- Qt 5.15
- Qt Serial Bus
- Using SocketCAN Plugin
- Qt 5.15.19 Reference Documentation

# Using SocketCAN Plugin

The SocketCAN plugin encapsulates the Linux sockets API for accessing the CAN devices. This API is a set of open source CAN drivers and a networking stack contributed by Volkswagen Research to the Linux kernel.

This plugin requires a Linux Kernel with SocketCAN support and SocketCAN device drivers for the used CAN hardware.

## SocketCAN usage

To list all (including unconfigured) network interfaces, the command `ifconfig -a` can be used.

To use SocketCAN, the corresponding Linux Kernel modules must be loaded and the network interface must be configured.

### Setting up real CAN hardware

This section assumes, that the device driver is already loaded (most likely automatically when connecting the CAN hardware).

#### Default settings

To set the device can0 to a bitrate of 250 kBit/s:

     sudo ip link set up can0 type can bitrate 250000

To automatically recover from "bus off" errors after 100 milliseconds, the following command can be used:

     sudo ip link set up can0 type can bitrate 250000 restart-ms 100

#### CAN FD settings

To set the device can0 to an arbitration bitrate of 500 kBit/s and a data bitrate of 4 MBit/s (for frames with bitrate switch flag):

     sudo ip link set can0 up type can bitrate 500000 dbitrate 4000000 fd on

### Setting up a virtual CAN bus

Note: For CAN FD usage, the MTU (Maximum Transmission Unit) has to be set to 72 byte.

     sudo modprobe vcan
     sudo ip link add dev vcan0 type vcan
     sudo ip link set up vcan0 mtu 72

The command line test programs used in the following are from the can-utils package:

     # Display received CAN messages with absolute timestamps and flags
     candump -ta -x vcan0

     # Send a CAN FD message with flags BRS and EFI set
     cansend vcan0 123##3112233445566778899aabbccddeeff

     # Generate random CAN messages
     cangen vcan0

## Creating CAN Bus Devices

At first it is necessary to check that QCanBus provides the desired plugin:

     if (QCanBus::instance()->plugins().contains(QStringLiteral("socketcan"))) {
         // plugin available
     }

Where socketcan is the plugin name.

Next, a connection to a specific interface can be established:

     QString errorString;
     QCanBusDevice *device = QCanBus::instance()->createDevice(
         QStringLiteral("socketcan"), QStringLiteral("can0"), &errorString);
     if (!device) {
         // Error handling goes here
         qDebug << errorString;
     } else {
         device->connectDevice();
     }

Where can0 is the active CAN interface name. CAN interfaces act like regular network interfaces on Linux systems and can be discovered using `ifconfig`. Also, the availableDevices() method returns a list of currently available devices.

The device is now open for writing and reading CAN frames:

     QCanBusFrame frame;
     frame.setFrameId(8);
     QByteArray payload("A36E");
     frame.setPayload(payload);
     device->writeFrame(frame);

The reading can be done using the readFrame() method. The framesReceived() signal is emitted when at least one new frame is available for reading:

     QCanBusFrame frame = device->readFrame();

SocketCAN supports the following configurations that can be controlled through setConfigurationParameter():

Configuration parameter key | Description
--- | ---
QCanBusDevice::LoopbackKey | To meet the multiple-user needs, the local loopback is enabled by default. This means, whenever a CAN frame is transmitted on the CAN bus, a local echo of this frame is sent to all applications connected to this CAN device. If this option is enabled, the therefore received frames are marked with QCanBusFrame::hasLocalEcho()
QCanBusDevice::ReceiveOwnKey | The reception of the CAN frames on the same socket that was sending the CAN frame is disabled by default. When enabling this option, all CAN frames sent to the CAN bus immediately appear in the receive buffer. This can be used to check if sending was successful. If this option is enabled, the therefore received frames are marked with QCanBusFrame::hasLocalEcho()
QCanBusDevice::ErrorFilterKey | A CAN interface driver can generate so called Error Message Frames that can optionally be passed to the user application in the same way as other CAN frames. The possible errors are divided into different error classes that may be filtered using the appropriate error mask. The values for the error mask are defined in `linux/can/error.h`.
QCanBusDevice::RawFilterKey | This configuration can contain multiple filters of type QCanBusDevice::Filter. By default, the connection is configured to accept any CAN bus message.
QCanBusDevice::BitRateKey | Determines the bit rate of the CAN bus connection. The following bit rates are supported: 5000, 10000, 20000, 33000, 47000, 50000, 83000, 95000, 100000, 125000, 250000, 500000, 800000, 1000000. Note that this configuration parameter can only be adjusted while the QCanBusDevice is not connected. To set this configuration parameter, the library libsocketcan is needed at runtime http://www.pengutronix.de/software/libsocketcan Usually, root rights are needed to set the CAN bus bitrate.
QCanBusDevice::CanFdKey | This configuration option determines whether CANFD frames may be sent or received. By default, this option is disabled. It controls the CAN_RAW_FD_FRAMES option of the CAN socket.
QCanBusDevice::DataBitRateKey | This configuration is not supported by the socketcan plugin. However it is possible to set the data rate when configuring the CAN network interface using the `ip link` command.
QCanBusDevice::ProtocolKey | Allows to use another protocol inside the protocol family PF_CAN. The default value for this configuration option is CAN_RAW (1).

For example:

     QList<QCanBusDevice::Filter> list;
     QCanBusDevice::Filter f;

     // only accept odd numbered frame id of type remote request
     // frame can utilize extended or base format
     f.frameId = 0x1;
     f.frameIdMask = 0x1;
     f.format = QCanBusDevice::Filter::MatchBaseAndExtendedFormat;
     f.type = QCanBusFrame::RemoteRequestFrame;
     list.append(f);

     device->setConfigurationParameter(QCanBusDevice::RawFilterKey, QVariant::fromValue(list));
     device->setConfigurationParameter(QCanBusDevice::ErrorFilterKey,
                                       QVariant::fromValue(QCanBusFrame::FrameErrors(QCanBusFrame::AnyError)));

Extended frame format and flexible data-rate are supported in SocketCAN.

SocketCAN supports the following additional functions:

- QCanBusDevice::resetController() (needs libsocketcan)
- QCanBusDevice::busStatus() (needs libsocketcan)