linux/Documentation/networking/can.txt
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   1============================================================================
   2
   3can.txt
   4
   5Readme file for the Controller Area Network Protocol Family (aka Socket CAN)
   6
   7This file contains
   8
   9  1 Overview / What is Socket CAN
  10
  11  2 Motivation / Why using the socket API
  12
  13  3 Socket CAN concept
  14    3.1 receive lists
  15    3.2 local loopback of sent frames
  16    3.3 network security issues (capabilities)
  17    3.4 network problem notifications
  18
  19  4 How to use Socket CAN
  20    4.1 RAW protocol sockets with can_filters (SOCK_RAW)
  21      4.1.1 RAW socket option CAN_RAW_FILTER
  22      4.1.2 RAW socket option CAN_RAW_ERR_FILTER
  23      4.1.3 RAW socket option CAN_RAW_LOOPBACK
  24      4.1.4 RAW socket option CAN_RAW_RECV_OWN_MSGS
  25    4.2 Broadcast Manager protocol sockets (SOCK_DGRAM)
  26    4.3 connected transport protocols (SOCK_SEQPACKET)
  27    4.4 unconnected transport protocols (SOCK_DGRAM)
  28
  29  5 Socket CAN core module
  30    5.1 can.ko module params
  31    5.2 procfs content
  32    5.3 writing own CAN protocol modules
  33
  34  6 CAN network drivers
  35    6.1 general settings
  36    6.2 local loopback of sent frames
  37    6.3 CAN controller hardware filters
  38    6.4 The virtual CAN driver (vcan)
  39    6.5 currently supported CAN hardware
  40    6.6 todo
  41
  42  7 Credits
  43
  44============================================================================
  45
  461. Overview / What is Socket CAN
  47--------------------------------
  48
  49The socketcan package is an implementation of CAN protocols
  50(Controller Area Network) for Linux.  CAN is a networking technology
  51which has widespread use in automation, embedded devices, and
  52automotive fields.  While there have been other CAN implementations
  53for Linux based on character devices, Socket CAN uses the Berkeley
  54socket API, the Linux network stack and implements the CAN device
  55drivers as network interfaces.  The CAN socket API has been designed
  56as similar as possible to the TCP/IP protocols to allow programmers,
  57familiar with network programming, to easily learn how to use CAN
  58sockets.
  59
  602. Motivation / Why using the socket API
  61----------------------------------------
  62
  63There have been CAN implementations for Linux before Socket CAN so the
  64question arises, why we have started another project.  Most existing
  65implementations come as a device driver for some CAN hardware, they
  66are based on character devices and provide comparatively little
  67functionality.  Usually, there is only a hardware-specific device
  68driver which provides a character device interface to send and
  69receive raw CAN frames, directly to/from the controller hardware.
  70Queueing of frames and higher-level transport protocols like ISO-TP
  71have to be implemented in user space applications.  Also, most
  72character-device implementations support only one single process to
  73open the device at a time, similar to a serial interface.  Exchanging
  74the CAN controller requires employment of another device driver and
  75often the need for adaption of large parts of the application to the
  76new driver's API.
  77
  78Socket CAN was designed to overcome all of these limitations.  A new
  79protocol family has been implemented which provides a socket interface
  80to user space applications and which builds upon the Linux network
  81layer, so to use all of the provided queueing functionality.  A device
  82driver for CAN controller hardware registers itself with the Linux
  83network layer as a network device, so that CAN frames from the
  84controller can be passed up to the network layer and on to the CAN
  85protocol family module and also vice-versa.  Also, the protocol family
  86module provides an API for transport protocol modules to register, so
  87that any number of transport protocols can be loaded or unloaded
  88dynamically.  In fact, the can core module alone does not provide any
  89protocol and cannot be used without loading at least one additional
  90protocol module.  Multiple sockets can be opened at the same time,
  91on different or the same protocol module and they can listen/send
  92frames on different or the same CAN IDs.  Several sockets listening on
  93the same interface for frames with the same CAN ID are all passed the
  94same received matching CAN frames.  An application wishing to
  95communicate using a specific transport protocol, e.g. ISO-TP, just
  96selects that protocol when opening the socket, and then can read and
  97write application data byte streams, without having to deal with
  98CAN-IDs, frames, etc.
  99
 100Similar functionality visible from user-space could be provided by a
 101character device, too, but this would lead to a technically inelegant
 102solution for a couple of reasons:
 103
 104* Intricate usage.  Instead of passing a protocol argument to
 105  socket(2) and using bind(2) to select a CAN interface and CAN ID, an
 106  application would have to do all these operations using ioctl(2)s.
 107
 108* Code duplication.  A character device cannot make use of the Linux
 109  network queueing code, so all that code would have to be duplicated
 110  for CAN networking.
 111
 112* Abstraction.  In most existing character-device implementations, the
 113  hardware-specific device driver for a CAN controller directly
 114  provides the character device for the application to work with.
 115  This is at least very unusual in Unix systems for both, char and
 116  block devices.  For example you don't have a character device for a
 117  certain UART of a serial interface, a certain sound chip in your
 118  computer, a SCSI or IDE controller providing access to your hard
 119  disk or tape streamer device.  Instead, you have abstraction layers
 120  which provide a unified character or block device interface to the
 121  application on the one hand, and a interface for hardware-specific
 122  device drivers on the other hand.  These abstractions are provided
 123  by subsystems like the tty layer, the audio subsystem or the SCSI
 124  and IDE subsystems for the devices mentioned above.
 125
 126  The easiest way to implement a CAN device driver is as a character
 127  device without such a (complete) abstraction layer, as is done by most
 128  existing drivers.  The right way, however, would be to add such a
 129  layer with all the functionality like registering for certain CAN
 130  IDs, supporting several open file descriptors and (de)multiplexing
 131  CAN frames between them, (sophisticated) queueing of CAN frames, and
 132  providing an API for device drivers to register with.  However, then
 133  it would be no more difficult, or may be even easier, to use the
 134  networking framework provided by the Linux kernel, and this is what
 135  Socket CAN does.
 136
 137  The use of the networking framework of the Linux kernel is just the
 138  natural and most appropriate way to implement CAN for Linux.
 139
 1403. Socket CAN concept
 141---------------------
 142
 143  As described in chapter 2 it is the main goal of Socket CAN to
 144  provide a socket interface to user space applications which builds
 145  upon the Linux network layer. In contrast to the commonly known
 146  TCP/IP and ethernet networking, the CAN bus is a broadcast-only(!)
 147  medium that has no MAC-layer addressing like ethernet. The CAN-identifier
 148  (can_id) is used for arbitration on the CAN-bus. Therefore the CAN-IDs
 149  have to be chosen uniquely on the bus. When designing a CAN-ECU
 150  network the CAN-IDs are mapped to be sent by a specific ECU.
 151  For this reason a CAN-ID can be treated best as a kind of source address.
 152
 153  3.1 receive lists
 154
 155  The network transparent access of multiple applications leads to the
 156  problem that different applications may be interested in the same
 157  CAN-IDs from the same CAN network interface. The Socket CAN core
 158  module - which implements the protocol family CAN - provides several
 159  high efficient receive lists for this reason. If e.g. a user space
 160  application opens a CAN RAW socket, the raw protocol module itself
 161  requests the (range of) CAN-IDs from the Socket CAN core that are
 162  requested by the user. The subscription and unsubscription of
 163  CAN-IDs can be done for specific CAN interfaces or for all(!) known
 164  CAN interfaces with the can_rx_(un)register() functions provided to
 165  CAN protocol modules by the SocketCAN core (see chapter 5).
 166  To optimize the CPU usage at runtime the receive lists are split up
 167  into several specific lists per device that match the requested
 168  filter complexity for a given use-case.
 169
 170  3.2 local loopback of sent frames
 171
 172  As known from other networking concepts the data exchanging
 173  applications may run on the same or different nodes without any
 174  change (except for the according addressing information):
 175
 176         ___   ___   ___                   _______   ___
 177        | _ | | _ | | _ |                 | _   _ | | _ |
 178        ||A|| ||B|| ||C||                 ||A| |B|| ||C||
 179        |___| |___| |___|                 |_______| |___|
 180          |     |     |                       |       |
 181        -----------------(1)- CAN bus -(2)---------------
 182
 183  To ensure that application A receives the same information in the
 184  example (2) as it would receive in example (1) there is need for
 185  some kind of local loopback of the sent CAN frames on the appropriate
 186  node.
 187
 188  The Linux network devices (by default) just can handle the
 189  transmission and reception of media dependent frames. Due to the
 190  arbitration on the CAN bus the transmission of a low prio CAN-ID
 191  may be delayed by the reception of a high prio CAN frame. To
 192  reflect the correct* traffic on the node the loopback of the sent
 193  data has to be performed right after a successful transmission. If
 194  the CAN network interface is not capable of performing the loopback for
 195  some reason the SocketCAN core can do this task as a fallback solution.
 196  See chapter 6.2 for details (recommended).
 197
 198  The loopback functionality is enabled by default to reflect standard
 199  networking behaviour for CAN applications. Due to some requests from
 200  the RT-SocketCAN group the loopback optionally may be disabled for each
 201  separate socket. See sockopts from the CAN RAW sockets in chapter 4.1.
 202
 203  * = you really like to have this when you're running analyser tools
 204      like 'candump' or 'cansniffer' on the (same) node.
 205
 206  3.3 network security issues (capabilities)
 207
 208  The Controller Area Network is a local field bus transmitting only
 209  broadcast messages without any routing and security concepts.
 210  In the majority of cases the user application has to deal with
 211  raw CAN frames. Therefore it might be reasonable NOT to restrict
 212  the CAN access only to the user root, as known from other networks.
 213  Since the currently implemented CAN_RAW and CAN_BCM sockets can only
 214  send and receive frames to/from CAN interfaces it does not affect
 215  security of others networks to allow all users to access the CAN.
 216  To enable non-root users to access CAN_RAW and CAN_BCM protocol
 217  sockets the Kconfig options CAN_RAW_USER and/or CAN_BCM_USER may be
 218  selected at kernel compile time.
 219
 220  3.4 network problem notifications
 221
 222  The use of the CAN bus may lead to several problems on the physical
 223  and media access control layer. Detecting and logging of these lower
 224  layer problems is a vital requirement for CAN users to identify
 225  hardware issues on the physical transceiver layer as well as
 226  arbitration problems and error frames caused by the different
 227  ECUs. The occurrence of detected errors are important for diagnosis
 228  and have to be logged together with the exact timestamp. For this
 229  reason the CAN interface driver can generate so called Error Frames
 230  that can optionally be passed to the user application in the same
 231  way as other CAN frames. Whenever an error on the physical layer
 232  or the MAC layer is detected (e.g. by the CAN controller) the driver
 233  creates an appropriate error frame. Error frames can be requested by
 234  the user application using the common CAN filter mechanisms. Inside
 235  this filter definition the (interested) type of errors may be
 236  selected. The reception of error frames is disabled by default.
 237
 2384. How to use Socket CAN
 239------------------------
 240
 241  Like TCP/IP, you first need to open a socket for communicating over a
 242  CAN network. Since Socket CAN implements a new protocol family, you
 243  need to pass PF_CAN as the first argument to the socket(2) system
 244  call. Currently, there are two CAN protocols to choose from, the raw
 245  socket protocol and the broadcast manager (BCM). So to open a socket,
 246  you would write
 247
 248    s = socket(PF_CAN, SOCK_RAW, CAN_RAW);
 249
 250  and
 251
 252    s = socket(PF_CAN, SOCK_DGRAM, CAN_BCM);
 253
 254  respectively.  After the successful creation of the socket, you would
 255  normally use the bind(2) system call to bind the socket to a CAN
 256  interface (which is different from TCP/IP due to different addressing
 257  - see chapter 3). After binding (CAN_RAW) or connecting (CAN_BCM)
 258  the socket, you can read(2) and write(2) from/to the socket or use
 259  send(2), sendto(2), sendmsg(2) and the recv* counterpart operations
 260  on the socket as usual. There are also CAN specific socket options
 261  described below.
 262
 263  The basic CAN frame structure and the sockaddr structure are defined
 264  in include/linux/can.h:
 265
 266    struct can_frame {
 267            canid_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */
 268            __u8    can_dlc; /* data length code: 0 .. 8 */
 269            __u8    data[8] __attribute__((aligned(8)));
 270    };
 271
 272  The alignment of the (linear) payload data[] to a 64bit boundary
 273  allows the user to define own structs and unions to easily access the
 274  CAN payload. There is no given byteorder on the CAN bus by
 275  default. A read(2) system call on a CAN_RAW socket transfers a
 276  struct can_frame to the user space.
 277
 278  The sockaddr_can structure has an interface index like the
 279  PF_PACKET socket, that also binds to a specific interface:
 280
 281    struct sockaddr_can {
 282            sa_family_t can_family;
 283            int         can_ifindex;
 284            union {
 285                    /* transport protocol class address info (e.g. ISOTP) */
 286                    struct { canid_t rx_id, tx_id; } tp;
 287
 288                    /* reserved for future CAN protocols address information */
 289            } can_addr;
 290    };
 291
 292  To determine the interface index an appropriate ioctl() has to
 293  be used (example for CAN_RAW sockets without error checking):
 294
 295    int s;
 296    struct sockaddr_can addr;
 297    struct ifreq ifr;
 298
 299    s = socket(PF_CAN, SOCK_RAW, CAN_RAW);
 300
 301    strcpy(ifr.ifr_name, "can0" );
 302    ioctl(s, SIOCGIFINDEX, &ifr);
 303
 304    addr.can_family = AF_CAN;
 305    addr.can_ifindex = ifr.ifr_ifindex;
 306
 307    bind(s, (struct sockaddr *)&addr, sizeof(addr));
 308
 309    (..)
 310
 311  To bind a socket to all(!) CAN interfaces the interface index must
 312  be 0 (zero). In this case the socket receives CAN frames from every
 313  enabled CAN interface. To determine the originating CAN interface
 314  the system call recvfrom(2) may be used instead of read(2). To send
 315  on a socket that is bound to 'any' interface sendto(2) is needed to
 316  specify the outgoing interface.
 317
 318  Reading CAN frames from a bound CAN_RAW socket (see above) consists
 319  of reading a struct can_frame:
 320
 321    struct can_frame frame;
 322
 323    nbytes = read(s, &frame, sizeof(struct can_frame));
 324
 325    if (nbytes < 0) {
 326            perror("can raw socket read");
 327            return 1;
 328    }
 329
 330    /* paraniod check ... */
 331    if (nbytes < sizeof(struct can_frame)) {
 332            fprintf(stderr, "read: incomplete CAN frame\n");
 333            return 1;
 334    }
 335
 336    /* do something with the received CAN frame */
 337
 338  Writing CAN frames can be done similarly, with the write(2) system call:
 339
 340    nbytes = write(s, &frame, sizeof(struct can_frame));
 341
 342  When the CAN interface is bound to 'any' existing CAN interface
 343  (addr.can_ifindex = 0) it is recommended to use recvfrom(2) if the
 344  information about the originating CAN interface is needed:
 345
 346    struct sockaddr_can addr;
 347    struct ifreq ifr;
 348    socklen_t len = sizeof(addr);
 349    struct can_frame frame;
 350
 351    nbytes = recvfrom(s, &frame, sizeof(struct can_frame),
 352                      0, (struct sockaddr*)&addr, &len);
 353
 354    /* get interface name of the received CAN frame */
 355    ifr.ifr_ifindex = addr.can_ifindex;
 356    ioctl(s, SIOCGIFNAME, &ifr);
 357    printf("Received a CAN frame from interface %s", ifr.ifr_name);
 358
 359  To write CAN frames on sockets bound to 'any' CAN interface the
 360  outgoing interface has to be defined certainly.
 361
 362    strcpy(ifr.ifr_name, "can0");
 363    ioctl(s, SIOCGIFINDEX, &ifr);
 364    addr.can_ifindex = ifr.ifr_ifindex;
 365    addr.can_family  = AF_CAN;
 366
 367    nbytes = sendto(s, &frame, sizeof(struct can_frame),
 368                    0, (struct sockaddr*)&addr, sizeof(addr));
 369
 370  4.1 RAW protocol sockets with can_filters (SOCK_RAW)
 371
 372  Using CAN_RAW sockets is extensively comparable to the commonly
 373  known access to CAN character devices. To meet the new possibilities
 374  provided by the multi user SocketCAN approach, some reasonable
 375  defaults are set at RAW socket binding time:
 376
 377  - The filters are set to exactly one filter receiving everything
 378  - The socket only receives valid data frames (=> no error frames)
 379  - The loopback of sent CAN frames is enabled (see chapter 3.2)
 380  - The socket does not receive its own sent frames (in loopback mode)
 381
 382  These default settings may be changed before or after binding the socket.
 383  To use the referenced definitions of the socket options for CAN_RAW
 384  sockets, include <linux/can/raw.h>.
 385
 386  4.1.1 RAW socket option CAN_RAW_FILTER
 387
 388  The reception of CAN frames using CAN_RAW sockets can be controlled
 389  by defining 0 .. n filters with the CAN_RAW_FILTER socket option.
 390
 391  The CAN filter structure is defined in include/linux/can.h:
 392
 393    struct can_filter {
 394            canid_t can_id;
 395            canid_t can_mask;
 396    };
 397
 398  A filter matches, when
 399
 400    <received_can_id> & mask == can_id & mask
 401
 402  which is analogous to known CAN controllers hardware filter semantics.
 403  The filter can be inverted in this semantic, when the CAN_INV_FILTER
 404  bit is set in can_id element of the can_filter structure. In
 405  contrast to CAN controller hardware filters the user may set 0 .. n
 406  receive filters for each open socket separately:
 407
 408    struct can_filter rfilter[2];
 409
 410    rfilter[0].can_id   = 0x123;
 411    rfilter[0].can_mask = CAN_SFF_MASK;
 412    rfilter[1].can_id   = 0x200;
 413    rfilter[1].can_mask = 0x700;
 414
 415    setsockopt(s, SOL_CAN_RAW, CAN_RAW_FILTER, &rfilter, sizeof(rfilter));
 416
 417  To disable the reception of CAN frames on the selected CAN_RAW socket:
 418
 419    setsockopt(s, SOL_CAN_RAW, CAN_RAW_FILTER, NULL, 0);
 420
 421  To set the filters to zero filters is quite obsolete as not read
 422  data causes the raw socket to discard the received CAN frames. But
 423  having this 'send only' use-case we may remove the receive list in the
 424  Kernel to save a little (really a very little!) CPU usage.
 425
 426  4.1.2 RAW socket option CAN_RAW_ERR_FILTER
 427
 428  As described in chapter 3.4 the CAN interface driver can generate so
 429  called Error Frames that can optionally be passed to the user
 430  application in the same way as other CAN frames. The possible
 431  errors are divided into different error classes that may be filtered
 432  using the appropriate error mask. To register for every possible
 433  error condition CAN_ERR_MASK can be used as value for the error mask.
 434  The values for the error mask are defined in linux/can/error.h .
 435
 436    can_err_mask_t err_mask = ( CAN_ERR_TX_TIMEOUT | CAN_ERR_BUSOFF );
 437
 438    setsockopt(s, SOL_CAN_RAW, CAN_RAW_ERR_FILTER,
 439               &err_mask, sizeof(err_mask));
 440
 441  4.1.3 RAW socket option CAN_RAW_LOOPBACK
 442
 443  To meet multi user needs the local loopback is enabled by default
 444  (see chapter 3.2 for details). But in some embedded use-cases
 445  (e.g. when only one application uses the CAN bus) this loopback
 446  functionality can be disabled (separately for each socket):
 447
 448    int loopback = 0; /* 0 = disabled, 1 = enabled (default) */
 449
 450    setsockopt(s, SOL_CAN_RAW, CAN_RAW_LOOPBACK, &loopback, sizeof(loopback));
 451
 452  4.1.4 RAW socket option CAN_RAW_RECV_OWN_MSGS
 453
 454  When the local loopback is enabled, all the sent CAN frames are
 455  looped back to the open CAN sockets that registered for the CAN
 456  frames' CAN-ID on this given interface to meet the multi user
 457  needs. The reception of the CAN frames on the same socket that was
 458  sending the CAN frame is assumed to be unwanted and therefore
 459  disabled by default. This default behaviour may be changed on
 460  demand:
 461
 462    int recv_own_msgs = 1; /* 0 = disabled (default), 1 = enabled */
 463
 464    setsockopt(s, SOL_CAN_RAW, CAN_RAW_RECV_OWN_MSGS,
 465               &recv_own_msgs, sizeof(recv_own_msgs));
 466
 467  4.2 Broadcast Manager protocol sockets (SOCK_DGRAM)
 468  4.3 connected transport protocols (SOCK_SEQPACKET)
 469  4.4 unconnected transport protocols (SOCK_DGRAM)
 470
 471
 4725. Socket CAN core module
 473-------------------------
 474
 475  The Socket CAN core module implements the protocol family
 476  PF_CAN. CAN protocol modules are loaded by the core module at
 477  runtime. The core module provides an interface for CAN protocol
 478  modules to subscribe needed CAN IDs (see chapter 3.1).
 479
 480  5.1 can.ko module params
 481
 482  - stats_timer: To calculate the Socket CAN core statistics
 483    (e.g. current/maximum frames per second) this 1 second timer is
 484    invoked at can.ko module start time by default. This timer can be
 485    disabled by using stattimer=0 on the module commandline.
 486
 487  - debug: (removed since SocketCAN SVN r546)
 488
 489  5.2 procfs content
 490
 491  As described in chapter 3.1 the Socket CAN core uses several filter
 492  lists to deliver received CAN frames to CAN protocol modules. These
 493  receive lists, their filters and the count of filter matches can be
 494  checked in the appropriate receive list. All entries contain the
 495  device and a protocol module identifier:
 496
 497    foo@bar:~$ cat /proc/net/can/rcvlist_all
 498
 499    receive list 'rx_all':
 500      (vcan3: no entry)
 501      (vcan2: no entry)
 502      (vcan1: no entry)
 503      device   can_id   can_mask  function  userdata   matches  ident
 504       vcan0     000    00000000  f88e6370  f6c6f400         0  raw
 505      (any: no entry)
 506
 507  In this example an application requests any CAN traffic from vcan0.
 508
 509    rcvlist_all - list for unfiltered entries (no filter operations)
 510    rcvlist_eff - list for single extended frame (EFF) entries
 511    rcvlist_err - list for error frames masks
 512    rcvlist_fil - list for mask/value filters
 513    rcvlist_inv - list for mask/value filters (inverse semantic)
 514    rcvlist_sff - list for single standard frame (SFF) entries
 515
 516  Additional procfs files in /proc/net/can
 517
 518    stats       - Socket CAN core statistics (rx/tx frames, match ratios, ...)
 519    reset_stats - manual statistic reset
 520    version     - prints the Socket CAN core version and the ABI version
 521
 522  5.3 writing own CAN protocol modules
 523
 524  To implement a new protocol in the protocol family PF_CAN a new
 525  protocol has to be defined in include/linux/can.h .
 526  The prototypes and definitions to use the Socket CAN core can be
 527  accessed by including include/linux/can/core.h .
 528  In addition to functions that register the CAN protocol and the
 529  CAN device notifier chain there are functions to subscribe CAN
 530  frames received by CAN interfaces and to send CAN frames:
 531
 532    can_rx_register   - subscribe CAN frames from a specific interface
 533    can_rx_unregister - unsubscribe CAN frames from a specific interface
 534    can_send          - transmit a CAN frame (optional with local loopback)
 535
 536  For details see the kerneldoc documentation in net/can/af_can.c or
 537  the source code of net/can/raw.c or net/can/bcm.c .
 538
 5396. CAN network drivers
 540----------------------
 541
 542  Writing a CAN network device driver is much easier than writing a
 543  CAN character device driver. Similar to other known network device
 544  drivers you mainly have to deal with:
 545
 546  - TX: Put the CAN frame from the socket buffer to the CAN controller.
 547  - RX: Put the CAN frame from the CAN controller to the socket buffer.
 548
 549  See e.g. at Documentation/networking/netdevices.txt . The differences
 550  for writing CAN network device driver are described below:
 551
 552  6.1 general settings
 553
 554    dev->type  = ARPHRD_CAN; /* the netdevice hardware type */
 555    dev->flags = IFF_NOARP;  /* CAN has no arp */
 556
 557    dev->mtu   = sizeof(struct can_frame);
 558
 559  The struct can_frame is the payload of each socket buffer in the
 560  protocol family PF_CAN.
 561
 562  6.2 local loopback of sent frames
 563
 564  As described in chapter 3.2 the CAN network device driver should
 565  support a local loopback functionality similar to the local echo
 566  e.g. of tty devices. In this case the driver flag IFF_ECHO has to be
 567  set to prevent the PF_CAN core from locally echoing sent frames
 568  (aka loopback) as fallback solution:
 569
 570    dev->flags = (IFF_NOARP | IFF_ECHO);
 571
 572  6.3 CAN controller hardware filters
 573
 574  To reduce the interrupt load on deep embedded systems some CAN
 575  controllers support the filtering of CAN IDs or ranges of CAN IDs.
 576  These hardware filter capabilities vary from controller to
 577  controller and have to be identified as not feasible in a multi-user
 578  networking approach. The use of the very controller specific
 579  hardware filters could make sense in a very dedicated use-case, as a
 580  filter on driver level would affect all users in the multi-user
 581  system. The high efficient filter sets inside the PF_CAN core allow
 582  to set different multiple filters for each socket separately.
 583  Therefore the use of hardware filters goes to the category 'handmade
 584  tuning on deep embedded systems'. The author is running a MPC603e
 585  @133MHz with four SJA1000 CAN controllers from 2002 under heavy bus
 586  load without any problems ...
 587
 588  6.4 The virtual CAN driver (vcan)
 589
 590  Similar to the network loopback devices, vcan offers a virtual local
 591  CAN interface. A full qualified address on CAN consists of
 592
 593  - a unique CAN Identifier (CAN ID)
 594  - the CAN bus this CAN ID is transmitted on (e.g. can0)
 595
 596  so in common use cases more than one virtual CAN interface is needed.
 597
 598  The virtual CAN interfaces allow the transmission and reception of CAN
 599  frames without real CAN controller hardware. Virtual CAN network
 600  devices are usually named 'vcanX', like vcan0 vcan1 vcan2 ...
 601  When compiled as a module the virtual CAN driver module is called vcan.ko
 602
 603  Since Linux Kernel version 2.6.24 the vcan driver supports the Kernel
 604  netlink interface to create vcan network devices. The creation and
 605  removal of vcan network devices can be managed with the ip(8) tool:
 606
 607  - Create a virtual CAN network interface:
 608       ip link add type vcan
 609
 610  - Create a virtual CAN network interface with a specific name 'vcan42':
 611       ip link add dev vcan42 type vcan
 612
 613  - Remove a (virtual CAN) network interface 'vcan42':
 614       ip link del vcan42
 615
 616  The tool 'vcan' from the SocketCAN SVN repository on BerliOS is obsolete.
 617
 618  Virtual CAN network device creation in older Kernels:
 619  In Linux Kernel versions < 2.6.24 the vcan driver creates 4 vcan
 620  netdevices at module load time by default. This value can be changed
 621  with the module parameter 'numdev'. E.g. 'modprobe vcan numdev=8'
 622
 623  6.5 currently supported CAN hardware
 624
 625  On the project website http://developer.berlios.de/projects/socketcan
 626  there are different drivers available:
 627
 628    vcan:    Virtual CAN interface driver (if no real hardware is available)
 629    sja1000: Philips SJA1000 CAN controller (recommended)
 630    i82527:  Intel i82527 CAN controller
 631    mscan:   Motorola/Freescale CAN controller (e.g. inside SOC MPC5200)
 632    ccan:    CCAN controller core (e.g. inside SOC h7202)
 633    slcan:   For a bunch of CAN adaptors that are attached via a
 634             serial line ASCII protocol (for serial / USB adaptors)
 635
 636  Additionally the different CAN adaptors (ISA/PCI/PCMCIA/USB/Parport)
 637  from PEAK Systemtechnik support the CAN netdevice driver model
 638  since Linux driver v6.0: http://www.peak-system.com/linux/index.htm
 639
 640  Please check the Mailing Lists on the berlios OSS project website.
 641
 642  6.6 todo
 643
 644  The configuration interface for CAN network drivers is still an open
 645  issue that has not been finalized in the socketcan project. Also the
 646  idea of having a library module (candev.ko) that holds functions
 647  that are needed by all CAN netdevices is not ready to ship.
 648  Your contribution is welcome.
 649
 6507. Credits
 651----------
 652
 653  Oliver Hartkopp (PF_CAN core, filters, drivers, bcm)
 654  Urs Thuermann (PF_CAN core, kernel integration, socket interfaces, raw, vcan)
 655  Jan Kizka (RT-SocketCAN core, Socket-API reconciliation)
 656  Wolfgang Grandegger (RT-SocketCAN core & drivers, Raw Socket-API reviews)
 657  Robert Schwebel (design reviews, PTXdist integration)
 658  Marc Kleine-Budde (design reviews, Kernel 2.6 cleanups, drivers)
 659  Benedikt Spranger (reviews)
 660  Thomas Gleixner (LKML reviews, coding style, posting hints)
 661  Andrey Volkov (kernel subtree structure, ioctls, mscan driver)
 662  Matthias Brukner (first SJA1000 CAN netdevice implementation Q2/2003)
 663  Klaus Hitschler (PEAK driver integration)
 664  Uwe Koppe (CAN netdevices with PF_PACKET approach)
 665  Michael Schulze (driver layer loopback requirement, RT CAN drivers review)
 666
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