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+.. 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 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 a 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 the 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 vice 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 its 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 sends,
+without its 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_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 ``j1939acd`` 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;
+ .addr = 0x30,
+ .pgn = 0x12300,
+ },
+ };
+
+ sendto(sock, dat, sizeof(dat), 0, (const struct sockaddr *)&saddr, sizeof(saddr));
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