Setup
I have two nodes connected to one CAN bus. The first node is a black-box, controlled by some real-time hardware. The second node is a Linux machine with attached PEAK-USB CAN controller:
+--------+ +----------+
| HW CAN |--- CAN BUS ---| Linux PC |
+--------+ +----------+
In order to investigate some problem related to occasional frame loss I want to mimic the CAN arbitration process. To do that I am setting the CAN bit-rate to 125Kb/s and flooding it with random CAN frames with 1ms delay, controlling the bus load with canbusload from can-utils. I also monitor CAN error frames running candump can0,0~0,#ffffffff and the overall can statistics with ip -s -d link show can:
26: can0: <NOARP,UP,LOWER_UP,ECHO> mtu 16 qdisc pfifo_fast state UNKNOWN mode DEFAULT group default qlen 10
link/can promiscuity 0
can state ERROR-ACTIVE restart-ms 0
bitrate 125000 sample-point 0.875
tq 500 prop-seg 6 phase-seg1 7 phase-seg2 2 sjw 1
pcan_usb: tseg1 1..16 tseg2 1..8 sjw 1..4 brp 1..64 brp-inc 1
clock 8000000
re-started bus-errors arbit-lost error-warn error-pass bus-off
0 0 0 0 0 0
RX: bytes packets errors dropped overrun mcast
120880 15110 0 0 0 0
TX: bytes packets errors dropped carrier collsns
234123 123412 0 0 0 0
Problem
Now the problem is that the given setup works for hours with zero collisions (arbitration) or any other kind of error frames when the load is at 99%. When I reduce the delay to increase the bus load write(2) fails with either "ENOBUFS 105 No buffer space available" or "EAGAIN 11 Resource temporarily unavailable" - the actual error depends on whether I modify the qlen parameter or set to to defaults.
As I understand it, the load I put is either not enough or too much. What would be the right way to make two nodes enter the arbitration? A successful result would be a received CAN error frame corresponding to the CAN_ERR_LOSTARB constant from can/error.h and a value of collsns other than 0.
Source code
HW Node (Arduino Due with CAN board)
#include <due_can.h>
CAN_FRAME input, output;
// the setup function runs once when you press reset or power the board
void setup() {
Serial.begin(9600);
Serial.println("start");
// Can0.begin(CAN_BPS_10K);
Can0.begin(CAN_BPS_125K);
// Can0.begin(CAN_BPS_250K);
output.id = 0x303;
output.length = 8;
output.data.low = 0x12abcdef;
output.data.high = 0x24abcdef;
}
// the loop function runs over and over again forever
void loop() {
Can0.sendFrame(output);
Can0.read(input);
delay(1);
}
Linux node
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <net/if.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <sys/ioctl.h>
#include <linux/can.h>
#include <linux/can/raw.h>
int main(int argc, char *argv[])
{
int s;
int nbytes;
struct sockaddr_can addr;
struct can_frame frame;
struct ifreq ifr;
const char *ifname = "can0";
if((s = socket(PF_CAN, SOCK_RAW, CAN_RAW)) < 0) {
perror("Error while opening socket");
return -1;
}
strcpy(ifr.ifr_name, ifname);
ioctl(s, SIOCGIFINDEX, &ifr);
addr.can_family = AF_CAN;
addr.can_ifindex = ifr.ifr_ifindex;
printf("%s at index %d\n", ifname, ifr.ifr_ifindex);
if(bind(s, (struct sockaddr *)&addr, sizeof(addr)) < 0) {
perror("Error in socket bind");
return -2;
}
frame.can_id = 0x304;
frame.can_dlc = 2;
frame.data[0] = 0x11;
frame.data[1] = 0x22;
int sleep_ms = atoi(argv[1]) * 1000;
for (;;) {
nbytes = write(s, &frame, sizeof(struct can_frame));
if (nbytes == -1) {
perror("write");
return 1;
}
usleep(sleep_ms);
}
return 0;
}