423 lines
15 KiB
ReStructuredText
423 lines
15 KiB
ReStructuredText
.. SPDX-License-Identifier: (GPL-2.0 OR MIT)
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===================
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J1939 Documentation
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===================
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Overview / What Is J1939
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========================
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SAE J1939 defines a higher layer protocol on CAN. It implements a more
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sophisticated addressing scheme and extends the maximum packet size above 8
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bytes. Several derived specifications exist, which differ from the original
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J1939 on the application level, like MilCAN A, NMEA2000 and especially
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ISO-11783 (ISOBUS). This last one specifies the so-called ETP (Extended
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Transport Protocol) which is has been included in this implementation. This
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results in a maximum packet size of ((2 ^ 24) - 1) * 7 bytes == 111 MiB.
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Specifications used
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-------------------
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* SAE J1939-21 : data link layer
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* SAE J1939-81 : network management
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* ISO 11783-6 : Virtual Terminal (Extended Transport Protocol)
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.. _j1939-motivation:
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Motivation
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==========
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Given the fact there's something like SocketCAN with an API similar to BSD
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sockets, we found some reasons to justify a kernel implementation for the
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addressing and transport methods used by J1939.
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* **Addressing:** when a process on an ECU communicates via J1939, it should
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not necessarily know its source address. Although at least one process per
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ECU should know the source address. Other processes should be able to reuse
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that address. This way, address parameters for different processes
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cooperating for the same ECU, are not duplicated. This way of working is
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closely related to the UNIX concept where programs do just one thing, and do
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it well.
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* **Dynamic addressing:** Address Claiming in J1939 is time critical.
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Furthermore data transport should be handled properly during the address
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negotiation. Putting this functionality in the kernel eliminates it as a
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requirement for _every_ user space process that communicates via J1939. This
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results in a consistent J1939 bus with proper addressing.
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* **Transport:** both TP & ETP reuse some PGNs to relay big packets over them.
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Different processes may thus use the same TP & ETP PGNs without actually
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knowing it. The individual TP & ETP sessions _must_ be serialized
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(synchronized) between different processes. The kernel solves this problem
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properly and eliminates the serialization (synchronization) as a requirement
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for _every_ user space process that communicates via J1939.
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J1939 defines some other features (relaying, gateway, fast packet transport,
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...). In-kernel code for these would not contribute to protocol stability.
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Therefore, these parts are left to user space.
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The J1939 sockets operate on CAN network devices (see SocketCAN). Any J1939
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user space library operating on CAN raw sockets will still operate properly.
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Since such library does not communicate with the in-kernel implementation, care
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must be taken that these two do not interfere. In practice, this means they
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cannot share ECU addresses. A single ECU (or virtual ECU) address is used by
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the library exclusively, or by the in-kernel system exclusively.
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J1939 concepts
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==============
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PGN
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---
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The PGN (Parameter Group Number) is a number to identify a packet. The PGN
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is composed as follows:
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1 bit : Reserved Bit
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1 bit : Data Page
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8 bits : PF (PDU Format)
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8 bits : PS (PDU Specific)
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In J1939-21 distinction is made between PDU1 format (where PF < 240) and PDU2
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format (where PF >= 240). Furthermore, when using PDU2 format, the PS-field
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contains a so-called Group Extension, which is part of the PGN. When using PDU2
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format, the Group Extension is set in the PS-field.
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On the other hand, when using PDU1 format, the PS-field contains a so-called
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Destination Address, which is _not_ part of the PGN. When communicating a PGN
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from user space to kernel (or visa versa) and PDU2 format is used, the PS-field
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of the PGN shall be set to zero. The Destination Address shall be set
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elsewhere.
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Regarding PGN mapping to 29-bit CAN identifier, the Destination Address shall
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be get/set from/to the appropriate bits of the identifier by the kernel.
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Addressing
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----------
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Both static and dynamic addressing methods can be used.
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For static addresses, no extra checks are made by the kernel, and provided
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addresses are considered right. This responsibility is for the OEM or system
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integrator.
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For dynamic addressing, so-called Address Claiming, extra support is foreseen
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in the kernel. In J1939 any ECU is known by it's 64-bit NAME. At the moment of
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a successful address claim, the kernel keeps track of both NAME and source
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address being claimed. This serves as a base for filter schemes. By default,
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packets with a destination that is not locally, will be rejected.
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Mixed mode packets (from a static to a dynamic address or vice versa) are
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allowed. The BSD sockets define separate API calls for getting/setting the
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local & remote address and are applicable for J1939 sockets.
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Filtering
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---------
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J1939 defines white list filters per socket that a user can set in order to
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receive a subset of the J1939 traffic. Filtering can be based on:
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* SA
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* SOURCE_NAME
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* PGN
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When multiple filters are in place for a single socket, and a packet comes in
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that matches several of those filters, the packet is only received once for
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that socket.
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How to Use J1939
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================
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API Calls
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---------
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On CAN, you first need to open a socket for communicating over a CAN network.
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To use J1939, #include <linux/can/j1939.h>. From there, <linux/can.h> will be
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included too. To open a socket, use:
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.. code-block:: C
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s = socket(PF_CAN, SOCK_DGRAM, CAN_J1939);
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J1939 does use SOCK_DGRAM sockets. In the J1939 specification, connections are
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mentioned in the context of transport protocol sessions. These still deliver
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packets to the other end (using several CAN packets). SOCK_STREAM is not
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supported.
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After the successful creation of the socket, you would normally use the bind(2)
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and/or connect(2) system call to bind the socket to a CAN interface. After
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binding and/or connecting the socket, you can read(2) and write(2) from/to the
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socket or use send(2), sendto(2), sendmsg(2) and the recv*() counterpart
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operations on the socket as usual. There are also J1939 specific socket options
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described below.
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In order to send data, a bind(2) must have been successful. bind(2) assigns a
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local address to a socket.
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Different from CAN is that the payload data is just the data that get send,
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without it's header info. The header info is derived from the sockaddr supplied
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to bind(2), connect(2), sendto(2) and recvfrom(2). A write(2) with size 4 will
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result in a packet with 4 bytes.
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The sockaddr structure has extensions for use with J1939 as specified below:
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.. code-block:: C
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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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/* pgn:
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* 8 bit: PS in PDU2 case, else 0
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* 8 bit: PF
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* 1 bit: DP
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* 1 bit: reserved
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*/
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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_family & can_ifindex serve the same purpose as for other SocketCAN sockets.
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can_addr.j1939.pgn specifies the PGN (max 0x3ffff). Individual bits are
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specified above.
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can_addr.j1939.name contains the 64-bit J1939 NAME.
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can_addr.j1939.addr contains the address.
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The bind(2) system call assigns the local address, i.e. the source address when
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sending packages. If a PGN during bind(2) is set, it's used as a RX filter.
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I.e. only packets with a matching PGN are received. If an ADDR or NAME is set
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it is used as a receive filter, too. It will match the destination NAME or ADDR
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of the incoming packet. The NAME filter will work only if appropriate Address
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Claiming for this name was done on the CAN bus and registered/cached by the
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kernel.
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On the other hand connect(2) assigns the remote address, i.e. the destination
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address. The PGN from connect(2) is used as the default PGN when sending
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packets. If ADDR or NAME is set it will be used as the default destination ADDR
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or NAME. Further a set ADDR or NAME during connect(2) is used as a receive
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filter. It will match the source NAME or ADDR of the incoming packet.
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Both write(2) and send(2) will send a packet with local address from bind(2) and
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the remote address from connect(2). Use sendto(2) to overwrite the destination
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address.
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If can_addr.j1939.name is set (!= 0) the NAME is looked up by the kernel and
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the corresponding ADDR is used. If can_addr.j1939.name is not set (== 0),
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can_addr.j1939.addr is used.
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When creating a socket, reasonable defaults are set. Some options can be
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modified with setsockopt(2) & getsockopt(2).
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RX path related options:
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- SO_J1939_FILTER - configure array of filters
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- SO_J1939_PROMISC - disable filters set by bind(2) and connect(2)
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By default no broadcast packets can be send or received. To enable sending or
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receiving broadcast packets use the socket option SO_BROADCAST:
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.. code-block:: C
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int value = 1;
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setsockopt(sock, SOL_SOCKET, SO_BROADCAST, &value, sizeof(value));
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The following diagram illustrates the RX path:
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.. code::
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+--------------------+
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| incoming packet |
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+--------------------+
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V
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+--------------------+
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| SO_J1939_PROMISC? |
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+--------------------+
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no | | yes
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.---------' `---------.
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+---------------------------+ |
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| bind() + connect() + | |
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| SOCK_BROADCAST filter | |
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+---------------------------+ |
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|<---------------------'
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V
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+---------------------------+
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| SO_J1939_FILTER |
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+---------------------------+
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V
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+---------------------------+
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| socket recv() |
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+---------------------------+
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TX path related options:
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SO_J1939_SEND_PRIO - change default send priority for the socket
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Message Flags during send() and Related System Calls
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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send(2), sendto(2) and sendmsg(2) take a 'flags' argument. Currently
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supported flags are:
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* MSG_DONTWAIT, i.e. non-blocking operation.
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recvmsg(2)
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^^^^^^^^^^
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In most cases recvmsg(2) is needed if you want to extract more information than
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recvfrom(2) can provide. For example package priority and timestamp. The
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Destination Address, name and packet priority (if applicable) are attached to
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the msghdr in the recvmsg(2) call. They can be extracted using cmsg(3) macros,
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with cmsg_level == SOL_J1939 && cmsg_type == SCM_J1939_DEST_ADDR,
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SCM_J1939_DEST_NAME or SCM_J1939_PRIO. The returned data is a uint8_t for
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priority and dst_addr, and uint64_t for dst_name.
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.. code-block:: C
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uint8_t priority, dst_addr;
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uint64_t dst_name;
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for (cmsg = CMSG_FIRSTHDR(&msg); cmsg; cmsg = CMSG_NXTHDR(&msg, cmsg)) {
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switch (cmsg->cmsg_level) {
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case SOL_CAN_J1939:
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if (cmsg->cmsg_type == SCM_J1939_DEST_ADDR)
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dst_addr = *CMSG_DATA(cmsg);
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else if (cmsg->cmsg_type == SCM_J1939_DEST_NAME)
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memcpy(&dst_name, CMSG_DATA(cmsg), cmsg->cmsg_len - CMSG_LEN(0));
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else if (cmsg->cmsg_type == SCM_J1939_PRIO)
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priority = *CMSG_DATA(cmsg);
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break;
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}
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}
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Dynamic Addressing
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------------------
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Distinction has to be made between using the claimed address and doing an
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address claim. To use an already claimed address, one has to fill in the
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j1939.name member and provide it to bind(2). If the name had claimed an address
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earlier, all further messages being sent will use that address. And the
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j1939.addr member will be ignored.
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An exception on this is PGN 0x0ee00. This is the "Address Claim/Cannot Claim
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Address" message and the kernel will use the j1939.addr member for that PGN if
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necessary.
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To claim an address following code example can be used:
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.. code-block:: C
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struct sockaddr_can baddr = {
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.can_family = AF_CAN,
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.can_addr.j1939 = {
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.name = name,
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.addr = J1939_IDLE_ADDR,
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.pgn = J1939_NO_PGN, /* to disable bind() rx filter for PGN */
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},
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.can_ifindex = if_nametoindex("can0"),
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};
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bind(sock, (struct sockaddr *)&baddr, sizeof(baddr));
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/* for Address Claiming broadcast must be allowed */
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int value = 1;
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setsockopt(sock, SOL_SOCKET, SO_BROADCAST, &value, sizeof(value));
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/* configured advanced RX filter with PGN needed for Address Claiming */
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const struct j1939_filter filt[] = {
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{
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.pgn = J1939_PGN_ADDRESS_CLAIMED,
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.pgn_mask = J1939_PGN_PDU1_MAX,
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}, {
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.pgn = J1939_PGN_REQUEST,
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.pgn_mask = J1939_PGN_PDU1_MAX,
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}, {
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.pgn = J1939_PGN_ADDRESS_COMMANDED,
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.pgn_mask = J1939_PGN_MAX,
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},
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};
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setsockopt(sock, SOL_CAN_J1939, SO_J1939_FILTER, &filt, sizeof(filt));
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uint64_t dat = htole64(name);
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const struct sockaddr_can saddr = {
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.can_family = AF_CAN,
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.can_addr.j1939 = {
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.pgn = J1939_PGN_ADDRESS_CLAIMED,
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.addr = J1939_NO_ADDR,
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},
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};
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/* Afterwards do a sendto(2) with data set to the NAME (Little Endian). If the
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* NAME provided, does not match the j1939.name provided to bind(2), EPROTO
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* will be returned.
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*/
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sendto(sock, dat, sizeof(dat), 0, (const struct sockaddr *)&saddr, sizeof(saddr));
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If no-one else contests the address claim within 250ms after transmission, the
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kernel marks the NAME-SA assignment as valid. The valid assignment will be kept
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among other valid NAME-SA assignments. From that point, any socket bound to the
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NAME can send packets.
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If another ECU claims the address, the kernel will mark the NAME-SA expired.
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No socket bound to the NAME can send packets (other than address claims). To
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claim another address, some socket bound to NAME, must bind(2) again, but with
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only j1939.addr changed to the new SA, and must then send a valid address claim
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packet. This restarts the state machine in the kernel (and any other
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participant on the bus) for this NAME.
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can-utils also include the jacd tool, so it can be used as code example or as
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default Address Claiming daemon.
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Send Examples
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-------------
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Static Addressing
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^^^^^^^^^^^^^^^^^
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This example will send a PGN (0x12300) from SA 0x20 to DA 0x30.
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Bind:
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.. code-block:: C
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struct sockaddr_can baddr = {
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.can_family = AF_CAN,
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.can_addr.j1939 = {
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.name = J1939_NO_NAME,
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.addr = 0x20,
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.pgn = J1939_NO_PGN,
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},
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.can_ifindex = if_nametoindex("can0"),
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};
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bind(sock, (struct sockaddr *)&baddr, sizeof(baddr));
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Now, the socket 'sock' is bound to the SA 0x20. Since no connect(2) was called,
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at this point we can use only sendto(2) or sendmsg(2).
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Send:
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.. code-block:: C
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const struct sockaddr_can saddr = {
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.can_family = AF_CAN,
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.can_addr.j1939 = {
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.name = J1939_NO_NAME;
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.addr = 0x30,
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.pgn = 0x12300,
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},
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};
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sendto(sock, dat, sizeof(dat), 0, (const struct sockaddr *)&saddr, sizeof(saddr));
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