rt-can-test

Overview

The main purpose of rt-can-test is to provide an easy way to test the performance of the Linux PREEMPT_RT patches together with CAN.

In many automotive applications, a node on a CAN network is expected to deliver data at fixed time intervals. Sometimes the timing of the CAN messages is very important for the functionality of the system.

Since a standard Linux kernel is not a real time system, there is risk of latency spikes in the CAN bus traffic. These latency spikes can potentially make the system fail. The latency spikes might not be caused by the CAN subsystem at all, as any subsystem that causes kernel preemption to be disabled can block the CAN threads from execution.

rt-can-test strives to implement all the recommendations for user space real time applications on a PREEMPT_RT system (this essentially means taking measures in order to avoid page faults after all threads have been created).

In average, rt-can-test will have the same performance (if not even better) when running on a standard Linux kernel, but the worst case will be better with a real time kernel.

The program uses the Linux socket CAN API (there is a small wrapper interface defined by can.h), and thus it has no dependencies to any external libraries.

Usage

To run a TX test, follow the below steps:

1. Connect the CAN bus of the device to some kind of CAN-dongle (like a Vector CANCase or similar) and start recording.
2. Configure the CAN interface of the device using the ip tool. See Configure CAN interface for more info.
3. Launch rt-can-test (see below).

A transmit example command can look like this:

rt-can-test --if can0 --tx 123#1122334455667788 -i 100000

The format of the TX message is identical to the format used by the cansend tool from can-utils.

-i is the CAN frame interval in microseconds. Thus, the above example will transmit CAN frames every 100 ms (10 Hz)

--if is the CAN interface we will send on (can0 in the example)

--tx must be followed by the CAN frame to send. In the example, a CAN frame with ID 0x123 and data 1122334455667788 will be transmitted.

An RX test might look like this:

rt-can-test --if can0 --rx

All received messages will be printed to stdout together with a time stamp.

It is important that the signal source (typically a CAN-dongle) transmits without any jitter.

Configure CAN interface

Use the tool ip from iproute2 and make sure the bitrate matches the CAN-dongle configuration:

ip link set can0 type can bitrate 500000
ip link set up can0

Build

Most likely you would want to cross compile the tool since not that many x86 PC's have native CAN interfaces.

Clone this repo, cd into the working dir and cross compile:

make CC=/path/to/my/cross-gcc

In case an explicit --sysroot is needed (the cross compiler was built without the --with-sysroot option), it can be passed with the EXTRA_CFLAGS variable:

make CC=/path/to/my/cross-gcc EXTRA_CFLAGS="--sysroot /path/to/cross/sysroot"