About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Documentation / networking / can.txt




Custom Search

Based on kernel version 3.13. Page generated on 2014-01-20 22:03 EST.

1	============================================================================
2	
3	can.txt
4	
5	Readme file for the Controller Area Network Protocol Family (aka Socket CAN)
6	
7	This 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	      4.1.5 RAW socket option CAN_RAW_FD_FRAMES
26	      4.1.6 RAW socket returned message flags
27	    4.2 Broadcast Manager protocol sockets (SOCK_DGRAM)
28	      4.2.1 Broadcast Manager operations
29	      4.2.2 Broadcast Manager message flags
30	      4.2.3 Broadcast Manager transmission timers
31	      4.2.4 Broadcast Manager message sequence transmission
32	      4.2.5 Broadcast Manager receive filter timers
33	      4.2.6 Broadcast Manager multiplex message receive filter
34	    4.3 connected transport protocols (SOCK_SEQPACKET)
35	    4.4 unconnected transport protocols (SOCK_DGRAM)
36	
37	  5 Socket CAN core module
38	    5.1 can.ko module params
39	    5.2 procfs content
40	    5.3 writing own CAN protocol modules
41	
42	  6 CAN network drivers
43	    6.1 general settings
44	    6.2 local loopback of sent frames
45	    6.3 CAN controller hardware filters
46	    6.4 The virtual CAN driver (vcan)
47	    6.5 The CAN network device driver interface
48	      6.5.1 Netlink interface to set/get devices properties
49	      6.5.2 Setting the CAN bit-timing
50	      6.5.3 Starting and stopping the CAN network device
51	    6.6 CAN FD (flexible data rate) driver support
52	    6.7 supported CAN hardware
53	
54	  7 Socket CAN resources
55	
56	  8 Credits
57	
58	============================================================================
59	
60	1. Overview / What is Socket CAN
61	--------------------------------
62	
63	The socketcan package is an implementation of CAN protocols
64	(Controller Area Network) for Linux.  CAN is a networking technology
65	which has widespread use in automation, embedded devices, and
66	automotive fields.  While there have been other CAN implementations
67	for Linux based on character devices, Socket CAN uses the Berkeley
68	socket API, the Linux network stack and implements the CAN device
69	drivers as network interfaces.  The CAN socket API has been designed
70	as similar as possible to the TCP/IP protocols to allow programmers,
71	familiar with network programming, to easily learn how to use CAN
72	sockets.
73	
74	2. Motivation / Why using the socket API
75	----------------------------------------
76	
77	There have been CAN implementations for Linux before Socket CAN so the
78	question arises, why we have started another project.  Most existing
79	implementations come as a device driver for some CAN hardware, they
80	are based on character devices and provide comparatively little
81	functionality.  Usually, there is only a hardware-specific device
82	driver which provides a character device interface to send and
83	receive raw CAN frames, directly to/from the controller hardware.
84	Queueing of frames and higher-level transport protocols like ISO-TP
85	have to be implemented in user space applications.  Also, most
86	character-device implementations support only one single process to
87	open the device at a time, similar to a serial interface.  Exchanging
88	the CAN controller requires employment of another device driver and
89	often the need for adaption of large parts of the application to the
90	new driver's API.
91	
92	Socket CAN was designed to overcome all of these limitations.  A new
93	protocol family has been implemented which provides a socket interface
94	to user space applications and which builds upon the Linux network
95	layer, so to use all of the provided queueing functionality.  A device
96	driver for CAN controller hardware registers itself with the Linux
97	network layer as a network device, so that CAN frames from the
98	controller can be passed up to the network layer and on to the CAN
99	protocol family module and also vice-versa.  Also, the protocol family
100	module provides an API for transport protocol modules to register, so
101	that any number of transport protocols can be loaded or unloaded
102	dynamically.  In fact, the can core module alone does not provide any
103	protocol and cannot be used without loading at least one additional
104	protocol module.  Multiple sockets can be opened at the same time,
105	on different or the same protocol module and they can listen/send
106	frames on different or the same CAN IDs.  Several sockets listening on
107	the same interface for frames with the same CAN ID are all passed the
108	same received matching CAN frames.  An application wishing to
109	communicate using a specific transport protocol, e.g. ISO-TP, just
110	selects that protocol when opening the socket, and then can read and
111	write application data byte streams, without having to deal with
112	CAN-IDs, frames, etc.
113	
114	Similar functionality visible from user-space could be provided by a
115	character device, too, but this would lead to a technically inelegant
116	solution for a couple of reasons:
117	
118	* Intricate usage.  Instead of passing a protocol argument to
119	  socket(2) and using bind(2) to select a CAN interface and CAN ID, an
120	  application would have to do all these operations using ioctl(2)s.
121	
122	* Code duplication.  A character device cannot make use of the Linux
123	  network queueing code, so all that code would have to be duplicated
124	  for CAN networking.
125	
126	* Abstraction.  In most existing character-device implementations, the
127	  hardware-specific device driver for a CAN controller directly
128	  provides the character device for the application to work with.
129	  This is at least very unusual in Unix systems for both, char and
130	  block devices.  For example you don't have a character device for a
131	  certain UART of a serial interface, a certain sound chip in your
132	  computer, a SCSI or IDE controller providing access to your hard
133	  disk or tape streamer device.  Instead, you have abstraction layers
134	  which provide a unified character or block device interface to the
135	  application on the one hand, and a interface for hardware-specific
136	  device drivers on the other hand.  These abstractions are provided
137	  by subsystems like the tty layer, the audio subsystem or the SCSI
138	  and IDE subsystems for the devices mentioned above.
139	
140	  The easiest way to implement a CAN device driver is as a character
141	  device without such a (complete) abstraction layer, as is done by most
142	  existing drivers.  The right way, however, would be to add such a
143	  layer with all the functionality like registering for certain CAN
144	  IDs, supporting several open file descriptors and (de)multiplexing
145	  CAN frames between them, (sophisticated) queueing of CAN frames, and
146	  providing an API for device drivers to register with.  However, then
147	  it would be no more difficult, or may be even easier, to use the
148	  networking framework provided by the Linux kernel, and this is what
149	  Socket CAN does.
150	
151	  The use of the networking framework of the Linux kernel is just the
152	  natural and most appropriate way to implement CAN for Linux.
153	
154	3. Socket CAN concept
155	---------------------
156	
157	  As described in chapter 2 it is the main goal of Socket CAN to
158	  provide a socket interface to user space applications which builds
159	  upon the Linux network layer. In contrast to the commonly known
160	  TCP/IP and ethernet networking, the CAN bus is a broadcast-only(!)
161	  medium that has no MAC-layer addressing like ethernet. The CAN-identifier
162	  (can_id) is used for arbitration on the CAN-bus. Therefore the CAN-IDs
163	  have to be chosen uniquely on the bus. When designing a CAN-ECU
164	  network the CAN-IDs are mapped to be sent by a specific ECU.
165	  For this reason a CAN-ID can be treated best as a kind of source address.
166	
167	  3.1 receive lists
168	
169	  The network transparent access of multiple applications leads to the
170	  problem that different applications may be interested in the same
171	  CAN-IDs from the same CAN network interface. The Socket CAN core
172	  module - which implements the protocol family CAN - provides several
173	  high efficient receive lists for this reason. If e.g. a user space
174	  application opens a CAN RAW socket, the raw protocol module itself
175	  requests the (range of) CAN-IDs from the Socket CAN core that are
176	  requested by the user. The subscription and unsubscription of
177	  CAN-IDs can be done for specific CAN interfaces or for all(!) known
178	  CAN interfaces with the can_rx_(un)register() functions provided to
179	  CAN protocol modules by the SocketCAN core (see chapter 5).
180	  To optimize the CPU usage at runtime the receive lists are split up
181	  into several specific lists per device that match the requested
182	  filter complexity for a given use-case.
183	
184	  3.2 local loopback of sent frames
185	
186	  As known from other networking concepts the data exchanging
187	  applications may run on the same or different nodes without any
188	  change (except for the according addressing information):
189	
190	         ___   ___   ___                   _______   ___
191	        | _ | | _ | | _ |                 | _   _ | | _ |
192	        ||A|| ||B|| ||C||                 ||A| |B|| ||C||
193	        |___| |___| |___|                 |_______| |___|
194	          |     |     |                       |       |
195	        -----------------(1)- CAN bus -(2)---------------
196	
197	  To ensure that application A receives the same information in the
198	  example (2) as it would receive in example (1) there is need for
199	  some kind of local loopback of the sent CAN frames on the appropriate
200	  node.
201	
202	  The Linux network devices (by default) just can handle the
203	  transmission and reception of media dependent frames. Due to the
204	  arbitration on the CAN bus the transmission of a low prio CAN-ID
205	  may be delayed by the reception of a high prio CAN frame. To
206	  reflect the correct* traffic on the node the loopback of the sent
207	  data has to be performed right after a successful transmission. If
208	  the CAN network interface is not capable of performing the loopback for
209	  some reason the SocketCAN core can do this task as a fallback solution.
210	  See chapter 6.2 for details (recommended).
211	
212	  The loopback functionality is enabled by default to reflect standard
213	  networking behaviour for CAN applications. Due to some requests from
214	  the RT-SocketCAN group the loopback optionally may be disabled for each
215	  separate socket. See sockopts from the CAN RAW sockets in chapter 4.1.
216	
217	  * = you really like to have this when you're running analyser tools
218	      like 'candump' or 'cansniffer' on the (same) node.
219	
220	  3.3 network security issues (capabilities)
221	
222	  The Controller Area Network is a local field bus transmitting only
223	  broadcast messages without any routing and security concepts.
224	  In the majority of cases the user application has to deal with
225	  raw CAN frames. Therefore it might be reasonable NOT to restrict
226	  the CAN access only to the user root, as known from other networks.
227	  Since the currently implemented CAN_RAW and CAN_BCM sockets can only
228	  send and receive frames to/from CAN interfaces it does not affect
229	  security of others networks to allow all users to access the CAN.
230	  To enable non-root users to access CAN_RAW and CAN_BCM protocol
231	  sockets the Kconfig options CAN_RAW_USER and/or CAN_BCM_USER may be
232	  selected at kernel compile time.
233	
234	  3.4 network problem notifications
235	
236	  The use of the CAN bus may lead to several problems on the physical
237	  and media access control layer. Detecting and logging of these lower
238	  layer problems is a vital requirement for CAN users to identify
239	  hardware issues on the physical transceiver layer as well as
240	  arbitration problems and error frames caused by the different
241	  ECUs. The occurrence of detected errors are important for diagnosis
242	  and have to be logged together with the exact timestamp. For this
243	  reason the CAN interface driver can generate so called Error Message
244	  Frames that can optionally be passed to the user application in the
245	  same way as other CAN frames. Whenever an error on the physical layer
246	  or the MAC layer is detected (e.g. by the CAN controller) the driver
247	  creates an appropriate error message frame. Error messages frames can
248	  be requested by the user application using the common CAN filter
249	  mechanisms. Inside this filter definition the (interested) type of
250	  errors may be selected. The reception of error messages is disabled
251	  by default. The format of the CAN error message frame is briefly
252	  described in the Linux header file "include/linux/can/error.h".
253	
254	4. How to use Socket CAN
255	------------------------
256	
257	  Like TCP/IP, you first need to open a socket for communicating over a
258	  CAN network. Since Socket CAN implements a new protocol family, you
259	  need to pass PF_CAN as the first argument to the socket(2) system
260	  call. Currently, there are two CAN protocols to choose from, the raw
261	  socket protocol and the broadcast manager (BCM). So to open a socket,
262	  you would write
263	
264	    s = socket(PF_CAN, SOCK_RAW, CAN_RAW);
265	
266	  and
267	
268	    s = socket(PF_CAN, SOCK_DGRAM, CAN_BCM);
269	
270	  respectively.  After the successful creation of the socket, you would
271	  normally use the bind(2) system call to bind the socket to a CAN
272	  interface (which is different from TCP/IP due to different addressing
273	  - see chapter 3). After binding (CAN_RAW) or connecting (CAN_BCM)
274	  the socket, you can read(2) and write(2) from/to the socket or use
275	  send(2), sendto(2), sendmsg(2) and the recv* counterpart operations
276	  on the socket as usual. There are also CAN specific socket options
277	  described below.
278	
279	  The basic CAN frame structure and the sockaddr structure are defined
280	  in include/linux/can.h:
281	
282	    struct can_frame {
283	            canid_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */
284	            __u8    can_dlc; /* frame payload length in byte (0 .. 8) */
285	            __u8    data[8] __attribute__((aligned(8)));
286	    };
287	
288	  The alignment of the (linear) payload data[] to a 64bit boundary
289	  allows the user to define own structs and unions to easily access the
290	  CAN payload. There is no given byteorder on the CAN bus by
291	  default. A read(2) system call on a CAN_RAW socket transfers a
292	  struct can_frame to the user space.
293	
294	  The sockaddr_can structure has an interface index like the
295	  PF_PACKET socket, that also binds to a specific interface:
296	
297	    struct sockaddr_can {
298	            sa_family_t can_family;
299	            int         can_ifindex;
300	            union {
301	                    /* transport protocol class address info (e.g. ISOTP) */
302	                    struct { canid_t rx_id, tx_id; } tp;
303	
304	                    /* reserved for future CAN protocols address information */
305	            } can_addr;
306	    };
307	
308	  To determine the interface index an appropriate ioctl() has to
309	  be used (example for CAN_RAW sockets without error checking):
310	
311	    int s;
312	    struct sockaddr_can addr;
313	    struct ifreq ifr;
314	
315	    s = socket(PF_CAN, SOCK_RAW, CAN_RAW);
316	
317	    strcpy(ifr.ifr_name, "can0" );
318	    ioctl(s, SIOCGIFINDEX, &ifr);
319	
320	    addr.can_family = AF_CAN;
321	    addr.can_ifindex = ifr.ifr_ifindex;
322	
323	    bind(s, (struct sockaddr *)&addr, sizeof(addr));
324	
325	    (..)
326	
327	  To bind a socket to all(!) CAN interfaces the interface index must
328	  be 0 (zero). In this case the socket receives CAN frames from every
329	  enabled CAN interface. To determine the originating CAN interface
330	  the system call recvfrom(2) may be used instead of read(2). To send
331	  on a socket that is bound to 'any' interface sendto(2) is needed to
332	  specify the outgoing interface.
333	
334	  Reading CAN frames from a bound CAN_RAW socket (see above) consists
335	  of reading a struct can_frame:
336	
337	    struct can_frame frame;
338	
339	    nbytes = read(s, &frame, sizeof(struct can_frame));
340	
341	    if (nbytes < 0) {
342	            perror("can raw socket read");
343	            return 1;
344	    }
345	
346	    /* paranoid check ... */
347	    if (nbytes < sizeof(struct can_frame)) {
348	            fprintf(stderr, "read: incomplete CAN frame\n");
349	            return 1;
350	    }
351	
352	    /* do something with the received CAN frame */
353	
354	  Writing CAN frames can be done similarly, with the write(2) system call:
355	
356	    nbytes = write(s, &frame, sizeof(struct can_frame));
357	
358	  When the CAN interface is bound to 'any' existing CAN interface
359	  (addr.can_ifindex = 0) it is recommended to use recvfrom(2) if the
360	  information about the originating CAN interface is needed:
361	
362	    struct sockaddr_can addr;
363	    struct ifreq ifr;
364	    socklen_t len = sizeof(addr);
365	    struct can_frame frame;
366	
367	    nbytes = recvfrom(s, &frame, sizeof(struct can_frame),
368	                      0, (struct sockaddr*)&addr, &len);
369	
370	    /* get interface name of the received CAN frame */
371	    ifr.ifr_ifindex = addr.can_ifindex;
372	    ioctl(s, SIOCGIFNAME, &ifr);
373	    printf("Received a CAN frame from interface %s", ifr.ifr_name);
374	
375	  To write CAN frames on sockets bound to 'any' CAN interface the
376	  outgoing interface has to be defined certainly.
377	
378	    strcpy(ifr.ifr_name, "can0");
379	    ioctl(s, SIOCGIFINDEX, &ifr);
380	    addr.can_ifindex = ifr.ifr_ifindex;
381	    addr.can_family  = AF_CAN;
382	
383	    nbytes = sendto(s, &frame, sizeof(struct can_frame),
384	                    0, (struct sockaddr*)&addr, sizeof(addr));
385	
386	  Remark about CAN FD (flexible data rate) support:
387	
388	  Generally the handling of CAN FD is very similar to the formerly described
389	  examples. The new CAN FD capable CAN controllers support two different
390	  bitrates for the arbitration phase and the payload phase of the CAN FD frame
391	  and up to 64 bytes of payload. This extended payload length breaks all the
392	  kernel interfaces (ABI) which heavily rely on the CAN frame with fixed eight
393	  bytes of payload (struct can_frame) like the CAN_RAW socket. Therefore e.g.
394	  the CAN_RAW socket supports a new socket option CAN_RAW_FD_FRAMES that
395	  switches the socket into a mode that allows the handling of CAN FD frames
396	  and (legacy) CAN frames simultaneously (see section 4.1.5).
397	
398	  The struct canfd_frame is defined in include/linux/can.h:
399	
400	    struct canfd_frame {
401	            canid_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */
402	            __u8    len;     /* frame payload length in byte (0 .. 64) */
403	            __u8    flags;   /* additional flags for CAN FD */
404	            __u8    __res0;  /* reserved / padding */
405	            __u8    __res1;  /* reserved / padding */
406	            __u8    data[64] __attribute__((aligned(8)));
407	    };
408	
409	  The struct canfd_frame and the existing struct can_frame have the can_id,
410	  the payload length and the payload data at the same offset inside their
411	  structures. This allows to handle the different structures very similar.
412	  When the content of a struct can_frame is copied into a struct canfd_frame
413	  all structure elements can be used as-is - only the data[] becomes extended.
414	
415	  When introducing the struct canfd_frame it turned out that the data length
416	  code (DLC) of the struct can_frame was used as a length information as the
417	  length and the DLC has a 1:1 mapping in the range of 0 .. 8. To preserve
418	  the easy handling of the length information the canfd_frame.len element
419	  contains a plain length value from 0 .. 64. So both canfd_frame.len and
420	  can_frame.can_dlc are equal and contain a length information and no DLC.
421	  For details about the distinction of CAN and CAN FD capable devices and
422	  the mapping to the bus-relevant data length code (DLC), see chapter 6.6.
423	
424	  The length of the two CAN(FD) frame structures define the maximum transfer
425	  unit (MTU) of the CAN(FD) network interface and skbuff data length. Two
426	  definitions are specified for CAN specific MTUs in include/linux/can.h :
427	
428	  #define CAN_MTU   (sizeof(struct can_frame))   == 16  => 'legacy' CAN frame
429	  #define CANFD_MTU (sizeof(struct canfd_frame)) == 72  => CAN FD frame
430	
431	  4.1 RAW protocol sockets with can_filters (SOCK_RAW)
432	
433	  Using CAN_RAW sockets is extensively comparable to the commonly
434	  known access to CAN character devices. To meet the new possibilities
435	  provided by the multi user SocketCAN approach, some reasonable
436	  defaults are set at RAW socket binding time:
437	
438	  - The filters are set to exactly one filter receiving everything
439	  - The socket only receives valid data frames (=> no error message frames)
440	  - The loopback of sent CAN frames is enabled (see chapter 3.2)
441	  - The socket does not receive its own sent frames (in loopback mode)
442	
443	  These default settings may be changed before or after binding the socket.
444	  To use the referenced definitions of the socket options for CAN_RAW
445	  sockets, include <linux/can/raw.h>.
446	
447	  4.1.1 RAW socket option CAN_RAW_FILTER
448	
449	  The reception of CAN frames using CAN_RAW sockets can be controlled
450	  by defining 0 .. n filters with the CAN_RAW_FILTER socket option.
451	
452	  The CAN filter structure is defined in include/linux/can.h:
453	
454	    struct can_filter {
455	            canid_t can_id;
456	            canid_t can_mask;
457	    };
458	
459	  A filter matches, when
460	
461	    <received_can_id> & mask == can_id & mask
462	
463	  which is analogous to known CAN controllers hardware filter semantics.
464	  The filter can be inverted in this semantic, when the CAN_INV_FILTER
465	  bit is set in can_id element of the can_filter structure. In
466	  contrast to CAN controller hardware filters the user may set 0 .. n
467	  receive filters for each open socket separately:
468	
469	    struct can_filter rfilter[2];
470	
471	    rfilter[0].can_id   = 0x123;
472	    rfilter[0].can_mask = CAN_SFF_MASK;
473	    rfilter[1].can_id   = 0x200;
474	    rfilter[1].can_mask = 0x700;
475	
476	    setsockopt(s, SOL_CAN_RAW, CAN_RAW_FILTER, &rfilter, sizeof(rfilter));
477	
478	  To disable the reception of CAN frames on the selected CAN_RAW socket:
479	
480	    setsockopt(s, SOL_CAN_RAW, CAN_RAW_FILTER, NULL, 0);
481	
482	  To set the filters to zero filters is quite obsolete as not read
483	  data causes the raw socket to discard the received CAN frames. But
484	  having this 'send only' use-case we may remove the receive list in the
485	  Kernel to save a little (really a very little!) CPU usage.
486	
487	  4.1.2 RAW socket option CAN_RAW_ERR_FILTER
488	
489	  As described in chapter 3.4 the CAN interface driver can generate so
490	  called Error Message Frames that can optionally be passed to the user
491	  application in the same way as other CAN frames. The possible
492	  errors are divided into different error classes that may be filtered
493	  using the appropriate error mask. To register for every possible
494	  error condition CAN_ERR_MASK can be used as value for the error mask.
495	  The values for the error mask are defined in linux/can/error.h .
496	
497	    can_err_mask_t err_mask = ( CAN_ERR_TX_TIMEOUT | CAN_ERR_BUSOFF );
498	
499	    setsockopt(s, SOL_CAN_RAW, CAN_RAW_ERR_FILTER,
500	               &err_mask, sizeof(err_mask));
501	
502	  4.1.3 RAW socket option CAN_RAW_LOOPBACK
503	
504	  To meet multi user needs the local loopback is enabled by default
505	  (see chapter 3.2 for details). But in some embedded use-cases
506	  (e.g. when only one application uses the CAN bus) this loopback
507	  functionality can be disabled (separately for each socket):
508	
509	    int loopback = 0; /* 0 = disabled, 1 = enabled (default) */
510	
511	    setsockopt(s, SOL_CAN_RAW, CAN_RAW_LOOPBACK, &loopback, sizeof(loopback));
512	
513	  4.1.4 RAW socket option CAN_RAW_RECV_OWN_MSGS
514	
515	  When the local loopback is enabled, all the sent CAN frames are
516	  looped back to the open CAN sockets that registered for the CAN
517	  frames' CAN-ID on this given interface to meet the multi user
518	  needs. The reception of the CAN frames on the same socket that was
519	  sending the CAN frame is assumed to be unwanted and therefore
520	  disabled by default. This default behaviour may be changed on
521	  demand:
522	
523	    int recv_own_msgs = 1; /* 0 = disabled (default), 1 = enabled */
524	
525	    setsockopt(s, SOL_CAN_RAW, CAN_RAW_RECV_OWN_MSGS,
526	               &recv_own_msgs, sizeof(recv_own_msgs));
527	
528	  4.1.5 RAW socket option CAN_RAW_FD_FRAMES
529	
530	  CAN FD support in CAN_RAW sockets can be enabled with a new socket option
531	  CAN_RAW_FD_FRAMES which is off by default. When the new socket option is
532	  not supported by the CAN_RAW socket (e.g. on older kernels), switching the
533	  CAN_RAW_FD_FRAMES option returns the error -ENOPROTOOPT.
534	
535	  Once CAN_RAW_FD_FRAMES is enabled the application can send both CAN frames
536	  and CAN FD frames. OTOH the application has to handle CAN and CAN FD frames
537	  when reading from the socket.
538	
539	    CAN_RAW_FD_FRAMES enabled:  CAN_MTU and CANFD_MTU are allowed
540	    CAN_RAW_FD_FRAMES disabled: only CAN_MTU is allowed (default)
541	
542	  Example:
543	    [ remember: CANFD_MTU == sizeof(struct canfd_frame) ]
544	
545	    struct canfd_frame cfd;
546	
547	    nbytes = read(s, &cfd, CANFD_MTU);
548	
549	    if (nbytes == CANFD_MTU) {
550	            printf("got CAN FD frame with length %d\n", cfd.len);
551		    /* cfd.flags contains valid data */
552	    } else if (nbytes == CAN_MTU) {
553	            printf("got legacy CAN frame with length %d\n", cfd.len);
554		    /* cfd.flags is undefined */
555	    } else {
556	            fprintf(stderr, "read: invalid CAN(FD) frame\n");
557	            return 1;
558	    }
559	
560	    /* the content can be handled independently from the received MTU size */
561	
562	    printf("can_id: %X data length: %d data: ", cfd.can_id, cfd.len);
563	    for (i = 0; i < cfd.len; i++)
564	            printf("%02X ", cfd.data[i]);
565	
566	  When reading with size CANFD_MTU only returns CAN_MTU bytes that have
567	  been received from the socket a legacy CAN frame has been read into the
568	  provided CAN FD structure. Note that the canfd_frame.flags data field is
569	  not specified in the struct can_frame and therefore it is only valid in
570	  CANFD_MTU sized CAN FD frames.
571	
572	  As long as the payload length is <=8 the received CAN frames from CAN FD
573	  capable CAN devices can be received and read by legacy sockets too. When
574	  user-generated CAN FD frames have a payload length <=8 these can be send
575	  by legacy CAN network interfaces too. Sending CAN FD frames with payload
576	  length > 8 to a legacy CAN network interface returns an -EMSGSIZE error.
577	
578	  Implementation hint for new CAN applications:
579	
580	  To build a CAN FD aware application use struct canfd_frame as basic CAN
581	  data structure for CAN_RAW based applications. When the application is
582	  executed on an older Linux kernel and switching the CAN_RAW_FD_FRAMES
583	  socket option returns an error: No problem. You'll get legacy CAN frames
584	  or CAN FD frames and can process them the same way.
585	
586	  When sending to CAN devices make sure that the device is capable to handle
587	  CAN FD frames by checking if the device maximum transfer unit is CANFD_MTU.
588	  The CAN device MTU can be retrieved e.g. with a SIOCGIFMTU ioctl() syscall.
589	
590	  4.1.6 RAW socket returned message flags
591	
592	  When using recvmsg() call, the msg->msg_flags may contain following flags:
593	
594	    MSG_DONTROUTE: set when the received frame was created on the local host.
595	
596	    MSG_CONFIRM: set when the frame was sent via the socket it is received on.
597	      This flag can be interpreted as a 'transmission confirmation' when the
598	      CAN driver supports the echo of frames on driver level, see 3.2 and 6.2.
599	      In order to receive such messages, CAN_RAW_RECV_OWN_MSGS must be set.
600	
601	  4.2 Broadcast Manager protocol sockets (SOCK_DGRAM)
602	
603	  The Broadcast Manager protocol provides a command based configuration
604	  interface to filter and send (e.g. cyclic) CAN messages in kernel space.
605	
606	  Receive filters can be used to down sample frequent messages; detect events
607	  such as message contents changes, packet length changes, and do time-out
608	  monitoring of received messages.
609	
610	  Periodic transmission tasks of CAN frames or a sequence of CAN frames can be
611	  created and modified at runtime; both the message content and the two
612	  possible transmit intervals can be altered.
613	
614	  A BCM socket is not intended for sending individual CAN frames using the
615	  struct can_frame as known from the CAN_RAW socket. Instead a special BCM
616	  configuration message is defined. The basic BCM configuration message used
617	  to communicate with the broadcast manager and the available operations are
618	  defined in the linux/can/bcm.h include. The BCM message consists of a
619	  message header with a command ('opcode') followed by zero or more CAN frames.
620	  The broadcast manager sends responses to user space in the same form:
621	
622	    struct bcm_msg_head {
623	            __u32 opcode;                   /* command */
624	            __u32 flags;                    /* special flags */
625	            __u32 count;                    /* run 'count' times with ival1 */
626	            struct timeval ival1, ival2;    /* count and subsequent interval */
627	            canid_t can_id;                 /* unique can_id for task */
628	            __u32 nframes;                  /* number of can_frames following */
629	            struct can_frame frames[0];
630	    };
631	
632	  The aligned payload 'frames' uses the same basic CAN frame structure defined
633	  at the beginning of section 4 and in the include/linux/can.h include. All
634	  messages to the broadcast manager from user space have this structure.
635	
636	  Note a CAN_BCM socket must be connected instead of bound after socket
637	  creation (example without error checking):
638	
639	    int s;
640	    struct sockaddr_can addr;
641	    struct ifreq ifr;
642	
643	    s = socket(PF_CAN, SOCK_DGRAM, CAN_BCM);
644	
645	    strcpy(ifr.ifr_name, "can0");
646	    ioctl(s, SIOCGIFINDEX, &ifr);
647	
648	    addr.can_family = AF_CAN;
649	    addr.can_ifindex = ifr.ifr_ifindex;
650	
651	    connect(s, (struct sockaddr *)&addr, sizeof(addr))
652	
653	    (..)
654	
655	  The broadcast manager socket is able to handle any number of in flight
656	  transmissions or receive filters concurrently. The different RX/TX jobs are
657	  distinguished by the unique can_id in each BCM message. However additional
658	  CAN_BCM sockets are recommended to communicate on multiple CAN interfaces.
659	  When the broadcast manager socket is bound to 'any' CAN interface (=> the
660	  interface index is set to zero) the configured receive filters apply to any
661	  CAN interface unless the sendto() syscall is used to overrule the 'any' CAN
662	  interface index. When using recvfrom() instead of read() to retrieve BCM
663	  socket messages the originating CAN interface is provided in can_ifindex.
664	
665	  4.2.1 Broadcast Manager operations
666	
667	  The opcode defines the operation for the broadcast manager to carry out,
668	  or details the broadcast managers response to several events, including
669	  user requests.
670	
671	  Transmit Operations (user space to broadcast manager):
672	
673	    TX_SETUP:   Create (cyclic) transmission task.
674	
675	    TX_DELETE:  Remove (cyclic) transmission task, requires only can_id.
676	
677	    TX_READ:    Read properties of (cyclic) transmission task for can_id.
678	
679	    TX_SEND:    Send one CAN frame.
680	
681	  Transmit Responses (broadcast manager to user space):
682	
683	    TX_STATUS:  Reply to TX_READ request (transmission task configuration).
684	
685	    TX_EXPIRED: Notification when counter finishes sending at initial interval
686	      'ival1'. Requires the TX_COUNTEVT flag to be set at TX_SETUP.
687	
688	  Receive Operations (user space to broadcast manager):
689	
690	    RX_SETUP:   Create RX content filter subscription.
691	
692	    RX_DELETE:  Remove RX content filter subscription, requires only can_id.
693	
694	    RX_READ:    Read properties of RX content filter subscription for can_id.
695	
696	  Receive Responses (broadcast manager to user space):
697	
698	    RX_STATUS:  Reply to RX_READ request (filter task configuration).
699	
700	    RX_TIMEOUT: Cyclic message is detected to be absent (timer ival1 expired).
701	
702	    RX_CHANGED: BCM message with updated CAN frame (detected content change).
703	      Sent on first message received or on receipt of revised CAN messages.
704	
705	  4.2.2 Broadcast Manager message flags
706	
707	  When sending a message to the broadcast manager the 'flags' element may
708	  contain the following flag definitions which influence the behaviour:
709	
710	    SETTIMER:           Set the values of ival1, ival2 and count
711	
712	    STARTTIMER:         Start the timer with the actual values of ival1, ival2
713	      and count. Starting the timer leads simultaneously to emit a CAN frame.
714	
715	    TX_COUNTEVT:        Create the message TX_EXPIRED when count expires
716	
717	    TX_ANNOUNCE:        A change of data by the process is emitted immediately.
718	
719	    TX_CP_CAN_ID:       Copies the can_id from the message header to each
720	      subsequent frame in frames. This is intended as usage simplification. For
721	      TX tasks the unique can_id from the message header may differ from the
722	      can_id(s) stored for transmission in the subsequent struct can_frame(s).
723	
724	    RX_FILTER_ID:       Filter by can_id alone, no frames required (nframes=0).
725	
726	    RX_CHECK_DLC:       A change of the DLC leads to an RX_CHANGED.
727	
728	    RX_NO_AUTOTIMER:    Prevent automatically starting the timeout monitor.
729	
730	    RX_ANNOUNCE_RESUME: If passed at RX_SETUP and a receive timeout occured, a
731	      RX_CHANGED message will be generated when the (cyclic) receive restarts.
732	
733	    TX_RESET_MULTI_IDX: Reset the index for the multiple frame transmission.
734	
735	    RX_RTR_FRAME:       Send reply for RTR-request (placed in op->frames[0]).
736	
737	  4.2.3 Broadcast Manager transmission timers
738	
739	  Periodic transmission configurations may use up to two interval timers.
740	  In this case the BCM sends a number of messages ('count') at an interval
741	  'ival1', then continuing to send at another given interval 'ival2'. When
742	  only one timer is needed 'count' is set to zero and only 'ival2' is used.
743	  When SET_TIMER and START_TIMER flag were set the timers are activated.
744	  The timer values can be altered at runtime when only SET_TIMER is set.
745	
746	  4.2.4 Broadcast Manager message sequence transmission
747	
748	  Up to 256 CAN frames can be transmitted in a sequence in the case of a cyclic
749	  TX task configuration. The number of CAN frames is provided in the 'nframes'
750	  element of the BCM message head. The defined number of CAN frames are added
751	  as array to the TX_SETUP BCM configuration message.
752	
753	    /* create a struct to set up a sequence of four CAN frames */
754	    struct {
755	            struct bcm_msg_head msg_head;
756	            struct can_frame frame[4];
757	    } mytxmsg;
758	
759	    (..)
760	    mytxmsg.nframes = 4;
761	    (..)
762	
763	    write(s, &mytxmsg, sizeof(mytxmsg));
764	
765	  With every transmission the index in the array of CAN frames is increased
766	  and set to zero at index overflow.
767	
768	  4.2.5 Broadcast Manager receive filter timers
769	
770	  The timer values ival1 or ival2 may be set to non-zero values at RX_SETUP.
771	  When the SET_TIMER flag is set the timers are enabled:
772	
773	  ival1: Send RX_TIMEOUT when a received message is not received again within
774	    the given time. When START_TIMER is set at RX_SETUP the timeout detection
775	    is activated directly - even without a former CAN frame reception.
776	
777	  ival2: Throttle the received message rate down to the value of ival2. This
778	    is useful to reduce messages for the application when the signal inside the
779	    CAN frame is stateless as state changes within the ival2 periode may get
780	    lost.
781	
782	  4.2.6 Broadcast Manager multiplex message receive filter
783	
784	  To filter for content changes in multiplex message sequences an array of more
785	  than one CAN frames can be passed in a RX_SETUP configuration message. The
786	  data bytes of the first CAN frame contain the mask of relevant bits that
787	  have to match in the subsequent CAN frames with the received CAN frame.
788	  If one of the subsequent CAN frames is matching the bits in that frame data
789	  mark the relevant content to be compared with the previous received content.
790	  Up to 257 CAN frames (multiplex filter bit mask CAN frame plus 256 CAN
791	  filters) can be added as array to the TX_SETUP BCM configuration message.
792	
793	    /* usually used to clear CAN frame data[] - beware of endian problems! */
794	    #define U64_DATA(p) (*(unsigned long long*)(p)->data)
795	
796	    struct {
797	            struct bcm_msg_head msg_head;
798	            struct can_frame frame[5];
799	    } msg;
800	
801	    msg.msg_head.opcode  = RX_SETUP;
802	    msg.msg_head.can_id  = 0x42;
803	    msg.msg_head.flags   = 0;
804	    msg.msg_head.nframes = 5;
805	    U64_DATA(&msg.frame[0]) = 0xFF00000000000000ULL; /* MUX mask */
806	    U64_DATA(&msg.frame[1]) = 0x01000000000000FFULL; /* data mask (MUX 0x01) */
807	    U64_DATA(&msg.frame[2]) = 0x0200FFFF000000FFULL; /* data mask (MUX 0x02) */
808	    U64_DATA(&msg.frame[3]) = 0x330000FFFFFF0003ULL; /* data mask (MUX 0x33) */
809	    U64_DATA(&msg.frame[4]) = 0x4F07FC0FF0000000ULL; /* data mask (MUX 0x4F) */
810	
811	    write(s, &msg, sizeof(msg));
812	
813	  4.3 connected transport protocols (SOCK_SEQPACKET)
814	  4.4 unconnected transport protocols (SOCK_DGRAM)
815	
816	
817	5. Socket CAN core module
818	-------------------------
819	
820	  The Socket CAN core module implements the protocol family
821	  PF_CAN. CAN protocol modules are loaded by the core module at
822	  runtime. The core module provides an interface for CAN protocol
823	  modules to subscribe needed CAN IDs (see chapter 3.1).
824	
825	  5.1 can.ko module params
826	
827	  - stats_timer: To calculate the Socket CAN core statistics
828	    (e.g. current/maximum frames per second) this 1 second timer is
829	    invoked at can.ko module start time by default. This timer can be
830	    disabled by using stattimer=0 on the module commandline.
831	
832	  - debug: (removed since SocketCAN SVN r546)
833	
834	  5.2 procfs content
835	
836	  As described in chapter 3.1 the Socket CAN core uses several filter
837	  lists to deliver received CAN frames to CAN protocol modules. These
838	  receive lists, their filters and the count of filter matches can be
839	  checked in the appropriate receive list. All entries contain the
840	  device and a protocol module identifier:
841	
842	    foo@bar:~$ cat /proc/net/can/rcvlist_all
843	
844	    receive list 'rx_all':
845	      (vcan3: no entry)
846	      (vcan2: no entry)
847	      (vcan1: no entry)
848	      device   can_id   can_mask  function  userdata   matches  ident
849	       vcan0     000    00000000  f88e6370  f6c6f400         0  raw
850	      (any: no entry)
851	
852	  In this example an application requests any CAN traffic from vcan0.
853	
854	    rcvlist_all - list for unfiltered entries (no filter operations)
855	    rcvlist_eff - list for single extended frame (EFF) entries
856	    rcvlist_err - list for error message frames masks
857	    rcvlist_fil - list for mask/value filters
858	    rcvlist_inv - list for mask/value filters (inverse semantic)
859	    rcvlist_sff - list for single standard frame (SFF) entries
860	
861	  Additional procfs files in /proc/net/can
862	
863	    stats       - Socket CAN core statistics (rx/tx frames, match ratios, ...)
864	    reset_stats - manual statistic reset
865	    version     - prints the Socket CAN core version and the ABI version
866	
867	  5.3 writing own CAN protocol modules
868	
869	  To implement a new protocol in the protocol family PF_CAN a new
870	  protocol has to be defined in include/linux/can.h .
871	  The prototypes and definitions to use the Socket CAN core can be
872	  accessed by including include/linux/can/core.h .
873	  In addition to functions that register the CAN protocol and the
874	  CAN device notifier chain there are functions to subscribe CAN
875	  frames received by CAN interfaces and to send CAN frames:
876	
877	    can_rx_register   - subscribe CAN frames from a specific interface
878	    can_rx_unregister - unsubscribe CAN frames from a specific interface
879	    can_send          - transmit a CAN frame (optional with local loopback)
880	
881	  For details see the kerneldoc documentation in net/can/af_can.c or
882	  the source code of net/can/raw.c or net/can/bcm.c .
883	
884	6. CAN network drivers
885	----------------------
886	
887	  Writing a CAN network device driver is much easier than writing a
888	  CAN character device driver. Similar to other known network device
889	  drivers you mainly have to deal with:
890	
891	  - TX: Put the CAN frame from the socket buffer to the CAN controller.
892	  - RX: Put the CAN frame from the CAN controller to the socket buffer.
893	
894	  See e.g. at Documentation/networking/netdevices.txt . The differences
895	  for writing CAN network device driver are described below:
896	
897	  6.1 general settings
898	
899	    dev->type  = ARPHRD_CAN; /* the netdevice hardware type */
900	    dev->flags = IFF_NOARP;  /* CAN has no arp */
901	
902	    dev->mtu = CAN_MTU; /* sizeof(struct can_frame) -> legacy CAN interface */
903	
904	    or alternative, when the controller supports CAN with flexible data rate:
905	    dev->mtu = CANFD_MTU; /* sizeof(struct canfd_frame) -> CAN FD interface */
906	
907	  The struct can_frame or struct canfd_frame is the payload of each socket
908	  buffer (skbuff) in the protocol family PF_CAN.
909	
910	  6.2 local loopback of sent frames
911	
912	  As described in chapter 3.2 the CAN network device driver should
913	  support a local loopback functionality similar to the local echo
914	  e.g. of tty devices. In this case the driver flag IFF_ECHO has to be
915	  set to prevent the PF_CAN core from locally echoing sent frames
916	  (aka loopback) as fallback solution:
917	
918	    dev->flags = (IFF_NOARP | IFF_ECHO);
919	
920	  6.3 CAN controller hardware filters
921	
922	  To reduce the interrupt load on deep embedded systems some CAN
923	  controllers support the filtering of CAN IDs or ranges of CAN IDs.
924	  These hardware filter capabilities vary from controller to
925	  controller and have to be identified as not feasible in a multi-user
926	  networking approach. The use of the very controller specific
927	  hardware filters could make sense in a very dedicated use-case, as a
928	  filter on driver level would affect all users in the multi-user
929	  system. The high efficient filter sets inside the PF_CAN core allow
930	  to set different multiple filters for each socket separately.
931	  Therefore the use of hardware filters goes to the category 'handmade
932	  tuning on deep embedded systems'. The author is running a MPC603e
933	  @133MHz with four SJA1000 CAN controllers from 2002 under heavy bus
934	  load without any problems ...
935	
936	  6.4 The virtual CAN driver (vcan)
937	
938	  Similar to the network loopback devices, vcan offers a virtual local
939	  CAN interface. A full qualified address on CAN consists of
940	
941	  - a unique CAN Identifier (CAN ID)
942	  - the CAN bus this CAN ID is transmitted on (e.g. can0)
943	
944	  so in common use cases more than one virtual CAN interface is needed.
945	
946	  The virtual CAN interfaces allow the transmission and reception of CAN
947	  frames without real CAN controller hardware. Virtual CAN network
948	  devices are usually named 'vcanX', like vcan0 vcan1 vcan2 ...
949	  When compiled as a module the virtual CAN driver module is called vcan.ko
950	
951	  Since Linux Kernel version 2.6.24 the vcan driver supports the Kernel
952	  netlink interface to create vcan network devices. The creation and
953	  removal of vcan network devices can be managed with the ip(8) tool:
954	
955	  - Create a virtual CAN network interface:
956	       $ ip link add type vcan
957	
958	  - Create a virtual CAN network interface with a specific name 'vcan42':
959	       $ ip link add dev vcan42 type vcan
960	
961	  - Remove a (virtual CAN) network interface 'vcan42':
962	       $ ip link del vcan42
963	
964	  6.5 The CAN network device driver interface
965	
966	  The CAN network device driver interface provides a generic interface
967	  to setup, configure and monitor CAN network devices. The user can then
968	  configure the CAN device, like setting the bit-timing parameters, via
969	  the netlink interface using the program "ip" from the "IPROUTE2"
970	  utility suite. The following chapter describes briefly how to use it.
971	  Furthermore, the interface uses a common data structure and exports a
972	  set of common functions, which all real CAN network device drivers
973	  should use. Please have a look to the SJA1000 or MSCAN driver to
974	  understand how to use them. The name of the module is can-dev.ko.
975	
976	  6.5.1 Netlink interface to set/get devices properties
977	
978	  The CAN device must be configured via netlink interface. The supported
979	  netlink message types are defined and briefly described in
980	  "include/linux/can/netlink.h". CAN link support for the program "ip"
981	  of the IPROUTE2 utility suite is available and it can be used as shown
982	  below:
983	
984	  - Setting CAN device properties:
985	
986	    $ ip link set can0 type can help
987	    Usage: ip link set DEVICE type can
988	    	[ bitrate BITRATE [ sample-point SAMPLE-POINT] ] |
989	    	[ tq TQ prop-seg PROP_SEG phase-seg1 PHASE-SEG1
990	     	  phase-seg2 PHASE-SEG2 [ sjw SJW ] ]
991	
992	    	[ loopback { on | off } ]
993	    	[ listen-only { on | off } ]
994	    	[ triple-sampling { on | off } ]
995	
996	    	[ restart-ms TIME-MS ]
997	    	[ restart ]
998	
999	    	Where: BITRATE       := { 1..1000000 }
1000	    	       SAMPLE-POINT  := { 0.000..0.999 }
1001	    	       TQ            := { NUMBER }
1002	    	       PROP-SEG      := { 1..8 }
1003	    	       PHASE-SEG1    := { 1..8 }
1004	    	       PHASE-SEG2    := { 1..8 }
1005	    	       SJW           := { 1..4 }
1006	    	       RESTART-MS    := { 0 | NUMBER }
1007	
1008	  - Display CAN device details and statistics:
1009	
1010	    $ ip -details -statistics link show can0
1011	    2: can0: <NOARP,UP,LOWER_UP,ECHO> mtu 16 qdisc pfifo_fast state UP qlen 10
1012	      link/can
1013	      can <TRIPLE-SAMPLING> state ERROR-ACTIVE restart-ms 100
1014	      bitrate 125000 sample_point 0.875
1015	      tq 125 prop-seg 6 phase-seg1 7 phase-seg2 2 sjw 1
1016	      sja1000: tseg1 1..16 tseg2 1..8 sjw 1..4 brp 1..64 brp-inc 1
1017	      clock 8000000
1018	      re-started bus-errors arbit-lost error-warn error-pass bus-off
1019	      41         17457      0          41         42         41
1020	      RX: bytes  packets  errors  dropped overrun mcast
1021	      140859     17608    17457   0       0       0
1022	      TX: bytes  packets  errors  dropped carrier collsns
1023	      861        112      0       41      0       0
1024	
1025	  More info to the above output:
1026	
1027	    "<TRIPLE-SAMPLING>"
1028		Shows the list of selected CAN controller modes: LOOPBACK,
1029		LISTEN-ONLY, or TRIPLE-SAMPLING.
1030	
1031	    "state ERROR-ACTIVE"
1032		The current state of the CAN controller: "ERROR-ACTIVE",
1033		"ERROR-WARNING", "ERROR-PASSIVE", "BUS-OFF" or "STOPPED"
1034	
1035	    "restart-ms 100"
1036		Automatic restart delay time. If set to a non-zero value, a
1037		restart of the CAN controller will be triggered automatically
1038		in case of a bus-off condition after the specified delay time
1039		in milliseconds. By default it's off.
1040	
1041	    "bitrate 125000 sample_point 0.875"
1042		Shows the real bit-rate in bits/sec and the sample-point in the
1043		range 0.000..0.999. If the calculation of bit-timing parameters
1044		is enabled in the kernel (CONFIG_CAN_CALC_BITTIMING=y), the
1045		bit-timing can be defined by setting the "bitrate" argument.
1046		Optionally the "sample-point" can be specified. By default it's
1047		0.000 assuming CIA-recommended sample-points.
1048	
1049	    "tq 125 prop-seg 6 phase-seg1 7 phase-seg2 2 sjw 1"
1050		Shows the time quanta in ns, propagation segment, phase buffer
1051		segment 1 and 2 and the synchronisation jump width in units of
1052		tq. They allow to define the CAN bit-timing in a hardware
1053		independent format as proposed by the Bosch CAN 2.0 spec (see
1054		chapter 8 of http://www.semiconductors.bosch.de/pdf/can2spec.pdf).
1055	
1056	    "sja1000: tseg1 1..16 tseg2 1..8 sjw 1..4 brp 1..64 brp-inc 1
1057	     clock 8000000"
1058		Shows the bit-timing constants of the CAN controller, here the
1059		"sja1000". The minimum and maximum values of the time segment 1
1060		and 2, the synchronisation jump width in units of tq, the
1061		bitrate pre-scaler and the CAN system clock frequency in Hz.
1062		These constants could be used for user-defined (non-standard)
1063		bit-timing calculation algorithms in user-space.
1064	
1065	    "re-started bus-errors arbit-lost error-warn error-pass bus-off"
1066		Shows the number of restarts, bus and arbitration lost errors,
1067		and the state changes to the error-warning, error-passive and
1068		bus-off state. RX overrun errors are listed in the "overrun"
1069		field of the standard network statistics.
1070	
1071	  6.5.2 Setting the CAN bit-timing
1072	
1073	  The CAN bit-timing parameters can always be defined in a hardware
1074	  independent format as proposed in the Bosch CAN 2.0 specification
1075	  specifying the arguments "tq", "prop_seg", "phase_seg1", "phase_seg2"
1076	  and "sjw":
1077	
1078	    $ ip link set canX type can tq 125 prop-seg 6 \
1079					phase-seg1 7 phase-seg2 2 sjw 1
1080	
1081	  If the kernel option CONFIG_CAN_CALC_BITTIMING is enabled, CIA
1082	  recommended CAN bit-timing parameters will be calculated if the bit-
1083	  rate is specified with the argument "bitrate":
1084	
1085	    $ ip link set canX type can bitrate 125000
1086	
1087	  Note that this works fine for the most common CAN controllers with
1088	  standard bit-rates but may *fail* for exotic bit-rates or CAN system
1089	  clock frequencies. Disabling CONFIG_CAN_CALC_BITTIMING saves some
1090	  space and allows user-space tools to solely determine and set the
1091	  bit-timing parameters. The CAN controller specific bit-timing
1092	  constants can be used for that purpose. They are listed by the
1093	  following command:
1094	
1095	    $ ip -details link show can0
1096	    ...
1097	      sja1000: clock 8000000 tseg1 1..16 tseg2 1..8 sjw 1..4 brp 1..64 brp-inc 1
1098	
1099	  6.5.3 Starting and stopping the CAN network device
1100	
1101	  A CAN network device is started or stopped as usual with the command
1102	  "ifconfig canX up/down" or "ip link set canX up/down". Be aware that
1103	  you *must* define proper bit-timing parameters for real CAN devices
1104	  before you can start it to avoid error-prone default settings:
1105	
1106	    $ ip link set canX up type can bitrate 125000
1107	
1108	  A device may enter the "bus-off" state if too much errors occurred on
1109	  the CAN bus. Then no more messages are received or sent. An automatic
1110	  bus-off recovery can be enabled by setting the "restart-ms" to a
1111	  non-zero value, e.g.:
1112	
1113	    $ ip link set canX type can restart-ms 100
1114	
1115	  Alternatively, the application may realize the "bus-off" condition
1116	  by monitoring CAN error message frames and do a restart when
1117	  appropriate with the command:
1118	
1119	    $ ip link set canX type can restart
1120	
1121	  Note that a restart will also create a CAN error message frame (see
1122	  also chapter 3.4).
1123	
1124	  6.6 CAN FD (flexible data rate) driver support
1125	
1126	  CAN FD capable CAN controllers support two different bitrates for the
1127	  arbitration phase and the payload phase of the CAN FD frame. Therefore a
1128	  second bittiming has to be specified in order to enable the CAN FD bitrate.
1129	
1130	  Additionally CAN FD capable CAN controllers support up to 64 bytes of
1131	  payload. The representation of this length in can_frame.can_dlc and
1132	  canfd_frame.len for userspace applications and inside the Linux network
1133	  layer is a plain value from 0 .. 64 instead of the CAN 'data length code'.
1134	  The data length code was a 1:1 mapping to the payload length in the legacy
1135	  CAN frames anyway. The payload length to the bus-relevant DLC mapping is
1136	  only performed inside the CAN drivers, preferably with the helper
1137	  functions can_dlc2len() and can_len2dlc().
1138	
1139	  The CAN netdevice driver capabilities can be distinguished by the network
1140	  devices maximum transfer unit (MTU):
1141	
1142	  MTU = 16 (CAN_MTU)   => sizeof(struct can_frame)   => 'legacy' CAN device
1143	  MTU = 72 (CANFD_MTU) => sizeof(struct canfd_frame) => CAN FD capable device
1144	
1145	  The CAN device MTU can be retrieved e.g. with a SIOCGIFMTU ioctl() syscall.
1146	  N.B. CAN FD capable devices can also handle and send legacy CAN frames.
1147	
1148	  FIXME: Add details about the CAN FD controller configuration when available.
1149	
1150	  6.7 Supported CAN hardware
1151	
1152	  Please check the "Kconfig" file in "drivers/net/can" to get an actual
1153	  list of the support CAN hardware. On the Socket CAN project website
1154	  (see chapter 7) there might be further drivers available, also for
1155	  older kernel versions.
1156	
1157	7. Socket CAN resources
1158	-----------------------
1159	
1160	  You can find further resources for Socket CAN like user space tools,
1161	  support for old kernel versions, more drivers, mailing lists, etc.
1162	  at the BerliOS OSS project website for Socket CAN:
1163	
1164	    http://developer.berlios.de/projects/socketcan
1165	
1166	  If you have questions, bug fixes, etc., don't hesitate to post them to
1167	  the Socketcan-Users mailing list. But please search the archives first.
1168	
1169	8. Credits
1170	----------
1171	
1172	  Oliver Hartkopp (PF_CAN core, filters, drivers, bcm, SJA1000 driver)
1173	  Urs Thuermann (PF_CAN core, kernel integration, socket interfaces, raw, vcan)
1174	  Jan Kizka (RT-SocketCAN core, Socket-API reconciliation)
1175	  Wolfgang Grandegger (RT-SocketCAN core & drivers, Raw Socket-API reviews,
1176	                       CAN device driver interface, MSCAN driver)
1177	  Robert Schwebel (design reviews, PTXdist integration)
1178	  Marc Kleine-Budde (design reviews, Kernel 2.6 cleanups, drivers)
1179	  Benedikt Spranger (reviews)
1180	  Thomas Gleixner (LKML reviews, coding style, posting hints)
1181	  Andrey Volkov (kernel subtree structure, ioctls, MSCAN driver)
1182	  Matthias Brukner (first SJA1000 CAN netdevice implementation Q2/2003)
1183	  Klaus Hitschler (PEAK driver integration)
1184	  Uwe Koppe (CAN netdevices with PF_PACKET approach)
1185	  Michael Schulze (driver layer loopback requirement, RT CAN drivers review)
1186	  Pavel Pisa (Bit-timing calculation)
1187	  Sascha Hauer (SJA1000 platform driver)
1188	  Sebastian Haas (SJA1000 EMS PCI driver)
1189	  Markus Plessing (SJA1000 EMS PCI driver)
1190	  Per Dalen (SJA1000 Kvaser PCI driver)
1191	  Sam Ravnborg (reviews, coding style, kbuild help)
Hide Line Numbers
About Kernel Documentation Linux Kernel Contact Linux Resources Linux Blog

Information is copyright its respective author. All material is available from the Linux Kernel Source distributed under a GPL License. This page is provided as a free service by mjmwired.net.