path: root/Documentation/networking/can.txt

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can.txt

Readme file for the Controller Area Network Protocol Family (aka Socket CAN)

This file contains

  1 Overview / What is Socket CAN

  2 Motivation / Why using the socket API

  3 Socket CAN concept
    3.1 receive lists
    3.2 local loopback of sent frames
    3.3 network security issues (capabilities)
    3.4 network problem notifications

  4 How to use Socket CAN
    4.1 RAW protocol sockets with can_filters (SOCK_RAW)
      4.1.1 RAW socket option CAN_RAW_FILTER
      4.1.2 RAW socket option CAN_RAW_ERR_FILTER
      4.1.3 RAW socket option CAN_RAW_LOOPBACK
      4.1.4 RAW socket option CAN_RAW_RECV_OWN_MSGS
      4.1.5 RAW socket option CAN_RAW_FD_FRAMES
      4.1.6 RAW socket returned message flags
    4.2 Broadcast Manager protocol sockets (SOCK_DGRAM)
      4.2.1 Broadcast Manager operations
      4.2.2 Broadcast Manager message flags
      4.2.3 Broadcast Manager transmission timers
      4.2.4 Broadcast Manager message sequence transmission
      4.2.5 Broadcast Manager receive filter timers
      4.2.6 Broadcast Manager multiplex message receive filter
    4.3 connected transport protocols (SOCK_SEQPACKET)
    4.4 unconnected transport protocols (SOCK_DGRAM)

  5 Socket CAN core module
    5.1 can.ko module params
    5.2 procfs content
    5.3 writing own CAN protocol modules

  6 CAN network drivers
    6.1 general settings
    6.2 local loopback of sent frames
    6.3 CAN controller hardware filters
    6.4 The virtual CAN driver (vcan)
    6.5 The CAN network device driver interface
      6.5.1 Netlink interface to set/get devices properties
      6.5.2 Setting the CAN bit-timing
      6.5.3 Starting and stopping the CAN network device
    6.6 CAN FD (flexible data rate) driver support
    6.7 supported CAN hardware

  7 Socket CAN resources

  8 Credits

============================================================================

1. Overview / What is Socket CAN
--------------------------------

The socketcan package is an implementation of CAN protocols
(Controller Area Network) for Linux.  CAN is a networking technology
which has widespread use in automation, embedded devices, and
automotive fields.  While there have been other CAN implementations
for Linux based on character devices, Socket CAN uses the Berkeley
socket API, the Linux network stack and implements the CAN device
drivers as network interfaces.  The CAN socket API has been designed
as similar as possible to the TCP/IP protocols to allow programmers,
familiar with network programming, to easily learn how to use CAN
sockets.

2. Motivation / Why using the socket API
----------------------------------------

There have been CAN implementations for Linux before Socket CAN so the
question arises, why we have started another project.  Most existing
implementations come as a device driver for some CAN hardware, they
are based on character devices and provide comparatively little
functionality.  Usually, there is only a hardware-specific device
driver which provides a character device interface to send and
receive raw CAN frames, directly to/from the controller hardware.
Queueing of frames and higher-level transport protocols like ISO-TP
have to be implemented in user space applications.  Also, most
character-device implementations support only one single process to
open the device at a time, similar to a serial interface.  Exchanging
the CAN controller requires employment of another device driver and
often the need for adaption of large parts of the application to the
new driver's API.

Socket CAN was designed to overcome all of these limitations.  A new
protocol family has been implemented which provides a socket interface
to user space applications and which builds upon the Linux network
layer, so to use all of the provided queueing functionality.  A device
driver for CAN controller hardware registers itself with the Linux
network layer as a network device, so that CAN frames from the
controller can be passed up to the network layer and on to the CAN
protocol family module and also vice-versa.  Also, the protocol family
module provides an API for transport protocol modules to register, so
that any number of transport protocols can be loaded or unloaded
dynamically.  In fact, the can core module alone does not provide any
protocol and cannot be used without loading at least one additional
protocol module.  Multiple sockets can be opened at the same time,
on different or the same protocol module and they can listen/send
frames on different or the same CAN IDs.  Several sockets listening on
the same interface for frames with the same CAN ID are all passed the
same received matching CAN frames.  An application wishing to
communicate using a specific transport protocol, e.g. ISO-TP, just
selects that protocol when opening the socket, and then can read and
write application data byte streams, without having to deal with
CAN-IDs, frames, etc.

Similar functionality visible from user-space could be provided by a
character device, too, but this would lead to a technically inelegant
solution for a couple of reasons:

* Intricate usage.  Instead of passing a protocol argument to
  socket(2) and using bind(2) to select a CAN interface and CAN ID, an
  application would have to do all these operations using ioctl(2)s.

* Code duplication.  A character device cannot make use of the Linux
  network queueing code, so all that code would have to be duplicated
  for CAN networking.

* Abstraction.  In most existing character-device implementations, the
  hardware-specific device driver for a CAN controller directly
  provides the character device for the application to work with.
  This is at least very unusual in Unix systems for both, char and
  block devices.  For example you don't have a character device for a
  certain UART of a serial interface, a certain sound chip in your
  computer, a SCSI or IDE controller providing access to your hard
  disk or tape streamer device.  Instead, you have abstraction layers
  which provide a unified character or block device interface to the
  application on the one hand, and a interface for hardware-specific
  device drivers on the other hand.  These abstractions are provided
  by subsystems like the tty layer, the audio subsystem or the SCSI
  and IDE subsystems for the devices mentioned above.

  The easiest way to implement a CAN device driver is as a character
  device without such a (complete) abstraction layer, as is done by most
  existing drivers.  The right way, however, would be to add such a
  layer with all the functionality like registering for certain CAN
  IDs, supporting several open file descriptors and (de)multiplexing
  CAN frames between them, (sophisticated) queueing of CAN frames, and
  providing an API for device drivers to register with.  However, then
  it would be no more difficult, or may be even easier, to use the
  networking framework provided by the Linux kernel, and this is what
  Socket CAN does.

  The use of the networking framework of the Linux kernel is just the
  natural and most appropriate way to implement CAN for Linux.

3. Socket CAN concept
---------------------

  As described in chapter 2 it is the main goal of Socket CAN to
  provide a socket interface to user space applications which builds
  upon the Linux network layer. In contrast to the commonly known
  TCP/IP and ethernet networking, the CAN bus is a broadcast-only(!)
  medium that has no MAC-layer addressing like ethernet. The CAN-identifier
  (can_id) is used for arbitration on the CAN-bus. Therefore the CAN-IDs
  have to be chosen uniquely on the bus. When designing a CAN-ECU
  network the CAN-IDs are mapped to be sent by a specific ECU.
  For this reason a CAN-ID can be treated best as a kind of source address.

  3.1 receive lists

  The network transparent access of multiple applications leads to the
  problem that different applications may be interested in the same
  CAN-IDs from the same CAN network interface. The Socket CAN core
  module - which implements the protocol family CAN - provides several
  high efficient receive lists for this reason. If e.g. a user space
  application opens a CAN RAW socket, the raw protocol module itself
  requests the (range of) CAN-IDs from the Socket CAN core that are
  requested by the user. The subscription and unsubscription of
  CAN-IDs can be done for specific CAN interfaces or for all(!) known
  CAN interfaces with the can_rx_(un)register() functions provided to
  CAN protocol modules by the SocketCAN core (see chapter 5).
  To optimize the CPU usage at runtime the receive lists are split up
  into several specific lists per device that match the requested
  filter complexity for a given use-case.

  3.2 local loopback of sent frames

  As known from other networking concepts the data exchanging
  applications may run on the same or different nodes without any
  change (except for the according addressing information):

         ___   ___   ___                   _______   ___
        | _ | | _ | | _ |                 | _   _ | | _ |
        ||A|| ||B|| ||C||                 ||A| |B|| ||C||
        |___| |___| |___|                 |_______| |___|
          |     |     |                       |       |
        -----------------(1)- CAN bus -(2)---------------

  To ensure that application A receives the same information in the
  example (2) as it would receive in example (1) there is need for
  some kind of local loopback of the sent CAN frames on the appropriate
  node.

  The Linux network devices (by default) just can handle the
  transmission and reception of media dependent frames. Due to the
  arbitration on the CAN bus the transmission of a low prio CAN-ID
  may be delayed by the reception of a high prio CAN frame. To
  reflect the correct* traffic on the node the loopback of the sent
  data has to be performed right after a successful transmission. If
  the CAN network interface is not capable of performing the loopback for
  some reason the SocketCAN core can do this task as a fallback solution.
  See chapter 6.2 for details (recommended).

  The loopback functionality is enabled by default to reflect standard
  networking behaviour for CAN applications. Due to some requests from
  the RT-SocketCAN group the loopback optionally may be disabled for each
  separate socket. See sockopts from the CAN RAW sockets in chapter 4.1.

  * = you really like to have this when you're running analyser tools
      like 'candump' or 'cansniffer' on the (same) node.

  3.3 network security issues (capabilities)

  The Controller Area Network is a local field bus transmitting only
  broadcast messages without any routing and security concepts.
  In the majority of cases the user application has to deal with
  raw CAN frames. Therefore it might be reasonable NOT to restrict
  the CAN access only to the user root, as known from other networks.
  Since the currently implemented CAN_RAW and CAN_BCM sockets can only
  send and receive frames to/from CAN interfaces it does not affect
  security of others networks to allow all users to access the CAN.
  To enable non-root users to access CAN_RAW and CAN_BCM protocol
  sockets the Kconfig options CAN_RAW_USER and/or CAN_BCM_USER may be
  selected at kernel compile time.

  3.4 network problem notifications

  The use of the CAN bus may lead to several problems on the physical
  and media access control layer. Detecting and logging of these lower
  layer problems is a vital requirement for CAN users to identify
  hardware issues on the physical transceiver layer as well as
  arbitration problems and error frames caused by the different
  ECUs. The occurrence of detected errors are important for diagnosis
  and have to be logged together with the exact timestamp. For this
  reason the CAN interface driver can generate so called Error Message
  Frames that can optionally be passed to the user application in the
  same way as other CAN frames. Whenever an error on the physical layer
  or the MAC layer is detected (e.g. by the CAN controller) the driver
  creates an appropriate error message frame. Error messages frames can
  be requested by the user application using the common CAN filter
  mechanisms. Inside this filter definition the (interested) type of
  errors may be selected. The reception of error messages is disabled
  by default. The format of the CAN error message frame is briefly
  described in the Linux header file "include/linux/can/error.h".

4. How to use Socket CAN
------------------------

  Like TCP/IP, you first need to open a socket for communicating over a
  CAN network. Since Socket CAN implements a new protocol family, you
  need to pass PF_CAN as the first argument to the socket(2) system
  call. Currently, there are two CAN protocols to choose from, the raw
  socket protocol and the broadcast manager (BCM). So to open a socket,
  you would write

    s = socket(PF_CAN, SOCK_RAW, CAN_RAW);

  and

    s = socket(PF_CAN, SOCK_DGRAM, CAN_BCM);

  respectively.  After the successful creation of the socket, you would
  normally use the bind(2) system call to bind the socket to a CAN
  interface (which is different from TCP/IP due to different addressing
  - see chapter 3). After binding (CAN_RAW) or connecting (CAN_BCM)
  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 CAN specific socket options
  described below.

  The basic CAN frame structure and the sockaddr structure are defined
  in include/linux/can.h:

    struct can_frame {
            canid_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */
            __u8    can_dlc; /* frame payload length in byte (0 .. 8) */
            __u8    data[8] __attribute__((aligned(8)));
    };

  The alignment of the (linear) payload data[] to a 64bit boundary
  allows the user to define own structs and unions to easily access the
  CAN payload. There is no given byteorder on the CAN bus by
  default. A read(2) system call on a CAN_RAW socket transfers a
  struct can_frame to the user space.

  The sockaddr_can structure has an interface index like the
  PF_PACKET socket, that also binds to a specific interface:

    struct sockaddr_can {
            sa_family_t can_family;
            int         can_ifindex;
            union {
                    /* transport protocol class address info (e.g. ISOTP) */
                    struct { canid_t rx_id, tx_id; } tp;

                    /* reserved for future CAN protocols address information */
            } can_addr;
    };

  To determine the interface index an appropriate ioctl() has to
  be used (example for CAN_RAW sockets without error checking):

    int s;
    struct sockaddr_can addr;
    struct ifreq ifr;

    s = socket(PF_CAN, SOCK_RAW, CAN_RAW);

    strcpy(ifr.ifr_name, "can0" );
    ioctl(s, SIOCGIFINDEX, &ifr);

    addr.can_family = AF_CAN;
    addr.can_ifindex = ifr.ifr_ifindex;

    bind(s, (struct sockaddr *)&addr, sizeof(addr));

    (..)

  To bind a socket to all(!) CAN interfaces the interface index must
  be 0 (zero). In this case the socket receives CAN frames from every
  enabled CAN interface. To determine the originating CAN interface
  the system call recvfrom(2) may be used instead of read(2). To send
  on a socket that is bound to 'any' interface sendto(2) is needed to
  specify the outgoing interface.

  Reading CAN frames from a bound CAN_RAW socket (see above) consists
  of reading a struct can_frame:

    struct can_frame frame;

    nbytes = read(s, &frame, sizeof(struct can_frame));

    if (nbytes < 0) {
            perror("can raw socket read");
            return 1;
    }

    /* paranoid check ... */
    if (nbytes < sizeof(struct can_frame)) {
            fprintf(stderr, "read: incomplete CAN frame\n");
            return 1;
    }

    /* do something with the received CAN frame */

  Writing CAN frames can be done similarly, with the write(2) system call:

    nbytes = write(s, &frame, sizeof(struct can_frame));

  When the CAN interface is bound to 'any' existing CAN interface
  (addr.can_ifindex = 0) it is recommended to use recvfrom(2) if the
  information about the originating CAN interface is needed:

    struct sockaddr_can addr;
    struct ifreq ifr;
    socklen_t len = sizeof(addr);
    struct can_frame frame;

    nbytes = recvfrom(s, &frame, sizeof(struct can_frame),
                      0, (struct sockaddr*)&addr, &len);

    /* get interface name of the received CAN frame */
    ifr.ifr_ifindex = addr.can_ifindex;
    ioctl(s, SIOCGIFNAME, &ifr);
    printf("Received a CAN frame from interface %s", ifr.ifr_name);

  To write CAN frames on sockets bound to 'any' CAN interface the
  outgoing interface has to be defined certainly.

    strcpy(ifr.ifr_name, "can0");
    ioctl(s, SIOCGIFINDEX, &ifr);
    addr.can_ifindex = ifr.ifr_ifindex;
    addr.can_family  = AF_CAN;

    nbytes = sendto(s, &frame, sizeof(struct can_frame),
                    0, (struct sockaddr*)&addr, sizeof(addr));

  Remark about CAN FD (flexible data rate) support:

  Generally the handling of CAN FD is very similar to the formerly described
  examples. The new CAN FD capable CAN controllers support two different
  bitrates for the arbitration phase and the payload phase of the CAN FD frame
  and up to 64 bytes of payload. This extended payload length breaks all the
  kernel interfaces (ABI) which heavily rely on the CAN frame with fixed eight
  bytes of payload (struct can_frame) like the CAN_RAW socket. Therefore e.g.
  the CAN_RAW socket supports a new socket option CAN_RAW_FD_FRAMES that
  switches the socket into a mode that allows the handling of CAN FD frames
  and (legacy) CAN frames simultaneously (see section 4.1.5).

  The struct canfd_frame is defined in include/linux/can.h:

    struct canfd_frame {
            canid_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */
            __u8    len;     /* frame payload length in byte (0 .. 64) */
            __u8    flags;   /* additional flags for CAN FD */
            __u8    __res0;  /* reserved / padding */
            __u8    __res1;  /* reserved / padding */
            __u8    data[64] __attribute__((aligned(8)));
    };

  The struct canfd_frame and the existing struct can_frame have the can_id,
  the payload length and the payload data at the same offset inside their
  structures. This allows to handle the different structures very similar.
  When the content of a struct can_frame is copied into a struct canfd_frame
  all structure elements can be used as-is - only the data[] becomes extended.

  When introducing the struct canfd_frame it turned out that the data length
  code (DLC) of the struct can_frame was used as a length information as the
  length and the DLC has a 1:1 mapping in the range of 0 .. 8. To preserve
  the easy handling of the length information the canfd_frame.len element
  contains a plain length value from 0 .. 64. So both canfd_frame.len and
  can_frame.can_dlc are equal and contain a length information and no DLC.
  For details about the distinction of CAN and CAN FD capable devices and
  the mapping to the bus-relevant data length code (DLC), see chapter 6.6.

  The length of the two CAN(FD) frame structures define the maximum transfer
  unit (MTU) of the CAN(FD) network interface and skbuff data length. Two
  definitions are specified for CAN specific MTUs in include/linux/can.h :

  #define CAN_MTU   (sizeof(struct can_frame))   == 16  => 'legacy' CAN frame
  #define CANFD_MTU (sizeof(struct canfd_frame)) == 72  => CAN FD frame

  4.1 RAW protocol sockets with can_filters (SOCK_RAW)

  Using CAN_RAW sockets is extensively comparable to the commonly
  known access to CAN character devices. To meet the new possibilities
  provided by the multi user SocketCAN approach, some reasonable
  defaults are set at RAW socket binding time:

  - The filters are set to exactly one filter receiving everything
  - The socket only receives valid data frames (=> no error message frames)
  - The loopback of sent CAN frames is enabled (see chapter 3.2)
  - The socket does not receive its own sent frames (in loopback mode)

  These default settings may be changed before or after binding the socket.
  To use the referenced definitions of the socket options for CAN_RAW
  sockets, include <linux/can/raw.h>.

  4.1.1 RAW socket option CAN_RAW_FILTER

  The reception of CAN frames using CAN_RAW sockets can be controlled
  by defining 0 .. n filters with the CAN_RAW_FILTER socket option.

  The CAN filter structure is defined in include/linux/can.h:

    struct can_filter {
            canid_t can_id;
            canid_t can_mask;
    };

  A filter matches, when

    <received_can_id> & mask == can_id & mask

  which is analogous to known CAN controllers hardware filter semantics.
  The filter can be inverted in this semantic, when the CAN_INV_FILTER
  bit is set in can_id element of the can_filter structure. In
  contrast to CAN controller hardware filters the user may set 0 .. n
  receive filters for each open socket separately:

    struct can_filter rfilter[2];

    rfilter[0].can_id   = 0x123;
    rfilter[0].can_mask = CAN_SFF_MASK;
    rfilter[1].can_id   = 0x200;
    rfilter[1].can_mask = 0x700;

    setsockopt(s, SOL_CAN_RAW, CAN_RAW_FILTER, &rfilter, sizeof(rfilter));

  To disable the reception of CAN frames on the selected CAN_RAW socket:

    setsockopt(s, SOL_CAN_RAW, CAN_RAW_FILTER, NULL, 0);

  To set the filters to zero filters is quite obsolete as not read
  data causes the raw socket to discard the received CAN frames. But
  having this 'send only' use-case we may remove the receive list in the
  Kernel to save a little (really a very little!) CPU usage.

  4.1.2 RAW socket option CAN_RAW_ERR_FILTER

  As described in chapter 3.4 the CAN interface driver can generate so
  called Error Message Frames that can optionally be passed to the user
  application in the same way as other CAN frames. The possible
  errors are divided into different error classes that may be filtered
  using the appropriate error mask. To register for every possible
  error condition CAN_ERR_MASK can be used as value for the error mask.
  The values for the error mask are defined in linux/can/error.h .

    can_err_mask_t err_mask = ( CAN_ERR_TX_TIMEOUT | CAN_ERR_BUSOFF );

    setsockopt(s, SOL_CAN_RAW, CAN_RAW_ERR_FILTER,
               &err_mask, sizeof(err_mask));

  4.1.3 RAW socket option CAN_RAW_LOOPBACK

  To meet multi user needs the local loopback is enabled by default
  (see chapter 3.2 for details). But in some embedded use-cases
  (e.g. when only one application uses the CAN bus) this loopback
  functionality can be disabled (separately for each socket):

    int loopback = 0; /* 0 = disabled, 1 = enabled (default) */

    setsockopt(s, SOL_CAN_RAW, CAN_RAW_LOOPBACK, &loopback, sizeof(loopback));

  4.1.4 RAW socket option CAN_RAW_RECV_OWN_MSGS

  When the local loopback is enabled, all the sent CAN frames are
  looped back to the open CAN sockets that registered for the CAN
  frames' CAN-ID on this given interface to meet the multi user
  needs. The reception of the CAN frames on the same socket that was
  sending the CAN frame is assumed to be unwanted and therefore
  disabled by default. This default behaviour may be changed on
  demand:

    int recv_own_msgs = 1; /* 0 = disabled (default), 1 = enabled */

    setsockopt(s, SOL_CAN_RAW, CAN_RAW_RECV_OWN_MSGS,
               &recv_own_msgs, sizeof(recv_own_msgs));

  4.1.5 RAW socket option CAN_RAW_FD_FRAMES

  CAN FD support in CAN_RAW sockets can be enabled with a new socket option
  CAN_RAW_FD_FRAMES which is off by default. When the new socket option is
  not supported by the CAN_RAW socket (e.g. on older kernels), switching the
  CAN_RAW_FD_FRAMES option returns the error -ENOPROTOOPT.

  Once CAN_RAW_FD_FRAMES is enabled the application can send both CAN frames
  and CAN FD frames. OTOH the application has to handle CAN and CAN FD frames
  when reading from the socket.

    CAN_RAW_FD_FRAMES enabled:  CAN_MTU and CANFD_MTU are allowed
    CAN_RAW_FD_FRAMES disabled: only CAN_MTU is allowed (default)

  Example:
    [ remember: CANFD_MTU == sizeof(struct canfd_frame) ]

    struct canfd_frame cfd;

    nbytes = read(s, &cfd, CANFD_MTU);

    if (nbytes == CANFD_MTU) {
            printf("got CAN FD frame with length %d\n", cfd.len);
	    /* cfd.flags contains valid data */
    } else if (nbytes == CAN_MTU) {
            printf("got legacy CAN frame with length %d\n", cfd.len);
	    /* cfd.flags is undefined */
    } else {
            fprintf(stderr, "read: invalid CAN(FD) frame\n");
            return 1;
    }

    /* the content can be handled independently from the received MTU size */

    printf("can_id: %X data length: %d data: ", cfd.can_id, cfd.len);
    for (i = 0; i < cfd.len; i++)
            printf("%02X ", cfd.data[i]);

  When reading with size CANFD_MTU only returns CAN_MTU bytes that have
  been received from the socket a legacy CAN frame has been read into the
  provided CAN FD structure. Note that the canfd_frame.flags data field is
  not specified in the struct can_frame and therefore it is only valid in
  CANFD_MTU sized CAN FD frames.

  As long as the payload length is <=8 the received CAN frames from CAN FD
  capable CAN devices can be received and read by legacy sockets too. When
  user-generated CAN FD frames have a payload length <=8 these can be send
  by legacy CAN network interfaces too. Sending CAN FD frames with payload
  length > 8 to a legacy CAN network interface returns an -EMSGSIZE error.

  Implementation hint for new CAN applications:

  To build a CAN FD aware application use struct canfd_frame as basic CAN
  data structure for CAN_RAW based applications. When the application is
  executed on an older Linux kernel and switching the CAN_RAW_FD_FRAMES
  socket option returns an error: No problem. You'll get legacy CAN frames
  or CAN FD frames and can process them the same way.

  When sending to CAN devices make sure that the device is capable to handle
  CAN FD frames by checking if the device maximum transfer unit is CANFD_MTU.
  The CAN device MTU can be retrieved e.g. with a SIOCGIFMTU ioctl() syscall.

  4.1.6 RAW socket returned message flags

  When using recvmsg() call, the msg->msg_flags may contain following flags:

    MSG_DONTROUTE: set when the received frame was created on the local host.

    MSG_CONFIRM: set when the frame was sent via the socket it is received on.
      This flag can be interpreted as a 'transmission confirmation' when the
      CAN driver supports the echo of frames on driver level, see 3.2 and 6.2.
      In order to receive such messages, CAN_RAW_RECV_OWN_MSGS must be set.

  4.2 Broadcast Manager protocol sockets (SOCK_DGRAM)

  The Broadcast Manager protocol provides a command based configuration
  interface to filter and send (e.g. cyclic) CAN messages in kernel space.

  Receive filters can be used to down sample frequent messages; detect events
  such as message contents changes, packet length changes, and do time-out
  monitoring of received messages.

  Periodic transmission tasks of CAN frames or a sequence of CAN frames can be
  created and modified at runtime; both the message content and the two
  possible transmit intervals can be altered.

  A BCM socket is not intended for sending individual CAN frames using the
  struct can_frame as known from the CAN_RAW socket. Instead a special BCM
  configuration message is defined. The basic BCM configuration message used
  to communicate with the broadcast manager and the available operations are
  defined in the linux/can/bcm.h include. The BCM message consists of a
  message header with a command ('opcode') followed by zero or more CAN frames.
  The broadcast manager sends responses to user space in the same form:

    struct bcm_msg_head {
            __u32 opcode;                   /* command */
            __u32 flags;                    /* special flags */
            __u32 count;                    /* run 'count' times with ival1 */
            struct timeval ival1, ival2;    /* count and subsequent interval */
            canid_t can_id;                 /* unique can_id for task */
            __u32 nframes;                  /* number of can_frames following */
            struct can_frame frames[0];
    };

  The aligned payload 'frames' uses the same basic CAN frame structure defined
  at the beginning of section 4 and in the include/linux/can.h include. All
  messages to the broadcast manager from user space have this structure.

  Note a CAN_BCM socket must be connected instead of bound after socket
  creation (example without error checking):

    int s;
    struct sockaddr_can addr;
    struct ifreq ifr;

    s = socket(PF_CAN, SOCK_DGRAM, CAN_BCM);

    strcpy(ifr.ifr_name, "can0");
    ioctl(s, SIOCGIFINDEX, &ifr);

    addr.can_family = AF_CAN;
    addr.can_ifindex = ifr.ifr_ifindex;

    connect(s, (struct sockaddr *)&addr, sizeof(addr))

    (..)

  The broadcast manager socket is able to handle any number of in flight
  transmissions or receive filters concurrently. The different RX/TX jobs are
  distinguished by the unique can_id in each BCM message. However additional
  CAN_BCM sockets are recommended to communicate on multiple CAN interfaces.
  When the broadcast manager socket is bound to 'any' CAN interface (=> the
  interface index is set to zero) the configured receive filters apply to any
  CAN interface unless the sendto() syscall is used to overrule the 'any' CAN
  interface index. When using recvfrom() instead of read() to retrieve BCM
  socket messages the originating CAN interface is provided in can_ifindex.

  4.2.1 Broadcast Manager operations

  The opcode defines the operation for the broadcast manager to carry out,
  or details the broadcast managers response to several events, including
  user requests.

  Transmit Operations (user space to broadcast manager):

    TX_SETUP:   Create (cyclic) transmission task.

    TX_DELETE:  Remove (cyclic) transmission task, requires only can_id.

    TX_READ:    Read properties of (cyclic) transmission task for can_id.

    TX_SEND:    Send one CAN frame.

  Transmit Responses (broadcast manager to user space):

    TX_STATUS:  Reply to TX_READ request (transmission task configuration).

    TX_EXPIRED: Notification when counter finishes sending at initial interval
      'ival1'. Requires the TX_COUNTEVT flag to be set at TX_SETUP.

  Receive Operations (user space to broadcast manager):

    RX_SETUP:   Create RX content filter subscription.

    RX_DELETE:  Remove RX content filter subscription, requires only can_id.

    RX_READ:    Read properties of RX content filter subscription for can_id.

  Receive Responses (broadcast manager to user space):

    RX_STATUS:  Reply to RX_READ request (filter task configuration).

    RX_TIMEOUT: Cyclic message is detected to be absent (timer ival1 expired).

    RX_CHANGED: BCM message with updated CAN frame (detected content change).
      Sent on first message received or on receipt of revised CAN messages.

  4.2.2 Broadcast Manager message flags

  When sending a message to the broadcast manager the 'flags' element may
  contain the following flag definitions which influence the behaviour:

    SETTIMER:           Set the values of ival1, ival2 and count

    STARTTIMER:         Start the timer with the actual values of ival1, ival2
      and count. Starting the timer leads simultaneously to emit a CAN frame.

    TX_COUNTEVT:        Create the message TX_EXPIRED when count expires

    TX_ANNOUNCE:        A change of data by the process is emitted immediately.

    TX_CP_CAN_ID:       Copies the can_id from the message header to each
      subsequent frame in frames. This is intended as usage simplification. For
      TX tasks the unique can_id from the message header may differ from the
      can_id(s) stored for transmission in the subsequent struct can_frame(s).

    RX_FILTER_ID:       Filter by can_id alone, no frames required (nframes=0).

    RX_CHECK_DLC:       A change of the DLC leads to an RX_CHANGED.

    RX_NO_AUTOTIMER:    Prevent automatically starting the timeout monitor.

    RX_ANNOUNCE_RESUME: If passed at RX_SETUP and a receive timeout occured, a
      RX_CHANGED message will be generated when the (cyclic) receive restarts.

    TX_RESET_MULTI_IDX: Reset the index for the multiple frame transmission.

    RX_RTR_FRAME:       Send reply for RTR-request (placed in op->frames[0]).

  4.2.3 Broadcast Manager transmission timers

  Periodic transmission configurations may use up to two interval timers.
  In this case the BCM sends a number of messages ('count') at an interval
  'ival1', then continuing to send at another given interval 'ival2'. When
  only one timer is needed 'count' is set to zero and only 'ival2' is used.
  When SET_TIMER and START_TIMER flag were set the timers are activated.
  The timer values can be altered at runtime when only SET_TIMER is set.

  4.2.4 Broadcast Manager message sequence transmission

  Up to 256 CAN frames can be transmitted in a sequence in the case of a cyclic
  TX task configuration. The number of CAN frames is provided in the 'nframes'
  element of the BCM message head. The defined number of CAN frames are added
  as array to the TX_SETUP BCM configuration message.

    /* create a struct to set up a sequence of four CAN frames */
    struct {
            struct bcm_msg_head msg_head;
            struct can_frame frame[4];
    } mytxmsg;

    (..)
    mytxmsg.nframes = 4;
    (..)

    write(s, &mytxmsg, sizeof(mytxmsg));

  With every transmission the index in the array of CAN frames is increased
  and set to zero at index overflow.

  4.2.5 Broadcast Manager receive filter timers

  The timer values ival1 or ival2 may be set to non-zero values at RX_SETUP.
  When the SET_TIMER flag is set the timers are enabled:

  ival1: Send RX_TIMEOUT when a received message is not received again within
    the given time. When START_TIMER is set at RX_SETUP the timeout detection
    is activated directly - even without a former CAN frame reception.

  ival2: Throttle the received message rate down to the value of ival2. This
    is useful to reduce messages for the application when the signal inside the
    CAN frame is stateless as state changes within the ival2 periode may get
    lost.

  4.2.6 Broadcast Manager multiplex message receive filter

  To filter for content changes in multiplex message sequences an array of more
  than one CAN frames can be passed in a RX_SETUP configuration message. The
  data bytes of the first CAN frame contain the mask of relevant bits that
  have to match in the subsequent CAN frames with the received CAN frame.
  If one of the subsequent CAN frames is matching the bits in that frame data
  mark the relevant content to be compared with the previous received content.
  Up to 257 CAN frames (multiplex filter bit mask CAN frame plus 256 CAN
  filters) can be added as array to the TX_SETUP BCM configuration message.

    /* usually used to clear CAN frame data[] - beware of endian problems! */
    #define U64_DATA(p) (*(unsigned long long*)(p)->data)

    struct {
            struct bcm_msg_head msg_head;
            struct can_frame frame[5];
    } msg;

    msg.msg_head.opcode  = RX_SETUP;
    msg.msg_head.can_id  = 0x42;
    msg.msg_head.flags   = 0;
    msg.msg_head.nframes = 5;
    U64_DATA(&msg.frame[0]) = 0xFF00000000000000ULL; /* MUX mask */
    U64_DATA(&msg.frame[1]) = 0x01000000000000FFULL; /* data mask (MUX 0x01) */
    U64_DATA(&msg.frame[2]) = 0x0200FFFF000000FFULL; /* data mask (MUX 0x02) */
    U64_DATA(&msg.frame[3]) = 0x330000FFFFFF0003ULL; /* data mask (MUX 0x33) */
    U64_DATA(&msg.frame[4]) = 0x4F07FC0FF0000000ULL; /* data mask (MUX 0x4F) */

    write(s, &msg, sizeof(msg));

  4.3 connected transport protocols (SOCK_SEQPACKET)
  4.4 unconnected transport protocols (SOCK_DGRAM)


5. Socket CAN core module
-------------------------

  The Socket CAN core module implements the protocol family
  PF_CAN. CAN protocol modules are loaded by the core module at
  runtime. The core module provides an interface for CAN protocol
  modules to subscribe needed CAN IDs (see chapter 3.1).

  5.1 can.ko module params

  - stats_timer: To calculate the Socket CAN core statistics
    (e.g. current/maximum frames per second) this 1 second timer is
    invoked at can.ko module start time by default. This timer can be
    disabled by using stattimer=0 on the module commandline.

  - debug: (removed since SocketCAN SVN r546)

  5.2 procfs content

  As described in chapter 3.1 the Socket CAN core uses several filter
  lists to deliver received CAN frames to CAN protocol modules. These
  receive lists, their filters and the count of filter matches can be
  checked in the appropriate receive list. All entries contain the
  device and a protocol module identifier:

    foo@bar:~$ cat /proc/net/can/rcvlist_all

    receive list 'rx_all':
      (vcan3: no entry)
      (vcan2: no entry)
      (vcan1: no entry)
      device   can_id   can_mask  function  userdata   matches  ident
       vcan0     000    00000000  f88e6370  f6c6f400         0  raw
      (any: no entry)

  In this example an application requests any CAN traffic from vcan0.

    rcvlist_all - list for unfiltered entries (no filter operations)
    rcvlist_eff - list for single extended frame (EFF) entries
    rcvlist_err - list for error message frames masks
    rcvlist_fil - list for mask/value filters
    rcvlist_inv - list for mask/value filters (inverse semantic)
    rcvlist_sff - list for single standard frame (SFF) entries

  Additional procfs files in /proc/net/can

    stats       - Socket CAN core statistics (rx/tx frames, match ratios, ...)
    reset_stats - manual statistic reset
    version     - prints the Socket CAN core version and the ABI version

  5.3 writing own CAN protocol modules

  To implement a new protocol in the protocol family PF_CAN a new
  protocol has to be defined in include/linux/can.h .
  The prototypes and definitions to use the Socket CAN core can be
  accessed by including include/linux/can/core.h .
  In addition to functions that register the CAN protocol and the
  CAN device notifier chain there are functions to subscribe CAN
  frames received by CAN interfaces and to send CAN frames:

    can_rx_register   - subscribe CAN frames from a specific interface
    can_rx_unregister - unsubscribe CAN frames from a specific interface
    can_send          - transmit a CAN frame (optional with local loopback)

  For details see the kerneldoc documentation in net/can/af_can.c or
  the source code of net/can/raw.c or net/can/bcm.c .

6. CAN network drivers
----------------------

  Writing a CAN network device driver is much easier than writing a
  CAN character device driver. Similar to other known network device
  drivers you mainly have to deal with:

  - TX: Put the CAN frame from the socket buffer to the CAN controller.
  - RX: Put the CAN frame from the CAN controller to the socket buffer.

  See e.g. at Documentation/networking/netdevices.txt . The differences
  for writing CAN network device driver are described below:

  6.1 general settings

    dev->type  = ARPHRD_CAN; /* the netdevice hardware type */
    dev->flags = IFF_NOARP;  /* CAN has no arp */

    dev->mtu = CAN_MTU; /* sizeof(struct can_frame) -> legacy CAN interface */

    or alternative, when the controller supports CAN with flexible data rate:
    dev->mtu = CANFD_MTU; /* sizeof(struct canfd_frame) -> CAN FD interface */

  The struct can_frame or struct canfd_frame is the payload of each socket
  buffer (skbuff) in the protocol family PF_CAN.

  6.2 local loopback of sent frames

  As described in chapter 3.2 the CAN network device driver should
  support a local loopback functionality similar to the local echo
  e.g. of tty devices. In this case the driver flag IFF_ECHO has to be
  set to prevent the PF_CAN core from locally echoing sent frames
  (aka loopback) as fallback solution:

    dev->flags = (IFF_NOARP | IFF_ECHO);

  6.3 CAN controller hardware filters

  To reduce the interrupt load on deep embedded systems some CAN
  controllers support the filtering of CAN IDs or ranges of CAN IDs.
  These hardware filter capabilities vary from controller to
  controller and have to be identified as not feasible in a multi-user
  networking approach. The use of the very controller specific
  hardware filters could make sense in a very dedicated use-case, as a
  filter on driver level would affect all users in the multi-user
  system. The high efficient filter sets inside the PF_CAN core allow
  to set different multiple filters for each socket separately.
  Therefore the use of hardware filters goes to the category 'handmade
  tuning on deep embedded systems'. The author is running a MPC603e
  @133MHz with four SJA1000 CAN controllers from 2002 under heavy bus
  load without any problems ...

  6.4 The virtual CAN driver (vcan)

  Similar to the network loopback devices, vcan offers a virtual local
  CAN interface. A full qualified address on CAN consists of

  - a unique CAN Identifier (CAN ID)
  - the CAN bus this CAN ID is transmitted on (e.g. can0)

  so in common use cases more than one virtual CAN interface is needed.

  The virtual CAN interfaces allow the transmission and reception of CAN
  frames without real CAN controller hardware. Virtual CAN network
  devices are usually named 'vcanX', like vcan0 vcan1 vcan2 ...
  When compiled as a module the virtual CAN driver module is called vcan.ko

  Since Linux Kernel version 2.6.24 the vcan driver supports the Kernel
  netlink interface to create vcan network devices. The creation and
  removal of vcan network devices can be managed with the ip(8) tool:

  - Create a virtual CAN network interface:
       $ ip link add type vcan

  - Create a virtual CAN network interface with a specific name 'vcan42':
       $ ip link add dev vcan42 type vcan

  - Remove a (virtual CAN) network interface 'vcan42':
       $ ip link del vcan42

  6.5 The CAN network device driver interface

  The CAN network device driver interface provides a generic interface
  to setup, configure and monitor CAN network devices. The user can then
  configure the CAN device, like setting the bit-timing parameters, via
  the netlink interface using the program "ip" from the "IPROUTE2"
  utility suite. The following chapter describes briefly how to use it.
  Furthermore, the interface uses a common data structure and exports a
  set of common functions, which all real CAN network device drivers
  should use. Please have a look to the SJA1000 or MSCAN driver to
  understand how to use them. The name of the module is can-dev.ko.

  6.5.1 Netlink interface to set/get devices properties

  The CAN device must be configured via netlink interface. The supported
  netlink message types are defined and briefly described in
  "include/linux/can/netlink.h". CAN link support for the program "ip"
  of the IPROUTE2 utility suite is available and it can be used as shown
  below:

  - Setting CAN device properties:

    $ ip link set can0 type can help
    Usage: ip link set DEVICE type can
    	[ bitrate BITRATE [ sample-point SAMPLE-POINT] ] |
    	[ tq TQ prop-seg PROP_SEG phase-seg1 PHASE-SEG1
     	  phase-seg2 PHASE-SEG2 [ sjw SJW ] ]

    	[ loopback { on | off } ]
    	[ listen-only { on | off } ]
    	[ triple-sampling { on | off } ]

    	[ restart-ms TIME-MS ]
    	[ restart ]

    	Where: BITRATE       := { 1..1000000 }
    	       SAMPLE-POINT  := { 0.000..0.999 }
    	       TQ            := { NUMBER }
    	       PROP-SEG      := { 1..8 }
    	       PHASE-SEG1    := { 1..8 }
    	       PHASE-SEG2    := { 1..8 }
    	       SJW           := { 1..4 }
    	       RESTART-MS    := { 0 | NUMBER }

  - Display CAN device details and statistics:

    $ ip -details -statistics link show can0
    2: can0: <NOARP,UP,LOWER_UP,ECHO> mtu 16 qdisc pfifo_fast state UP qlen 10
      link/can
      can <TRIPLE-SAMPLING> state ERROR-ACTIVE restart-ms 100
      bitrate 125000 sample_point 0.875
      tq 125 prop-seg 6 phase-seg1 7 phase-seg2 2 sjw 1
      sja1000: tseg1 1..16 tseg2 1..8 sjw 1..4 brp 1..64 brp-inc 1
      clock 8000000
      re-started bus-errors arbit-lost error-warn error-pass bus-off
      41         17457      0          41         42         41
      RX: bytes  packets  errors  dropped overrun mcast
      140859     17608    17457   0       0       0
      TX: bytes  packets  errors  dropped carrier collsns
      861        112      0       41      0       0

  More info to the above output:

    "<TRIPLE-SAMPLING>"
	Shows the list of selected CAN controller modes: LOOPBACK,
	LISTEN-ONLY, or TRIPLE-SAMPLING.

    "state ERROR-ACTIVE"
	The current state of the CAN controller: "ERROR-ACTIVE",
	"ERROR-WARNING", "ERROR-PASSIVE", "BUS-OFF" or "STOPPED"

    "restart-ms 100"
	Automatic restart delay time. If set to a non-zero value, a
	restart of the CAN controller will be triggered automatically
	in case of a bus-off condition after the specified delay time
	in milliseconds. By default it's off.

    "bitrate 125000 sample_point 0.875"
	Shows the real bit-rate in bits/sec and the sample-point in the
	range 0.000..0.999. If the calculation of bit-timing parameters
	is enabled in the kernel (CONFIG_CAN_CALC_BITTIMING=y), the
	bit-timing can be defined by setting the "bitrate" argument.
	Optionally the "sample-point" can be specified. By default it's
	0.000 assuming CIA-recommended sample-points.

    "tq 125 prop-seg 6 phase-seg1 7 phase-seg2 2 sjw 1"
	Shows the time quanta in ns, propagation segment, phase buffer
	segment 1 and 2 and the synchronisation jump width in units of
	tq. They allow to define the CAN bit-timing in a hardware
	independent format as proposed by the Bosch CAN 2.0 spec (see
	chapter 8 of http://www.semiconductors.bosch.de/pdf/can2spec.pdf).

    "sja1000: tseg1 1..16 tseg2 1..8 sjw 1..4 brp 1..64 brp-inc 1
     clock 8000000"
	Shows the bit-timing constants of the CAN controller, here the
	"sja1000". The minimum and maximum values of the time segment 1
	and 2, the synchronisation jump width in units of tq, the
	bitrate pre-scaler and the CAN system clock frequency in Hz.
	These constants could be used for user-defined (non-standard)
	bit-timing calculation algorithms in user-space.

    "re-started bus-errors arbit-lost error-warn error-pass bus-off"
	Shows the number of restarts, bus and arbitration lost errors,
	and the state changes to the error-warning, error-passive and
	bus-off state. RX overrun errors are listed in the "overrun"
	field of the standard network statistics.

  6.5.2 Setting the CAN bit-timing

  The CAN bit-timing parameters can always be defined in a hardware
  independent format as proposed in the Bosch CAN 2.0 specification
  specifying the arguments "tq", "prop_seg", "phase_seg1", "phase_seg2"
  and "sjw":

    $ ip link set canX type can tq 125 prop-seg 6 \
				phase-seg1 7 phase-seg2 2 sjw 1

  If the kernel option CONFIG_CAN_CALC_BITTIMING is enabled, CIA
  recommended CAN bit-timing parameters will be calculated if the bit-
  rate is specified with the argument "bitrate":

    $ ip link set canX type can bitrate 125000

  Note that this works fine for the most common CAN controllers with
  standard bit-rates but may *fail* for exotic bit-rates or CAN system
  clock frequencies. Disabling CONFIG_CAN_CALC_BITTIMING saves some
  space and allows user-space tools to solely determine and set the
  bit-timing parameters. The CAN controller specific bit-timing
  constants can be used for that purpose. They are listed by the
  following command:

    $ ip -details link show can0
    ...
      sja1000: clock 8000000 tseg1 1..16 tseg2 1..8 sjw 1..4 brp 1..64 brp-inc 1

  6.5.3 Starting and stopping the CAN network device

  A CAN network device is started or stopped as usual with the command
  "ifconfig canX up/down" or "ip link set canX up/down". Be aware that
  you *must* define proper bit-timing parameters for real CAN devices
  before you can start it to avoid error-prone default settings:

    $ ip link set canX up type can bitrate 125000

  A device may enter the "bus-off" state if too much errors occurred on
  the CAN bus. Then no more messages are received or sent. An automatic
  bus-off recovery can be enabled by setting the "restart-ms" to a
  non-zero value, e.g.:

    $ ip link set canX type can restart-ms 100

  Alternatively, the application may realize the "bus-off" condition
  by monitoring CAN error message frames and do a restart when
  appropriate with the command:

    $ ip link set canX type can restart

  Note that a restart will also create a CAN error message frame (see
  also chapter 3.4).

  6.6 CAN FD (flexible data rate) driver support

  CAN FD capable CAN controllers support two different bitrates for the
  arbitration phase and the payload phase of the CAN FD frame. Therefore a
  second bittiming has to be specified in order to enable the CAN FD bitrate.

  Additionally CAN FD capable CAN controllers support up to 64 bytes of
  payload. The representation of this length in can_frame.can_dlc and
  canfd_frame.len for userspace applications and inside the Linux network
  layer is a plain value from 0 .. 64 instead of the CAN 'data length code'.
  The data length code was a 1:1 mapping to the payload length in the legacy
  CAN frames anyway. The payload length to the bus-relevant DLC mapping is
  only performed inside the CAN drivers, preferably with the helper
  functions can_dlc2len() and can_len2dlc().

  The CAN netdevice driver capabilities can be distinguished by the network
  devices maximum transfer unit (MTU):

  MTU = 16 (CAN_MTU)   => sizeof(struct can_frame)   => 'legacy' CAN device
  MTU = 72 (CANFD_MTU) => sizeof(struct canfd_frame) => CAN FD capable device

  The CAN device MTU can be retrieved e.g. with a SIOCGIFMTU ioctl() syscall.
  N.B. CAN FD capable devices can also handle and send legacy CAN frames.

  FIXME: Add details about the CAN FD controller configuration when available.

  6.7 Supported CAN hardware

  Please check the "Kconfig" file in "drivers/net/can" to get an actual
  list of the support CAN hardware. On the Socket CAN project website
  (see chapter 7) there might be further drivers available, also for
  older kernel versions.

7. Socket CAN resources
-----------------------

  You can find further resources for Socket CAN like user space tools,
  support for old kernel versions, more drivers, mailing lists, etc.
  at the BerliOS OSS project website for Socket CAN:

    http://developer.berlios.de/projects/socketcan

  If you have questions, bug fixes, etc., don't hesitate to post them to
  the Socketcan-Users mailing list. But please search the archives first.

8. Credits
----------

  Oliver Hartkopp (PF_CAN core, filters, drivers, bcm, SJA1000 driver)
  Urs Thuermann (PF_CAN core, kernel integration, socket interfaces, raw, vcan)
  Jan Kizka (RT-SocketCAN core, Socket-API reconciliation)
  Wolfgang Grandegger (RT-SocketCAN core & drivers, Raw Socket-API reviews,
                       CAN device driver interface, MSCAN driver)
  Robert Schwebel (design reviews, PTXdist integration)
  Marc Kleine-Budde (design reviews, Kernel 2.6 cleanups, drivers)
  Benedikt Spranger (reviews)
  Thomas Gleixner (LKML reviews, coding style, posting hints)
  Andrey Volkov (kernel subtree structure, ioctls, MSCAN driver)
  Matthias Brukner (first SJA1000 CAN netdevice implementation Q2/2003)
  Klaus Hitschler (PEAK driver integration)
  Uwe Koppe (CAN netdevices with PF_PACKET approach)
  Michael Schulze (driver layer loopback requirement, RT CAN drivers review)
  Pavel Pisa (Bit-timing calculation)
  Sascha Hauer (SJA1000 platform driver)
  Sebastian Haas (SJA1000 EMS PCI driver)
  Markus Plessing (SJA1000 EMS PCI driver)
  Per Dalen (SJA1000 Kvaser PCI driver)
  Sam Ravnborg (reviews, coding style, kbuild help)