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diff --git a/Documentation/networking/j1939.rst b/Documentation/networking/j1939.rst new file mode 100644 index 000000000000..ce7e7a044e08 --- /dev/null +++ b/Documentation/networking/j1939.rst @@ -0,0 +1,422 @@ +.. SPDX-License-Identifier: (GPL-2.0 OR MIT) + +=================== +J1939 Documentation +=================== + +Overview / What Is J1939 +======================== + +SAE J1939 defines a higher layer protocol on CAN. It implements a more +sophisticated addressing scheme and extends the maximum packet size above 8 +bytes. Several derived specifications exist, which differ from the original +J1939 on the application level, like MilCAN A, NMEA2000 and especially +ISO-11783 (ISOBUS). This last one specifies the so-called ETP (Extended +Transport Protocol) which is has been included in this implementation. This +results in a maximum packet size of ((2 ^ 24) - 1) * 7 bytes == 111 MiB. + +Specifications used +------------------- + +* SAE J1939-21 : data link layer +* SAE J1939-81 : network management +* ISO 11783-6 : Virtual Terminal (Extended Transport Protocol) + +.. _j1939-motivation: + +Motivation +========== + +Given the fact there's something like SocketCAN with an API similar to BSD +sockets, we found some reasons to justify a kernel implementation for the +addressing and transport methods used by J1939. + +* **Addressing:** when a process on an ECU communicates via J1939, it should + not necessarily know its source address. Although at least one process per + ECU should know the source address. Other processes should be able to reuse + that address. This way, address parameters for different processes + cooperating for the same ECU, are not duplicated. This way of working is + closely related to the UNIX concept where programs do just one thing, and do + it well. + +* **Dynamic addressing:** Address Claiming in J1939 is time critical. + Furthermore data transport should be handled properly during the address + negotiation. Putting this functionality in the kernel eliminates it as a + requirement for _every_ user space process that communicates via J1939. This + results in a consistent J1939 bus with proper addressing. + +* **Transport:** both TP & ETP reuse some PGNs to relay big packets over them. + Different processes may thus use the same TP & ETP PGNs without actually + knowing it. The individual TP & ETP sessions _must_ be serialized + (synchronized) between different processes. The kernel solves this problem + properly and eliminates the serialization (synchronization) as a requirement + for _every_ user space process that communicates via J1939. + +J1939 defines some other features (relaying, gateway, fast packet transport, +...). In-kernel code for these would not contribute to protocol stability. +Therefore, these parts are left to user space. + +The J1939 sockets operate on CAN network devices (see SocketCAN). Any J1939 +user space library operating on CAN raw sockets will still operate properly. +Since such library does not communicate with the in-kernel implementation, care +must be taken that these two do not interfere. In practice, this means they +cannot share ECU addresses. A single ECU (or virtual ECU) address is used by +the library exclusively, or by the in-kernel system exclusively. + +J1939 concepts +============== + +PGN +--- + +The PGN (Parameter Group Number) is a number to identify a packet. The PGN +is composed as follows: +1 bit : Reserved Bit +1 bit : Data Page +8 bits : PF (PDU Format) +8 bits : PS (PDU Specific) + +In J1939-21 distinction is made between PDU1 format (where PF < 240) and PDU2 +format (where PF >= 240). Furthermore, when using PDU2 format, the PS-field +contains a so-called Group Extension, which is part of the PGN. When using PDU2 +format, the Group Extension is set in the PS-field. + +On the other hand, when using PDU1 format, the PS-field contains a so-called +Destination Address, which is _not_ part of the PGN. When communicating a PGN +from user space to kernel (or visa versa) and PDU2 format is used, the PS-field +of the PGN shall be set to zero. The Destination Address shall be set +elsewhere. + +Regarding PGN mapping to 29-bit CAN identifier, the Destination Address shall +be get/set from/to the appropriate bits of the identifier by the kernel. + + +Addressing +---------- + +Both static and dynamic addressing methods can be used. + +For static addresses, no extra checks are made by the kernel, and provided +addresses are considered right. This responsibility is for the OEM or system +integrator. + +For dynamic addressing, so-called Address Claiming, extra support is foreseen +in the kernel. In J1939 any ECU is known by it's 64-bit NAME. At the moment of +a successful address claim, the kernel keeps track of both NAME and source +address being claimed. This serves as a base for filter schemes. By default, +packets with a destination that is not locally, will be rejected. + +Mixed mode packets (from a static to a dynamic address or vice versa) are +allowed. The BSD sockets define separate API calls for getting/setting the +local & remote address and are applicable for J1939 sockets. + +Filtering +--------- + +J1939 defines white list filters per socket that a user can set in order to +receive a subset of the J1939 traffic. Filtering can be based on: + +* SA +* SOURCE_NAME +* PGN + +When multiple filters are in place for a single socket, and a packet comes in +that matches several of those filters, the packet is only received once for +that socket. + +How to Use J1939 +================ + +API Calls +--------- + +On CAN, you first need to open a socket for communicating over a CAN network. +To use J1939, #include <linux/can/j1939.h>. From there, <linux/can.h> will be +included too. To open a socket, use: + +.. code-block:: C + + s = socket(PF_CAN, SOCK_DGRAM, CAN_J1939); + +J1939 does use SOCK_DGRAM sockets. In the J1939 specification, connections are +mentioned in the context of transport protocol sessions. These still deliver +packets to the other end (using several CAN packets). SOCK_STREAM is not +supported. + +After the successful creation of the socket, you would normally use the bind(2) +and/or connect(2) system call to bind the socket to a CAN interface. After +binding and/or connecting the socket, you can read(2) and write(2) from/to the +socket or use send(2), sendto(2), sendmsg(2) and the recv*() counterpart +operations on the socket as usual. There are also J1939 specific socket options +described below. + +In order to send data, a bind(2) must have been successful. bind(2) assigns a +local address to a socket. + +Different from CAN is that the payload data is just the data that get send, +without it's header info. The header info is derived from the sockaddr supplied +to bind(2), connect(2), sendto(2) and recvfrom(2). A write(2) with size 4 will +result in a packet with 4 bytes. + +The sockaddr structure has extensions for use with J1939 as specified below: + +.. code-block:: C + + struct sockaddr_can { + sa_family_t can_family; + int can_ifindex; + union { + struct { + __u64 name; + /* pgn: + * 8 bit: PS in PDU2 case, else 0 + * 8 bit: PF + * 1 bit: DP + * 1 bit: reserved + */ + __u32 pgn; + __u8 addr; + } j1939; + } can_addr; + } + +can_family & can_ifindex serve the same purpose as for other SocketCAN sockets. + +can_addr.j1939.pgn specifies the PGN (max 0x3ffff). Individual bits are +specified above. + +can_addr.j1939.name contains the 64-bit J1939 NAME. + +can_addr.j1939.addr contains the address. + +The bind(2) system call assigns the local address, i.e. the source address when +sending packages. If a PGN during bind(2) is set, it's used as a RX filter. +I.e. only packets with a matching PGN are received. If an ADDR or NAME is set +it is used as a receive filter, too. It will match the destination NAME or ADDR +of the incoming packet. The NAME filter will work only if appropriate Address +Claiming for this name was done on the CAN bus and registered/cached by the +kernel. + +On the other hand connect(2) assigns the remote address, i.e. the destination +address. The PGN from connect(2) is used as the default PGN when sending +packets. If ADDR or NAME is set it will be used as the default destination ADDR +or NAME. Further a set ADDR or NAME during connect(2) is used as a receive +filter. It will match the source NAME or ADDR of the incoming packet. + +Both write(2) and send(2) will send a packet with local address from bind(2) and +the remote address from connect(2). Use sendto(2) to overwrite the destination +address. + +If can_addr.j1939.name is set (!= 0) the NAME is looked up by the kernel and +the corresponding ADDR is used. If can_addr.j1939.name is not set (== 0), +can_addr.j1939.addr is used. + +When creating a socket, reasonable defaults are set. Some options can be +modified with setsockopt(2) & getsockopt(2). + +RX path related options: + +- SO_J1939_FILTER - configure array of filters +- SO_J1939_PROMISC - disable filters set by bind(2) and connect(2) + +By default no broadcast packets can be send or received. To enable sending or +receiving broadcast packets use the socket option SO_BROADCAST: + +.. code-block:: C + + int value = 1; + setsockopt(sock, SOL_SOCKET, SO_BROADCAST, &value, sizeof(value)); + +The following diagram illustrates the RX path: + +.. code:: + + +--------------------+ + | incoming packet | + +--------------------+ + | + V + +--------------------+ + | SO_J1939_PROMISC? | + +--------------------+ + | | + no | | yes + | | + .---------' `---------. + | | + +---------------------------+ | + | bind() + connect() + | | + | SOCK_BROADCAST filter | | + +---------------------------+ | + | | + |<---------------------' + V + +---------------------------+ + | SO_J1939_FILTER | + +---------------------------+ + | + V + +---------------------------+ + | socket recv() | + +---------------------------+ + +TX path related options: +SO_J1939_SEND_PRIO - change default send priority for the socket + +Message Flags during send() and Related System Calls +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +send(2), sendto(2) and sendmsg(2) take a 'flags' argument. Currently +supported flags are: + +* MSG_DONTWAIT, i.e. non-blocking operation. + +recvmsg(2) +^^^^^^^^^ + +In most cases recvmsg(2) is needed if you want to extract more information than +recvfrom(2) can provide. For example package priority and timestamp. The +Destination Address, name and packet priority (if applicable) are attached to +the msghdr in the recvmsg(2) call. They can be extracted using cmsg(3) macros, +with cmsg_level == SOL_J1939 && cmsg_type == SCM_J1939_DEST_ADDR, +SCM_J1939_DEST_NAME or SCM_J1939_PRIO. The returned data is a uint8_t for +priority and dst_addr, and uint64_t for dst_name. + +.. code-block:: C + + uint8_t priority, dst_addr; + uint64_t dst_name; + + for (cmsg = CMSG_FIRSTHDR(&msg); cmsg; cmsg = CMSG_NXTHDR(&msg, cmsg)) { + switch (cmsg->cmsg_level) { + case SOL_CAN_J1939: + if (cmsg->cmsg_type == SCM_J1939_DEST_ADDR) + dst_addr = *CMSG_DATA(cmsg); + else if (cmsg->cmsg_type == SCM_J1939_DEST_NAME) + memcpy(&dst_name, CMSG_DATA(cmsg), cmsg->cmsg_len - CMSG_LEN(0)); + else if (cmsg->cmsg_type == SCM_J1939_PRIO) + priority = *CMSG_DATA(cmsg); + break; + } + } + +Dynamic Addressing +------------------ + +Distinction has to be made between using the claimed address and doing an +address claim. To use an already claimed address, one has to fill in the +j1939.name member and provide it to bind(2). If the name had claimed an address +earlier, all further messages being sent will use that address. And the +j1939.addr member will be ignored. + +An exception on this is PGN 0x0ee00. This is the "Address Claim/Cannot Claim +Address" message and the kernel will use the j1939.addr member for that PGN if +necessary. + +To claim an address following code example can be used: + +.. code-block:: C + + struct sockaddr_can baddr = { + .can_family = AF_CAN, + .can_addr.j1939 = { + .name = name, + .addr = J1939_IDLE_ADDR, + .pgn = J1939_NO_PGN, /* to disable bind() rx filter for PGN */ + }, + .can_ifindex = if_nametoindex("can0"), + }; + + bind(sock, (struct sockaddr *)&baddr, sizeof(baddr)); + + /* for Address Claiming broadcast must be allowed */ + int value = 1; + setsockopt(sock, SOL_SOCKET, SO_BROADCAST, &value, sizeof(value)); + + /* configured advanced RX filter with PGN needed for Address Claiming */ + const struct j1939_filter filt[] = { + { + .pgn = J1939_PGN_ADDRESS_CLAIMED, + .pgn_mask = J1939_PGN_PDU1_MAX, + }, { + .pgn = J1939_PGN_ADDRESS_REQUEST, + .pgn_mask = J1939_PGN_PDU1_MAX, + }, { + .pgn = J1939_PGN_ADDRESS_COMMANDED, + .pgn_mask = J1939_PGN_MAX, + }, + }; + + setsockopt(sock, SOL_CAN_J1939, SO_J1939_FILTER, &filt, sizeof(filt)); + + uint64_t dat = htole64(name); + const struct sockaddr_can saddr = { + .can_family = AF_CAN, + .can_addr.j1939 = { + .pgn = J1939_PGN_ADDRESS_CLAIMED, + .addr = J1939_NO_ADDR, + }, + }; + + /* Afterwards do a sendto(2) with data set to the NAME (Little Endian). If the + * NAME provided, does not match the j1939.name provided to bind(2), EPROTO + * will be returned. + */ + sendto(sock, dat, sizeof(dat), 0, (const struct sockaddr *)&saddr, sizeof(saddr)); + +If no-one else contests the address claim within 250ms after transmission, the +kernel marks the NAME-SA assignment as valid. The valid assignment will be kept +among other valid NAME-SA assignments. From that point, any socket bound to the +NAME can send packets. + +If another ECU claims the address, the kernel will mark the NAME-SA expired. +No socket bound to the NAME can send packets (other than address claims). To +claim another address, some socket bound to NAME, must bind(2) again, but with +only j1939.addr changed to the new SA, and must then send a valid address claim +packet. This restarts the state machine in the kernel (and any other +participant on the bus) for this NAME. + +can-utils also include the jacd tool, so it can be used as code example or as +default Address Claiming daemon. + +Send Examples +------------- + +Static Addressing +^^^^^^^^^^^^^^^^^ + +This example will send a PGN (0x12300) from SA 0x20 to DA 0x30. + +Bind: + +.. code-block:: C + + struct sockaddr_can baddr = { + .can_family = AF_CAN, + .can_addr.j1939 = { + .name = J1939_NO_NAME, + .addr = 0x20, + .pgn = J1939_NO_PGN, + }, + .can_ifindex = if_nametoindex("can0"), + }; + + bind(sock, (struct sockaddr *)&baddr, sizeof(baddr)); + +Now, the socket 'sock' is bound to the SA 0x20. Since no connect(2) was called, +at this point we can use only sendto(2) or sendmsg(2). + +Send: + +.. code-block:: C + + const struct sockaddr_can saddr = { + .can_family = AF_CAN, + .can_addr.j1939 = { + .name = J1939_NO_NAME; + .pgn = 0x30, + .addr = 0x12300, + }, + }; + + sendto(sock, dat, sizeof(dat), 0, (const struct sockaddr *)&saddr, sizeof(saddr)); |