99 lines
2.3 KiB
Markdown
99 lines
2.3 KiB
Markdown
# CAN-J1939 on linux
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The [Kickstart guide is here](can-j1939-kickstart.md)
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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 (destination 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 fragmentation, emitting 1+ CAN frames.
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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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