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

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
    4.2 Broadcast Manager protocol sockets (SOCK_DGRAM)
    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 currently supported CAN hardware
    6.5 todo

  7 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 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 frame. Error 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 frames is disabled by default.

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; /* data length code: 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;
    }

    /* paraniod 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));

  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 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 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));

  4.2 Broadcast Manager protocol sockets (SOCK_DGRAM)
  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 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   = sizeof(struct can_frame);

  The struct can_frame is the payload of each socket buffer 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 currently supported CAN hardware (September 2007)

  On the project website http://developer.berlios.de/projects/socketcan
  there are different drivers available:

    vcan:    Virtual CAN interface driver (if no real hardware is available)
    sja1000: Philips SJA1000 CAN controller (recommended)
    i82527:  Intel i82527 CAN controller
    mscan:   Motorola/Freescale CAN controller (e.g. inside SOC MPC5200)
    ccan:    CCAN controller core (e.g. inside SOC h7202)
    slcan:   For a bunch of CAN adaptors that are attached via a
             serial line ASCII protocol (for serial / USB adaptors)

  Additionally the different CAN adaptors (ISA/PCI/PCMCIA/USB/Parport)
  from PEAK Systemtechnik support the CAN netdevice driver model
  since Linux driver v6.0: http://www.peak-system.com/linux/index.htm

  Please check the Mailing Lists on the berlios OSS project website.

  6.5 todo (September 2007)

  The configuration interface for CAN network drivers is still an open
  issue that has not been finalized in the socketcan project. Also the
  idea of having a library module (candev.ko) that holds functions
  that are needed by all CAN netdevices is not ready to ship.
  Your contribution is welcome.

7. Credits
----------

  Oliver Hartkopp (PF_CAN core, filters, drivers, bcm)
  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)
  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)