355 lines
7.5 KiB
Plaintext
355 lines
7.5 KiB
Plaintext
# CAN-J1939 on linux
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## CAN on linux
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See [Wikipedia:socketcan](http://en.wikipedia.org/wiki/Socketcan)
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## J1939 networking in short
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* Add addressing on top of CAN (destination address & broadcast)
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* Any (max 1780) length packets.
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Packets of 9 or more use **Transport Protocol** (fragmentation)
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Such packets use different CANid for the same PGN.
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* only **29**bit, non-**RTR** CAN frames
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* CAN id is composed of
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* 0..8: SA (source address)
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* 9..26:
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* PDU1: PGN+DA (destionation address)
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* PDU2: PGN
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* 27..29: PRIO
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* SA / DA may be dynamically assigned via j1939-81
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Fixed rules of precedence in Specification, no master necessary
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## J1939 on SocketCAN
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J1939 is *just another protocol* that fits
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in the Berkely sockets.
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socket(AF_CAN, SOCK_DGRAM, CAN_J1939)
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## differences from CAN_RAW
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### addressing
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SA, DA & PGN are used, not CAN id.
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Berkeley socket API is used to communicate these to userspace:
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* SA+PGN is put in sockname ([getsockname](http://man7.org/linux/man-pages/man2/getsockname.2.html))
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* DA+PGN is put in peername ([getpeername](http://man7.org/linux/man-pages/man2/getpeername.2.html))
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PGN is put in both structs
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PRIO is a datalink property, and irrelevant for interpretation
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Therefore, PRIO is not in *sockname* or *peername*.
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The *data* that is [recv][recvfrom] or [send][sendto] is the real payload.
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Unlike CAN_RAW, where addressing info is data.
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### Packet size
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J1939 handles packets of 8+ bytes with **Transport Protocol** fragmentation transparently.
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No fixed data size is necessary.
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send(sock, data, 8, 0);
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will emit a single CAN frame.
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send(sock, data, 9, 0);
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will use fragementation, emitting 1+ CAN frames.
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# Enable j1939
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CAN has no protocol id field.
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Enabling protocols must be done manually
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### netlink
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ip link set can0 j1939 on
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### procfs for legacy kernel (2.6.25)
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This API is dropped for kernels with netlink support!
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echo can0 > /proc/net/can-j1939/net
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# Using J1939
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## BSD socket implementation
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* socket
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* bind / connect
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* recvfrom / sendto
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* getsockname / getpeername
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## Modified *struct sockaddr_can*
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struct sockaddr_can {
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sa_family_t can_family;
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int can_ifindex;
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union {
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struct {
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__u64 name;
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__u32 pgn;
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__u8 addr;
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} j1939;
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} can_addr;
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}
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* *can_addr.j1939.pgn* is PGN
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* *can_addr.j1939.addr* & *can_addr.j1939.name*
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determine the ECU
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* receiving address information,
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*addr* is always set,
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*name* is set when available.
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* When providing address information,
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*name* != 0 indicates dynamic addressing
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## iproute2
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### Static addressing
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ip addr add j1939 0x80 dev can0
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### Dynamic addressing
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ip addr add j1939 name 0x012345678abcdef dev can0
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# Kickstart guide to j1939 on linux
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## Prepare using VCAN
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You may skip this step entirely if you have a functional
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**can0** bus on your system.
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Load module, when *vcan* is not in-kernel
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modprobe vcan
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Create a virtual can0 device and start the device
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ip link add can0 type vcan
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ip link set can0 up
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## First steps with j1939
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Use [testj1939](testj1939.c)
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When *can-j1939* is compiled as module, load it.
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modprobe can-j1939
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Enable the j1939 protocol stack on the CAN device
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ip link set can0 j1939 on
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Most of the subsequent examples will use 2 sockets programs (in 2 terminals).
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One will use CAN_J1939 sockets using *testj1939*,
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and the other will use CAN_RAW sockets using cansend+candump.
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testj1939 can be told to print the used API calls by adding **-v** program argument.
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### receive without source address
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Do in terminal 1
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./testj1939 -r can0:
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Send raw CAN in terminal 2
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cansend can0 1823ff40#0123
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You should have this output in terminal 1
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40 02300: 01 23
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This means, from NAME 0, SA 40, PGN 02300 was received,
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with 2 databytes, *01* & *02*.
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now emit this CAN message:
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cansend can0 18234140#0123
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In J1939, this means that ECU 0x40 sends directly to ECU 0x41
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Since we did not bind to address 0x41, this traffic
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is not meant for us and *testj1939* does not receive it.
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### Use source address
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./testj1939 can0:0x80
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will say
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./testj1939: bind(): Cannot assign requested address
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Since J1939 maintains addressing, **0x80** has not yet been assigned
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as an address on **can0** . This behaviour is very similar to IP
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addressing: you cannot bind to an address that is not your own.
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Now tell the kernel that we *own* address 0x80.
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It will be available from now on.
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ip addr add j1939 0x80 dev can0
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./testj1939 can0:0x80
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now succeeds.
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### receive with source address
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Terminal 1:
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./testj1939 -r can0:0x80
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Terminal 2:
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cansend can0 18238040#0123
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Will emit this output
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40 02300: 01 23
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This is because the traffic had destination address __0x80__ .
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### send
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Open in terminal 1:
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candump -L can0
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And to these test in another terminal
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./testj1939 -s can0:0x80
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This produces **1BFFFF80#0123456789ABCDEF** on CAN.
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./testj1939 -s can0:
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will produce exactly the same because **0x80** is the only
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address currently assigned to **can0:** and is used by default.
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### Multiple source addresses on 1 CAN device
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ip addr add j1939 0x90 dev can0
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./testj1939 -s can0:0x90
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produces **1BFFFF90#0123456789ABCDEF** ,
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./testj1939 -s can0:
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still produces **1BFFFF80#0123456789ABCDEF** , since **0x80**
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is the default _source address_.
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Check
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ip addr show can0
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emits
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X: can0: <NOARP,UP,LOWER_UP> mtu 16 qdisc noqueue state UNKNOWN
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link/can
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can-j1939 0x80 scope link
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can-j1939 0x90 scope link
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0x80 is the first address on can0.
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### Use specific PGN
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./testj1939 -s can0:,0x12345
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emits **1923FF80#0123456789ABCDEF** .
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Note that the real PGN is **0x12300**, and destination address is **0xff**.
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### Emit destination specific packets
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The destination field may be set during sendto().
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*testj1939* implements that like this
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./testj1939 -s can0:,0x12345 can0:0x40
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emits **19234080#0123456789ABCDEF** .
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The destination CAN iface __must__ always match the source CAN iface.
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Specifing one during bind is therefore sufficient.
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./testj1939 -s can0:,0x12300 :0x40
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emits the very same.
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### Emit different PGNs using the same socket
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The PGN is provided in both __bind( *sockname* )__ and
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__sendto( *peername* )__ , and only one is used.
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*peername* PGN has highest precedence.
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For broadcasted transmissions
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./testj1939 -s can0:,0x12300 :,0x32100
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emits **1B21FF80#0123456789ABCDEF** rather than 1923FF80#012345678ABCDEF
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Desitination specific transmissions
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./testj1939 -s can0:,0x12300 :0x40,0x32100
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emits **1B214080#0123456789ABCDEF** .
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It makes sometimes sense to omit the PGN in __bind( *sockname* )__ .
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### Larger packets
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J1939 transparently switches to *Transport Protocol* when packets
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do not fit into single CAN packets.
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./testj1939 -s20 can0:0x80 :,0x12300
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emits:
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18ECFF80#20140003FF002301
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18EBFF80#010123456789ABCD
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18EBFF80#02EF0123456789AB
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18EBFF80#03CDEF01234567
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The fragments for broadcasted *Transport Protocol* are seperated
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__50ms__ from each other.
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Destination specific *Transport Protocol* applies flow control
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and may emit CAN packets much faster.
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./testj1939 -s20 can0:0x80 :0x90,0x12300
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emits:
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18EC9080#1014000303002301
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18EC8090#110301FFFF002301
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18EB9080#010123456789ABCD
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18EB9080#02EF0123456789AB
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18EB9080#03CDEF01234567
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18EC8090#13140003FF002301
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The flow control causes a bit overhead.
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This overhead scales very good for larger J1939 packets.
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## Advanced topics with j1939
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### Change priority of J1939 packets
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./testj1939 -s can0:0x80,0x0100
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./testj1939 -s -p3 can0:0x80,0x0200
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emits
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1801FF80#0123456789ABCDEF
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0C02FF80#0123456789ABCDEF
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### using connect
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### advanced filtering
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## dynamic addressing
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