
I'm talking to CAN with a PEAK PCI SJA1000 CAN adapter.
I have a control loop running at 200 Hz, and I use it to drive a motor
and read an encoder using rtcan. My code is based on the
rtcansend/rtcanrecv examples.
I found a bug where my control loop was hanging, and I've isolated the
problem so that it may be reproduced with Xenomai supplied tools,
without using my own code.
The problem is:
When I have a loop that is sending CAN packets, if I read
/proc/rtcan/rtcan0/registers at the same time, it causes the loop to
hang. The hang is in rt_dev_sendto.
To reproduce:
Configure the CAN controller
$ rtcanconfig rtcan0 --baudrate=1000000 start
Send packets, in this case, SYNC messages at 1000 Hz
with tty output 1 Hz.
$ rtcansend rtcan0 -i 0x80 -p 1000 -l 1000000
It will print CAN SYNC (0x80) messages:
<0x080> [0]
<0x080> [0]
...
In another window, read rtcan0/registers in a loop (10 Hz):
$ while :; do cat /proc/rtcan/rtcan0/registers; sleep .1; done
After a few seconds or a minute, the rtcansend window will stop printing
SYNC messages, because it is hung.
gdb stack trace from hung rtcansend:
(gdb) bt
#0 0xb7862813 in rt_dev_sendmsg (fd=0, msg=0xbfe4d3ec, flags=0) at
core.c:84
#1 0x08048b9b in rt_dev_sendto () at ../../../include/rtdm/rtdm.h:359
#2 rt_task () at rtcansend.c:89
#3 0x080492d3 in main (argc=8, argv=0xbfe4d654) at rtcansend.c:301
This example uses a (default) 1000 Hz rtcansend loop and a 10 Hz cat
from /proc, but in my code I found it with a 200 Hz rtcan I/O loop and a
/usr/bin/watch script that was running at .5 Hz (but it took longer to
reproduce the bug).
I think I see that the rtcan0/registers are read in:
ksrc/drivers/can/sja1000/rtcan_sja1000_proc.c:rtcan_sja_proc_regs()
but I don't understand the details enough to debug further.
For debugging convenience, I found that if I ^C out of the rtcansend and
try to rerun it, the rtcan0 I/O will still be hung, but if I rerun the
rtcanconfig command above, that will unhang the I/O. Of course, that's
not a workable fix.
I found this problem by running my CAN I/O app while running a "canstat"
shell script from a "watch" command. The canstat script looks like this:
#! /bin/bash
pcat() {
for i in $*
do
echo "###" $i
cat $i
echo
done
}
cd /proc/rtcan
pcat version devices sockets rtcan0/filters rtcan0/registers
echo "### rtps"
rtps
This script cats a bunch of data from /proc/rtcan/ but I believe the
rtcan0/registers data is the only part that causes the rt_dev_sendto to
hang.
-Andy
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