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Based on kernel version 3.16. Page generated on 2014-08-06 21:40 EST.

1	============================================================================
2	
3	can.txt
4	
5	Readme file for the Controller Area Network Protocol Family (aka SocketCAN)
6	
7	This file contains
8	
9	  1 Overview / What is SocketCAN
10	
11	  2 Motivation / Why using the socket API
12	
13	  3 SocketCAN concept
14	    3.1 receive lists
15	    3.2 local loopback of sent frames
16	    3.3 network problem notifications
17	
18	  4 How to use SocketCAN
19	    4.1 RAW protocol sockets with can_filters (SOCK_RAW)
20	      4.1.1 RAW socket option CAN_RAW_FILTER
21	      4.1.2 RAW socket option CAN_RAW_ERR_FILTER
22	      4.1.3 RAW socket option CAN_RAW_LOOPBACK
23	      4.1.4 RAW socket option CAN_RAW_RECV_OWN_MSGS
24	      4.1.5 RAW socket option CAN_RAW_FD_FRAMES
25	      4.1.6 RAW socket returned message flags
26	    4.2 Broadcast Manager protocol sockets (SOCK_DGRAM)
27	      4.2.1 Broadcast Manager operations
28	      4.2.2 Broadcast Manager message flags
29	      4.2.3 Broadcast Manager transmission timers
30	      4.2.4 Broadcast Manager message sequence transmission
31	      4.2.5 Broadcast Manager receive filter timers
32	      4.2.6 Broadcast Manager multiplex message receive filter
33	    4.3 connected transport protocols (SOCK_SEQPACKET)
34	    4.4 unconnected transport protocols (SOCK_DGRAM)
35	
36	  5 SocketCAN core module
37	    5.1 can.ko module params
38	    5.2 procfs content
39	    5.3 writing own CAN protocol modules
40	
41	  6 CAN network drivers
42	    6.1 general settings
43	    6.2 local loopback of sent frames
44	    6.3 CAN controller hardware filters
45	    6.4 The virtual CAN driver (vcan)
46	    6.5 The CAN network device driver interface
47	      6.5.1 Netlink interface to set/get devices properties
48	      6.5.2 Setting the CAN bit-timing
49	      6.5.3 Starting and stopping the CAN network device
50	    6.6 CAN FD (flexible data rate) driver support
51	    6.7 supported CAN hardware
52	
53	  7 SocketCAN resources
54	
55	  8 Credits
56	
57	============================================================================
58	
59	1. Overview / What is SocketCAN
60	--------------------------------
61	
62	The socketcan package is an implementation of CAN protocols
63	(Controller Area Network) for Linux.  CAN is a networking technology
64	which has widespread use in automation, embedded devices, and
65	automotive fields.  While there have been other CAN implementations
66	for Linux based on character devices, SocketCAN uses the Berkeley
67	socket API, the Linux network stack and implements the CAN device
68	drivers as network interfaces.  The CAN socket API has been designed
69	as similar as possible to the TCP/IP protocols to allow programmers,
70	familiar with network programming, to easily learn how to use CAN
71	sockets.
72	
73	2. Motivation / Why using the socket API
74	----------------------------------------
75	
76	There have been CAN implementations for Linux before SocketCAN so the
77	question arises, why we have started another project.  Most existing
78	implementations come as a device driver for some CAN hardware, they
79	are based on character devices and provide comparatively little
80	functionality.  Usually, there is only a hardware-specific device
81	driver which provides a character device interface to send and
82	receive raw CAN frames, directly to/from the controller hardware.
83	Queueing of frames and higher-level transport protocols like ISO-TP
84	have to be implemented in user space applications.  Also, most
85	character-device implementations support only one single process to
86	open the device at a time, similar to a serial interface.  Exchanging
87	the CAN controller requires employment of another device driver and
88	often the need for adaption of large parts of the application to the
89	new driver's API.
90	
91	SocketCAN was designed to overcome all of these limitations.  A new
92	protocol family has been implemented which provides a socket interface
93	to user space applications and which builds upon the Linux network
94	layer, enabling use all of the provided queueing functionality.  A device
95	driver for CAN controller hardware registers itself with the Linux
96	network layer as a network device, so that CAN frames from the
97	controller can be passed up to the network layer and on to the CAN
98	protocol family module and also vice-versa.  Also, the protocol family
99	module provides an API for transport protocol modules to register, so
100	that any number of transport protocols can be loaded or unloaded
101	dynamically.  In fact, the can core module alone does not provide any
102	protocol and cannot be used without loading at least one additional
103	protocol module.  Multiple sockets can be opened at the same time,
104	on different or the same protocol module and they can listen/send
105	frames on different or the same CAN IDs.  Several sockets listening on
106	the same interface for frames with the same CAN ID are all passed the
107	same received matching CAN frames.  An application wishing to
108	communicate using a specific transport protocol, e.g. ISO-TP, just
109	selects that protocol when opening the socket, and then can read and
110	write application data byte streams, without having to deal with
111	CAN-IDs, frames, etc.
112	
113	Similar functionality visible from user-space could be provided by a
114	character device, too, but this would lead to a technically inelegant
115	solution for a couple of reasons:
116	
117	* Intricate usage.  Instead of passing a protocol argument to
118	  socket(2) and using bind(2) to select a CAN interface and CAN ID, an
119	  application would have to do all these operations using ioctl(2)s.
120	
121	* Code duplication.  A character device cannot make use of the Linux
122	  network queueing code, so all that code would have to be duplicated
123	  for CAN networking.
124	
125	* Abstraction.  In most existing character-device implementations, the
126	  hardware-specific device driver for a CAN controller directly
127	  provides the character device for the application to work with.
128	  This is at least very unusual in Unix systems for both, char and
129	  block devices.  For example you don't have a character device for a
130	  certain UART of a serial interface, a certain sound chip in your
131	  computer, a SCSI or IDE controller providing access to your hard
132	  disk or tape streamer device.  Instead, you have abstraction layers
133	  which provide a unified character or block device interface to the
134	  application on the one hand, and a interface for hardware-specific
135	  device drivers on the other hand.  These abstractions are provided
136	  by subsystems like the tty layer, the audio subsystem or the SCSI
137	  and IDE subsystems for the devices mentioned above.
138	
139	  The easiest way to implement a CAN device driver is as a character
140	  device without such a (complete) abstraction layer, as is done by most
141	  existing drivers.  The right way, however, would be to add such a
142	  layer with all the functionality like registering for certain CAN
143	  IDs, supporting several open file descriptors and (de)multiplexing
144	  CAN frames between them, (sophisticated) queueing of CAN frames, and
145	  providing an API for device drivers to register with.  However, then
146	  it would be no more difficult, or may be even easier, to use the
147	  networking framework provided by the Linux kernel, and this is what
148	  SocketCAN does.
149	
150	  The use of the networking framework of the Linux kernel is just the
151	  natural and most appropriate way to implement CAN for Linux.
152	
153	3. SocketCAN concept
154	---------------------
155	
156	  As described in chapter 2 it is the main goal of SocketCAN to
157	  provide a socket interface to user space applications which builds
158	  upon the Linux network layer. In contrast to the commonly known
159	  TCP/IP and ethernet networking, the CAN bus is a broadcast-only(!)
160	  medium that has no MAC-layer addressing like ethernet. The CAN-identifier
161	  (can_id) is used for arbitration on the CAN-bus. Therefore the CAN-IDs
162	  have to be chosen uniquely on the bus. When designing a CAN-ECU
163	  network the CAN-IDs are mapped to be sent by a specific ECU.
164	  For this reason a CAN-ID can be treated best as a kind of source address.
165	
166	  3.1 receive lists
167	
168	  The network transparent access of multiple applications leads to the
169	  problem that different applications may be interested in the same
170	  CAN-IDs from the same CAN network interface. The SocketCAN core
171	  module - which implements the protocol family CAN - provides several
172	  high efficient receive lists for this reason. If e.g. a user space
173	  application opens a CAN RAW socket, the raw protocol module itself
174	  requests the (range of) CAN-IDs from the SocketCAN core that are
175	  requested by the user. The subscription and unsubscription of
176	  CAN-IDs can be done for specific CAN interfaces or for all(!) known
177	  CAN interfaces with the can_rx_(un)register() functions provided to
178	  CAN protocol modules by the SocketCAN core (see chapter 5).
179	  To optimize the CPU usage at runtime the receive lists are split up
180	  into several specific lists per device that match the requested
181	  filter complexity for a given use-case.
182	
183	  3.2 local loopback of sent frames
184	
185	  As known from other networking concepts the data exchanging
186	  applications may run on the same or different nodes without any
187	  change (except for the according addressing information):
188	
189	         ___   ___   ___                   _______   ___
190	        | _ | | _ | | _ |                 | _   _ | | _ |
191	        ||A|| ||B|| ||C||                 ||A| |B|| ||C||
192	        |___| |___| |___|                 |_______| |___|
193	          |     |     |                       |       |
194	        -----------------(1)- CAN bus -(2)---------------
195	
196	  To ensure that application A receives the same information in the
197	  example (2) as it would receive in example (1) there is need for
198	  some kind of local loopback of the sent CAN frames on the appropriate
199	  node.
200	
201	  The Linux network devices (by default) just can handle the
202	  transmission and reception of media dependent frames. Due to the
203	  arbitration on the CAN bus the transmission of a low prio CAN-ID
204	  may be delayed by the reception of a high prio CAN frame. To
205	  reflect the correct* traffic on the node the loopback of the sent
206	  data has to be performed right after a successful transmission. If
207	  the CAN network interface is not capable of performing the loopback for
208	  some reason the SocketCAN core can do this task as a fallback solution.
209	  See chapter 6.2 for details (recommended).
210	
211	  The loopback functionality is enabled by default to reflect standard
212	  networking behaviour for CAN applications. Due to some requests from
213	  the RT-SocketCAN group the loopback optionally may be disabled for each
214	  separate socket. See sockopts from the CAN RAW sockets in chapter 4.1.
215	
216	  * = you really like to have this when you're running analyser tools
217	      like 'candump' or 'cansniffer' on the (same) node.
218	
219	  3.3 network problem notifications
220	
221	  The use of the CAN bus may lead to several problems on the physical
222	  and media access control layer. Detecting and logging of these lower
223	  layer problems is a vital requirement for CAN users to identify
224	  hardware issues on the physical transceiver layer as well as
225	  arbitration problems and error frames caused by the different
226	  ECUs. The occurrence of detected errors are important for diagnosis
227	  and have to be logged together with the exact timestamp. For this
228	  reason the CAN interface driver can generate so called Error Message
229	  Frames that can optionally be passed to the user application in the
230	  same way as other CAN frames. Whenever an error on the physical layer
231	  or the MAC layer is detected (e.g. by the CAN controller) the driver
232	  creates an appropriate error message frame. Error messages frames can
233	  be requested by the user application using the common CAN filter
234	  mechanisms. Inside this filter definition the (interested) type of
235	  errors may be selected. The reception of error messages is disabled
236	  by default. The format of the CAN error message frame is briefly
237	  described in the Linux header file "include/linux/can/error.h".
238	
239	4. How to use SocketCAN
240	------------------------
241	
242	  Like TCP/IP, you first need to open a socket for communicating over a
243	  CAN network. Since SocketCAN implements a new protocol family, you
244	  need to pass PF_CAN as the first argument to the socket(2) system
245	  call. Currently, there are two CAN protocols to choose from, the raw
246	  socket protocol and the broadcast manager (BCM). So to open a socket,
247	  you would write
248	
249	    s = socket(PF_CAN, SOCK_RAW, CAN_RAW);
250	
251	  and
252	
253	    s = socket(PF_CAN, SOCK_DGRAM, CAN_BCM);
254	
255	  respectively.  After the successful creation of the socket, you would
256	  normally use the bind(2) system call to bind the socket to a CAN
257	  interface (which is different from TCP/IP due to different addressing
258	  - see chapter 3). After binding (CAN_RAW) or connecting (CAN_BCM)
259	  the socket, you can read(2) and write(2) from/to the socket or use
260	  send(2), sendto(2), sendmsg(2) and the recv* counterpart operations
261	  on the socket as usual. There are also CAN specific socket options
262	  described below.
263	
264	  The basic CAN frame structure and the sockaddr structure are defined
265	  in include/linux/can.h:
266	
267	    struct can_frame {
268	            canid_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */
269	            __u8    can_dlc; /* frame payload length in byte (0 .. 8) */
270	            __u8    data[8] __attribute__((aligned(8)));
271	    };
272	
273	  The alignment of the (linear) payload data[] to a 64bit boundary
274	  allows the user to define their own structs and unions to easily access
275	  the CAN payload. There is no given byteorder on the CAN bus by
276	  default. A read(2) system call on a CAN_RAW socket transfers a
277	  struct can_frame to the user space.
278	
279	  The sockaddr_can structure has an interface index like the
280	  PF_PACKET socket, that also binds to a specific interface:
281	
282	    struct sockaddr_can {
283	            sa_family_t can_family;
284	            int         can_ifindex;
285	            union {
286	                    /* transport protocol class address info (e.g. ISOTP) */
287	                    struct { canid_t rx_id, tx_id; } tp;
288	
289	                    /* reserved for future CAN protocols address information */
290	            } can_addr;
291	    };
292	
293	  To determine the interface index an appropriate ioctl() has to
294	  be used (example for CAN_RAW sockets without error checking):
295	
296	    int s;
297	    struct sockaddr_can addr;
298	    struct ifreq ifr;
299	
300	    s = socket(PF_CAN, SOCK_RAW, CAN_RAW);
301	
302	    strcpy(ifr.ifr_name, "can0" );
303	    ioctl(s, SIOCGIFINDEX, &ifr);
304	
305	    addr.can_family = AF_CAN;
306	    addr.can_ifindex = ifr.ifr_ifindex;
307	
308	    bind(s, (struct sockaddr *)&addr, sizeof(addr));
309	
310	    (..)
311	
312	  To bind a socket to all(!) CAN interfaces the interface index must
313	  be 0 (zero). In this case the socket receives CAN frames from every
314	  enabled CAN interface. To determine the originating CAN interface
315	  the system call recvfrom(2) may be used instead of read(2). To send
316	  on a socket that is bound to 'any' interface sendto(2) is needed to
317	  specify the outgoing interface.
318	
319	  Reading CAN frames from a bound CAN_RAW socket (see above) consists
320	  of reading a struct can_frame:
321	
322	    struct can_frame frame;
323	
324	    nbytes = read(s, &frame, sizeof(struct can_frame));
325	
326	    if (nbytes < 0) {
327	            perror("can raw socket read");
328	            return 1;
329	    }
330	
331	    /* paranoid check ... */
332	    if (nbytes < sizeof(struct can_frame)) {
333	            fprintf(stderr, "read: incomplete CAN frame\n");
334	            return 1;
335	    }
336	
337	    /* do something with the received CAN frame */
338	
339	  Writing CAN frames can be done similarly, with the write(2) system call:
340	
341	    nbytes = write(s, &frame, sizeof(struct can_frame));
342	
343	  When the CAN interface is bound to 'any' existing CAN interface
344	  (addr.can_ifindex = 0) it is recommended to use recvfrom(2) if the
345	  information about the originating CAN interface is needed:
346	
347	    struct sockaddr_can addr;
348	    struct ifreq ifr;
349	    socklen_t len = sizeof(addr);
350	    struct can_frame frame;
351	
352	    nbytes = recvfrom(s, &frame, sizeof(struct can_frame),
353	                      0, (struct sockaddr*)&addr, &len);
354	
355	    /* get interface name of the received CAN frame */
356	    ifr.ifr_ifindex = addr.can_ifindex;
357	    ioctl(s, SIOCGIFNAME, &ifr);
358	    printf("Received a CAN frame from interface %s", ifr.ifr_name);
359	
360	  To write CAN frames on sockets bound to 'any' CAN interface the
361	  outgoing interface has to be defined certainly.
362	
363	    strcpy(ifr.ifr_name, "can0");
364	    ioctl(s, SIOCGIFINDEX, &ifr);
365	    addr.can_ifindex = ifr.ifr_ifindex;
366	    addr.can_family  = AF_CAN;
367	
368	    nbytes = sendto(s, &frame, sizeof(struct can_frame),
369	                    0, (struct sockaddr*)&addr, sizeof(addr));
370	
371	  Remark about CAN FD (flexible data rate) support:
372	
373	  Generally the handling of CAN FD is very similar to the formerly described
374	  examples. The new CAN FD capable CAN controllers support two different
375	  bitrates for the arbitration phase and the payload phase of the CAN FD frame
376	  and up to 64 bytes of payload. This extended payload length breaks all the
377	  kernel interfaces (ABI) which heavily rely on the CAN frame with fixed eight
378	  bytes of payload (struct can_frame) like the CAN_RAW socket. Therefore e.g.
379	  the CAN_RAW socket supports a new socket option CAN_RAW_FD_FRAMES that
380	  switches the socket into a mode that allows the handling of CAN FD frames
381	  and (legacy) CAN frames simultaneously (see section 4.1.5).
382	
383	  The struct canfd_frame is defined in include/linux/can.h:
384	
385	    struct canfd_frame {
386	            canid_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */
387	            __u8    len;     /* frame payload length in byte (0 .. 64) */
388	            __u8    flags;   /* additional flags for CAN FD */
389	            __u8    __res0;  /* reserved / padding */
390	            __u8    __res1;  /* reserved / padding */
391	            __u8    data[64] __attribute__((aligned(8)));
392	    };
393	
394	  The struct canfd_frame and the existing struct can_frame have the can_id,
395	  the payload length and the payload data at the same offset inside their
396	  structures. This allows to handle the different structures very similar.
397	  When the content of a struct can_frame is copied into a struct canfd_frame
398	  all structure elements can be used as-is - only the data[] becomes extended.
399	
400	  When introducing the struct canfd_frame it turned out that the data length
401	  code (DLC) of the struct can_frame was used as a length information as the
402	  length and the DLC has a 1:1 mapping in the range of 0 .. 8. To preserve
403	  the easy handling of the length information the canfd_frame.len element
404	  contains a plain length value from 0 .. 64. So both canfd_frame.len and
405	  can_frame.can_dlc are equal and contain a length information and no DLC.
406	  For details about the distinction of CAN and CAN FD capable devices and
407	  the mapping to the bus-relevant data length code (DLC), see chapter 6.6.
408	
409	  The length of the two CAN(FD) frame structures define the maximum transfer
410	  unit (MTU) of the CAN(FD) network interface and skbuff data length. Two
411	  definitions are specified for CAN specific MTUs in include/linux/can.h :
412	
413	  #define CAN_MTU   (sizeof(struct can_frame))   == 16  => 'legacy' CAN frame
414	  #define CANFD_MTU (sizeof(struct canfd_frame)) == 72  => CAN FD frame
415	
416	  4.1 RAW protocol sockets with can_filters (SOCK_RAW)
417	
418	  Using CAN_RAW sockets is extensively comparable to the commonly
419	  known access to CAN character devices. To meet the new possibilities
420	  provided by the multi user SocketCAN approach, some reasonable
421	  defaults are set at RAW socket binding time:
422	
423	  - The filters are set to exactly one filter receiving everything
424	  - The socket only receives valid data frames (=> no error message frames)
425	  - The loopback of sent CAN frames is enabled (see chapter 3.2)
426	  - The socket does not receive its own sent frames (in loopback mode)
427	
428	  These default settings may be changed before or after binding the socket.
429	  To use the referenced definitions of the socket options for CAN_RAW
430	  sockets, include <linux/can/raw.h>.
431	
432	  4.1.1 RAW socket option CAN_RAW_FILTER
433	
434	  The reception of CAN frames using CAN_RAW sockets can be controlled
435	  by defining 0 .. n filters with the CAN_RAW_FILTER socket option.
436	
437	  The CAN filter structure is defined in include/linux/can.h:
438	
439	    struct can_filter {
440	            canid_t can_id;
441	            canid_t can_mask;
442	    };
443	
444	  A filter matches, when
445	
446	    <received_can_id> & mask == can_id & mask
447	
448	  which is analogous to known CAN controllers hardware filter semantics.
449	  The filter can be inverted in this semantic, when the CAN_INV_FILTER
450	  bit is set in can_id element of the can_filter structure. In
451	  contrast to CAN controller hardware filters the user may set 0 .. n
452	  receive filters for each open socket separately:
453	
454	    struct can_filter rfilter[2];
455	
456	    rfilter[0].can_id   = 0x123;
457	    rfilter[0].can_mask = CAN_SFF_MASK;
458	    rfilter[1].can_id   = 0x200;
459	    rfilter[1].can_mask = 0x700;
460	
461	    setsockopt(s, SOL_CAN_RAW, CAN_RAW_FILTER, &rfilter, sizeof(rfilter));
462	
463	  To disable the reception of CAN frames on the selected CAN_RAW socket:
464	
465	    setsockopt(s, SOL_CAN_RAW, CAN_RAW_FILTER, NULL, 0);
466	
467	  To set the filters to zero filters is quite obsolete as to not read
468	  data causes the raw socket to discard the received CAN frames. But
469	  having this 'send only' use-case we may remove the receive list in the
470	  Kernel to save a little (really a very little!) CPU usage.
471	
472	  4.1.1.1 CAN filter usage optimisation
473	
474	  The CAN filters are processed in per-device filter lists at CAN frame
475	  reception time. To reduce the number of checks that need to be performed
476	  while walking through the filter lists the CAN core provides an optimized
477	  filter handling when the filter subscription focusses on a single CAN ID.
478	
479	  For the possible 2048 SFF CAN identifiers the identifier is used as an index
480	  to access the corresponding subscription list without any further checks.
481	  For the 2^29 possible EFF CAN identifiers a 10 bit XOR folding is used as
482	  hash function to retrieve the EFF table index.
483	
484	  To benefit from the optimized filters for single CAN identifiers the
485	  CAN_SFF_MASK or CAN_EFF_MASK have to be set into can_filter.mask together
486	  with set CAN_EFF_FLAG and CAN_RTR_FLAG bits. A set CAN_EFF_FLAG bit in the
487	  can_filter.mask makes clear that it matters whether a SFF or EFF CAN ID is
488	  subscribed. E.g. in the example from above
489	
490	    rfilter[0].can_id   = 0x123;
491	    rfilter[0].can_mask = CAN_SFF_MASK;
492	
493	  both SFF frames with CAN ID 0x123 and EFF frames with 0xXXXXX123 can pass.
494	
495	  To filter for only 0x123 (SFF) and 0x12345678 (EFF) CAN identifiers the
496	  filter has to be defined in this way to benefit from the optimized filters:
497	
498	    struct can_filter rfilter[2];
499	
500	    rfilter[0].can_id   = 0x123;
501	    rfilter[0].can_mask = (CAN_EFF_FLAG | CAN_RTR_FLAG | CAN_SFF_MASK);
502	    rfilter[1].can_id   = 0x12345678 | CAN_EFF_FLAG;
503	    rfilter[1].can_mask = (CAN_EFF_FLAG | CAN_RTR_FLAG | CAN_EFF_MASK);
504	
505	    setsockopt(s, SOL_CAN_RAW, CAN_RAW_FILTER, &rfilter, sizeof(rfilter));
506	
507	  4.1.2 RAW socket option CAN_RAW_ERR_FILTER
508	
509	  As described in chapter 3.4 the CAN interface driver can generate so
510	  called Error Message Frames that can optionally be passed to the user
511	  application in the same way as other CAN frames. The possible
512	  errors are divided into different error classes that may be filtered
513	  using the appropriate error mask. To register for every possible
514	  error condition CAN_ERR_MASK can be used as value for the error mask.
515	  The values for the error mask are defined in linux/can/error.h .
516	
517	    can_err_mask_t err_mask = ( CAN_ERR_TX_TIMEOUT | CAN_ERR_BUSOFF );
518	
519	    setsockopt(s, SOL_CAN_RAW, CAN_RAW_ERR_FILTER,
520	               &err_mask, sizeof(err_mask));
521	
522	  4.1.3 RAW socket option CAN_RAW_LOOPBACK
523	
524	  To meet multi user needs the local loopback is enabled by default
525	  (see chapter 3.2 for details). But in some embedded use-cases
526	  (e.g. when only one application uses the CAN bus) this loopback
527	  functionality can be disabled (separately for each socket):
528	
529	    int loopback = 0; /* 0 = disabled, 1 = enabled (default) */
530	
531	    setsockopt(s, SOL_CAN_RAW, CAN_RAW_LOOPBACK, &loopback, sizeof(loopback));
532	
533	  4.1.4 RAW socket option CAN_RAW_RECV_OWN_MSGS
534	
535	  When the local loopback is enabled, all the sent CAN frames are
536	  looped back to the open CAN sockets that registered for the CAN
537	  frames' CAN-ID on this given interface to meet the multi user
538	  needs. The reception of the CAN frames on the same socket that was
539	  sending the CAN frame is assumed to be unwanted and therefore
540	  disabled by default. This default behaviour may be changed on
541	  demand:
542	
543	    int recv_own_msgs = 1; /* 0 = disabled (default), 1 = enabled */
544	
545	    setsockopt(s, SOL_CAN_RAW, CAN_RAW_RECV_OWN_MSGS,
546	               &recv_own_msgs, sizeof(recv_own_msgs));
547	
548	  4.1.5 RAW socket option CAN_RAW_FD_FRAMES
549	
550	  CAN FD support in CAN_RAW sockets can be enabled with a new socket option
551	  CAN_RAW_FD_FRAMES which is off by default. When the new socket option is
552	  not supported by the CAN_RAW socket (e.g. on older kernels), switching the
553	  CAN_RAW_FD_FRAMES option returns the error -ENOPROTOOPT.
554	
555	  Once CAN_RAW_FD_FRAMES is enabled the application can send both CAN frames
556	  and CAN FD frames. OTOH the application has to handle CAN and CAN FD frames
557	  when reading from the socket.
558	
559	    CAN_RAW_FD_FRAMES enabled:  CAN_MTU and CANFD_MTU are allowed
560	    CAN_RAW_FD_FRAMES disabled: only CAN_MTU is allowed (default)
561	
562	  Example:
563	    [ remember: CANFD_MTU == sizeof(struct canfd_frame) ]
564	
565	    struct canfd_frame cfd;
566	
567	    nbytes = read(s, &cfd, CANFD_MTU);
568	
569	    if (nbytes == CANFD_MTU) {
570	            printf("got CAN FD frame with length %d\n", cfd.len);
571		    /* cfd.flags contains valid data */
572	    } else if (nbytes == CAN_MTU) {
573	            printf("got legacy CAN frame with length %d\n", cfd.len);
574		    /* cfd.flags is undefined */
575	    } else {
576	            fprintf(stderr, "read: invalid CAN(FD) frame\n");
577	            return 1;
578	    }
579	
580	    /* the content can be handled independently from the received MTU size */
581	
582	    printf("can_id: %X data length: %d data: ", cfd.can_id, cfd.len);
583	    for (i = 0; i < cfd.len; i++)
584	            printf("%02X ", cfd.data[i]);
585	
586	  When reading with size CANFD_MTU only returns CAN_MTU bytes that have
587	  been received from the socket a legacy CAN frame has been read into the
588	  provided CAN FD structure. Note that the canfd_frame.flags data field is
589	  not specified in the struct can_frame and therefore it is only valid in
590	  CANFD_MTU sized CAN FD frames.
591	
592	  Implementation hint for new CAN applications:
593	
594	  To build a CAN FD aware application use struct canfd_frame as basic CAN
595	  data structure for CAN_RAW based applications. When the application is
596	  executed on an older Linux kernel and switching the CAN_RAW_FD_FRAMES
597	  socket option returns an error: No problem. You'll get legacy CAN frames
598	  or CAN FD frames and can process them the same way.
599	
600	  When sending to CAN devices make sure that the device is capable to handle
601	  CAN FD frames by checking if the device maximum transfer unit is CANFD_MTU.
602	  The CAN device MTU can be retrieved e.g. with a SIOCGIFMTU ioctl() syscall.
603	
604	  4.1.6 RAW socket returned message flags
605	
606	  When using recvmsg() call, the msg->msg_flags may contain following flags:
607	
608	    MSG_DONTROUTE: set when the received frame was created on the local host.
609	
610	    MSG_CONFIRM: set when the frame was sent via the socket it is received on.
611	      This flag can be interpreted as a 'transmission confirmation' when the
612	      CAN driver supports the echo of frames on driver level, see 3.2 and 6.2.
613	      In order to receive such messages, CAN_RAW_RECV_OWN_MSGS must be set.
614	
615	  4.2 Broadcast Manager protocol sockets (SOCK_DGRAM)
616	
617	  The Broadcast Manager protocol provides a command based configuration
618	  interface to filter and send (e.g. cyclic) CAN messages in kernel space.
619	
620	  Receive filters can be used to down sample frequent messages; detect events
621	  such as message contents changes, packet length changes, and do time-out
622	  monitoring of received messages.
623	
624	  Periodic transmission tasks of CAN frames or a sequence of CAN frames can be
625	  created and modified at runtime; both the message content and the two
626	  possible transmit intervals can be altered.
627	
628	  A BCM socket is not intended for sending individual CAN frames using the
629	  struct can_frame as known from the CAN_RAW socket. Instead a special BCM
630	  configuration message is defined. The basic BCM configuration message used
631	  to communicate with the broadcast manager and the available operations are
632	  defined in the linux/can/bcm.h include. The BCM message consists of a
633	  message header with a command ('opcode') followed by zero or more CAN frames.
634	  The broadcast manager sends responses to user space in the same form:
635	
636	    struct bcm_msg_head {
637	            __u32 opcode;                   /* command */
638	            __u32 flags;                    /* special flags */
639	            __u32 count;                    /* run 'count' times with ival1 */
640	            struct timeval ival1, ival2;    /* count and subsequent interval */
641	            canid_t can_id;                 /* unique can_id for task */
642	            __u32 nframes;                  /* number of can_frames following */
643	            struct can_frame frames[0];
644	    };
645	
646	  The aligned payload 'frames' uses the same basic CAN frame structure defined
647	  at the beginning of section 4 and in the include/linux/can.h include. All
648	  messages to the broadcast manager from user space have this structure.
649	
650	  Note a CAN_BCM socket must be connected instead of bound after socket
651	  creation (example without error checking):
652	
653	    int s;
654	    struct sockaddr_can addr;
655	    struct ifreq ifr;
656	
657	    s = socket(PF_CAN, SOCK_DGRAM, CAN_BCM);
658	
659	    strcpy(ifr.ifr_name, "can0");
660	    ioctl(s, SIOCGIFINDEX, &ifr);
661	
662	    addr.can_family = AF_CAN;
663	    addr.can_ifindex = ifr.ifr_ifindex;
664	
665	    connect(s, (struct sockaddr *)&addr, sizeof(addr))
666	
667	    (..)
668	
669	  The broadcast manager socket is able to handle any number of in flight
670	  transmissions or receive filters concurrently. The different RX/TX jobs are
671	  distinguished by the unique can_id in each BCM message. However additional
672	  CAN_BCM sockets are recommended to communicate on multiple CAN interfaces.
673	  When the broadcast manager socket is bound to 'any' CAN interface (=> the
674	  interface index is set to zero) the configured receive filters apply to any
675	  CAN interface unless the sendto() syscall is used to overrule the 'any' CAN
676	  interface index. When using recvfrom() instead of read() to retrieve BCM
677	  socket messages the originating CAN interface is provided in can_ifindex.
678	
679	  4.2.1 Broadcast Manager operations
680	
681	  The opcode defines the operation for the broadcast manager to carry out,
682	  or details the broadcast managers response to several events, including
683	  user requests.
684	
685	  Transmit Operations (user space to broadcast manager):
686	
687	    TX_SETUP:   Create (cyclic) transmission task.
688	
689	    TX_DELETE:  Remove (cyclic) transmission task, requires only can_id.
690	
691	    TX_READ:    Read properties of (cyclic) transmission task for can_id.
692	
693	    TX_SEND:    Send one CAN frame.
694	
695	  Transmit Responses (broadcast manager to user space):
696	
697	    TX_STATUS:  Reply to TX_READ request (transmission task configuration).
698	
699	    TX_EXPIRED: Notification when counter finishes sending at initial interval
700	      'ival1'. Requires the TX_COUNTEVT flag to be set at TX_SETUP.
701	
702	  Receive Operations (user space to broadcast manager):
703	
704	    RX_SETUP:   Create RX content filter subscription.
705	
706	    RX_DELETE:  Remove RX content filter subscription, requires only can_id.
707	
708	    RX_READ:    Read properties of RX content filter subscription for can_id.
709	
710	  Receive Responses (broadcast manager to user space):
711	
712	    RX_STATUS:  Reply to RX_READ request (filter task configuration).
713	
714	    RX_TIMEOUT: Cyclic message is detected to be absent (timer ival1 expired).
715	
716	    RX_CHANGED: BCM message with updated CAN frame (detected content change).
717	      Sent on first message received or on receipt of revised CAN messages.
718	
719	  4.2.2 Broadcast Manager message flags
720	
721	  When sending a message to the broadcast manager the 'flags' element may
722	  contain the following flag definitions which influence the behaviour:
723	
724	    SETTIMER:           Set the values of ival1, ival2 and count
725	
726	    STARTTIMER:         Start the timer with the actual values of ival1, ival2
727	      and count. Starting the timer leads simultaneously to emit a CAN frame.
728	
729	    TX_COUNTEVT:        Create the message TX_EXPIRED when count expires
730	
731	    TX_ANNOUNCE:        A change of data by the process is emitted immediately.
732	
733	    TX_CP_CAN_ID:       Copies the can_id from the message header to each
734	      subsequent frame in frames. This is intended as usage simplification. For
735	      TX tasks the unique can_id from the message header may differ from the
736	      can_id(s) stored for transmission in the subsequent struct can_frame(s).
737	
738	    RX_FILTER_ID:       Filter by can_id alone, no frames required (nframes=0).
739	
740	    RX_CHECK_DLC:       A change of the DLC leads to an RX_CHANGED.
741	
742	    RX_NO_AUTOTIMER:    Prevent automatically starting the timeout monitor.
743	
744	    RX_ANNOUNCE_RESUME: If passed at RX_SETUP and a receive timeout occurred, a
745	      RX_CHANGED message will be generated when the (cyclic) receive restarts.
746	
747	    TX_RESET_MULTI_IDX: Reset the index for the multiple frame transmission.
748	
749	    RX_RTR_FRAME:       Send reply for RTR-request (placed in op->frames[0]).
750	
751	  4.2.3 Broadcast Manager transmission timers
752	
753	  Periodic transmission configurations may use up to two interval timers.
754	  In this case the BCM sends a number of messages ('count') at an interval
755	  'ival1', then continuing to send at another given interval 'ival2'. When
756	  only one timer is needed 'count' is set to zero and only 'ival2' is used.
757	  When SET_TIMER and START_TIMER flag were set the timers are activated.
758	  The timer values can be altered at runtime when only SET_TIMER is set.
759	
760	  4.2.4 Broadcast Manager message sequence transmission
761	
762	  Up to 256 CAN frames can be transmitted in a sequence in the case of a cyclic
763	  TX task configuration. The number of CAN frames is provided in the 'nframes'
764	  element of the BCM message head. The defined number of CAN frames are added
765	  as array to the TX_SETUP BCM configuration message.
766	
767	    /* create a struct to set up a sequence of four CAN frames */
768	    struct {
769	            struct bcm_msg_head msg_head;
770	            struct can_frame frame[4];
771	    } mytxmsg;
772	
773	    (..)
774	    mytxmsg.nframes = 4;
775	    (..)
776	
777	    write(s, &mytxmsg, sizeof(mytxmsg));
778	
779	  With every transmission the index in the array of CAN frames is increased
780	  and set to zero at index overflow.
781	
782	  4.2.5 Broadcast Manager receive filter timers
783	
784	  The timer values ival1 or ival2 may be set to non-zero values at RX_SETUP.
785	  When the SET_TIMER flag is set the timers are enabled:
786	
787	  ival1: Send RX_TIMEOUT when a received message is not received again within
788	    the given time. When START_TIMER is set at RX_SETUP the timeout detection
789	    is activated directly - even without a former CAN frame reception.
790	
791	  ival2: Throttle the received message rate down to the value of ival2. This
792	    is useful to reduce messages for the application when the signal inside the
793	    CAN frame is stateless as state changes within the ival2 periode may get
794	    lost.
795	
796	  4.2.6 Broadcast Manager multiplex message receive filter
797	
798	  To filter for content changes in multiplex message sequences an array of more
799	  than one CAN frames can be passed in a RX_SETUP configuration message. The
800	  data bytes of the first CAN frame contain the mask of relevant bits that
801	  have to match in the subsequent CAN frames with the received CAN frame.
802	  If one of the subsequent CAN frames is matching the bits in that frame data
803	  mark the relevant content to be compared with the previous received content.
804	  Up to 257 CAN frames (multiplex filter bit mask CAN frame plus 256 CAN
805	  filters) can be added as array to the TX_SETUP BCM configuration message.
806	
807	    /* usually used to clear CAN frame data[] - beware of endian problems! */
808	    #define U64_DATA(p) (*(unsigned long long*)(p)->data)
809	
810	    struct {
811	            struct bcm_msg_head msg_head;
812	            struct can_frame frame[5];
813	    } msg;
814	
815	    msg.msg_head.opcode  = RX_SETUP;
816	    msg.msg_head.can_id  = 0x42;
817	    msg.msg_head.flags   = 0;
818	    msg.msg_head.nframes = 5;
819	    U64_DATA(&msg.frame[0]) = 0xFF00000000000000ULL; /* MUX mask */
820	    U64_DATA(&msg.frame[1]) = 0x01000000000000FFULL; /* data mask (MUX 0x01) */
821	    U64_DATA(&msg.frame[2]) = 0x0200FFFF000000FFULL; /* data mask (MUX 0x02) */
822	    U64_DATA(&msg.frame[3]) = 0x330000FFFFFF0003ULL; /* data mask (MUX 0x33) */
823	    U64_DATA(&msg.frame[4]) = 0x4F07FC0FF0000000ULL; /* data mask (MUX 0x4F) */
824	
825	    write(s, &msg, sizeof(msg));
826	
827	  4.3 connected transport protocols (SOCK_SEQPACKET)
828	  4.4 unconnected transport protocols (SOCK_DGRAM)
829	
830	
831	5. SocketCAN core module
832	-------------------------
833	
834	  The SocketCAN core module implements the protocol family
835	  PF_CAN. CAN protocol modules are loaded by the core module at
836	  runtime. The core module provides an interface for CAN protocol
837	  modules to subscribe needed CAN IDs (see chapter 3.1).
838	
839	  5.1 can.ko module params
840	
841	  - stats_timer: To calculate the SocketCAN core statistics
842	    (e.g. current/maximum frames per second) this 1 second timer is
843	    invoked at can.ko module start time by default. This timer can be
844	    disabled by using stattimer=0 on the module commandline.
845	
846	  - debug: (removed since SocketCAN SVN r546)
847	
848	  5.2 procfs content
849	
850	  As described in chapter 3.1 the SocketCAN core uses several filter
851	  lists to deliver received CAN frames to CAN protocol modules. These
852	  receive lists, their filters and the count of filter matches can be
853	  checked in the appropriate receive list. All entries contain the
854	  device and a protocol module identifier:
855	
856	    foo@bar:~$ cat /proc/net/can/rcvlist_all
857	
858	    receive list 'rx_all':
859	      (vcan3: no entry)
860	      (vcan2: no entry)
861	      (vcan1: no entry)
862	      device   can_id   can_mask  function  userdata   matches  ident
863	       vcan0     000    00000000  f88e6370  f6c6f400         0  raw
864	      (any: no entry)
865	
866	  In this example an application requests any CAN traffic from vcan0.
867	
868	    rcvlist_all - list for unfiltered entries (no filter operations)
869	    rcvlist_eff - list for single extended frame (EFF) entries
870	    rcvlist_err - list for error message frames masks
871	    rcvlist_fil - list for mask/value filters
872	    rcvlist_inv - list for mask/value filters (inverse semantic)
873	    rcvlist_sff - list for single standard frame (SFF) entries
874	
875	  Additional procfs files in /proc/net/can
876	
877	    stats       - SocketCAN core statistics (rx/tx frames, match ratios, ...)
878	    reset_stats - manual statistic reset
879	    version     - prints the SocketCAN core version and the ABI version
880	
881	  5.3 writing own CAN protocol modules
882	
883	  To implement a new protocol in the protocol family PF_CAN a new
884	  protocol has to be defined in include/linux/can.h .
885	  The prototypes and definitions to use the SocketCAN core can be
886	  accessed by including include/linux/can/core.h .
887	  In addition to functions that register the CAN protocol and the
888	  CAN device notifier chain there are functions to subscribe CAN
889	  frames received by CAN interfaces and to send CAN frames:
890	
891	    can_rx_register   - subscribe CAN frames from a specific interface
892	    can_rx_unregister - unsubscribe CAN frames from a specific interface
893	    can_send          - transmit a CAN frame (optional with local loopback)
894	
895	  For details see the kerneldoc documentation in net/can/af_can.c or
896	  the source code of net/can/raw.c or net/can/bcm.c .
897	
898	6. CAN network drivers
899	----------------------
900	
901	  Writing a CAN network device driver is much easier than writing a
902	  CAN character device driver. Similar to other known network device
903	  drivers you mainly have to deal with:
904	
905	  - TX: Put the CAN frame from the socket buffer to the CAN controller.
906	  - RX: Put the CAN frame from the CAN controller to the socket buffer.
907	
908	  See e.g. at Documentation/networking/netdevices.txt . The differences
909	  for writing CAN network device driver are described below:
910	
911	  6.1 general settings
912	
913	    dev->type  = ARPHRD_CAN; /* the netdevice hardware type */
914	    dev->flags = IFF_NOARP;  /* CAN has no arp */
915	
916	    dev->mtu = CAN_MTU; /* sizeof(struct can_frame) -> legacy CAN interface */
917	
918	    or alternative, when the controller supports CAN with flexible data rate:
919	    dev->mtu = CANFD_MTU; /* sizeof(struct canfd_frame) -> CAN FD interface */
920	
921	  The struct can_frame or struct canfd_frame is the payload of each socket
922	  buffer (skbuff) in the protocol family PF_CAN.
923	
924	  6.2 local loopback of sent frames
925	
926	  As described in chapter 3.2 the CAN network device driver should
927	  support a local loopback functionality similar to the local echo
928	  e.g. of tty devices. In this case the driver flag IFF_ECHO has to be
929	  set to prevent the PF_CAN core from locally echoing sent frames
930	  (aka loopback) as fallback solution:
931	
932	    dev->flags = (IFF_NOARP | IFF_ECHO);
933	
934	  6.3 CAN controller hardware filters
935	
936	  To reduce the interrupt load on deep embedded systems some CAN
937	  controllers support the filtering of CAN IDs or ranges of CAN IDs.
938	  These hardware filter capabilities vary from controller to
939	  controller and have to be identified as not feasible in a multi-user
940	  networking approach. The use of the very controller specific
941	  hardware filters could make sense in a very dedicated use-case, as a
942	  filter on driver level would affect all users in the multi-user
943	  system. The high efficient filter sets inside the PF_CAN core allow
944	  to set different multiple filters for each socket separately.
945	  Therefore the use of hardware filters goes to the category 'handmade
946	  tuning on deep embedded systems'. The author is running a MPC603e
947	  @133MHz with four SJA1000 CAN controllers from 2002 under heavy bus
948	  load without any problems ...
949	
950	  6.4 The virtual CAN driver (vcan)
951	
952	  Similar to the network loopback devices, vcan offers a virtual local
953	  CAN interface. A full qualified address on CAN consists of
954	
955	  - a unique CAN Identifier (CAN ID)
956	  - the CAN bus this CAN ID is transmitted on (e.g. can0)
957	
958	  so in common use cases more than one virtual CAN interface is needed.
959	
960	  The virtual CAN interfaces allow the transmission and reception of CAN
961	  frames without real CAN controller hardware. Virtual CAN network
962	  devices are usually named 'vcanX', like vcan0 vcan1 vcan2 ...
963	  When compiled as a module the virtual CAN driver module is called vcan.ko
964	
965	  Since Linux Kernel version 2.6.24 the vcan driver supports the Kernel
966	  netlink interface to create vcan network devices. The creation and
967	  removal of vcan network devices can be managed with the ip(8) tool:
968	
969	  - Create a virtual CAN network interface:
970	       $ ip link add type vcan
971	
972	  - Create a virtual CAN network interface with a specific name 'vcan42':
973	       $ ip link add dev vcan42 type vcan
974	
975	  - Remove a (virtual CAN) network interface 'vcan42':
976	       $ ip link del vcan42
977	
978	  6.5 The CAN network device driver interface
979	
980	  The CAN network device driver interface provides a generic interface
981	  to setup, configure and monitor CAN network devices. The user can then
982	  configure the CAN device, like setting the bit-timing parameters, via
983	  the netlink interface using the program "ip" from the "IPROUTE2"
984	  utility suite. The following chapter describes briefly how to use it.
985	  Furthermore, the interface uses a common data structure and exports a
986	  set of common functions, which all real CAN network device drivers
987	  should use. Please have a look to the SJA1000 or MSCAN driver to
988	  understand how to use them. The name of the module is can-dev.ko.
989	
990	  6.5.1 Netlink interface to set/get devices properties
991	
992	  The CAN device must be configured via netlink interface. The supported
993	  netlink message types are defined and briefly described in
994	  "include/linux/can/netlink.h". CAN link support for the program "ip"
995	  of the IPROUTE2 utility suite is available and it can be used as shown
996	  below:
997	
998	  - Setting CAN device properties:
999	
1000	    $ ip link set can0 type can help
1001	    Usage: ip link set DEVICE type can
1002	    	[ bitrate BITRATE [ sample-point SAMPLE-POINT] ] |
1003	    	[ tq TQ prop-seg PROP_SEG phase-seg1 PHASE-SEG1
1004	     	  phase-seg2 PHASE-SEG2 [ sjw SJW ] ]
1005	
1006	    	[ loopback { on | off } ]
1007	    	[ listen-only { on | off } ]
1008	    	[ triple-sampling { on | off } ]
1009	
1010	    	[ restart-ms TIME-MS ]
1011	    	[ restart ]
1012	
1013	    	Where: BITRATE       := { 1..1000000 }
1014	    	       SAMPLE-POINT  := { 0.000..0.999 }
1015	    	       TQ            := { NUMBER }
1016	    	       PROP-SEG      := { 1..8 }
1017	    	       PHASE-SEG1    := { 1..8 }
1018	    	       PHASE-SEG2    := { 1..8 }
1019	    	       SJW           := { 1..4 }
1020	    	       RESTART-MS    := { 0 | NUMBER }
1021	
1022	  - Display CAN device details and statistics:
1023	
1024	    $ ip -details -statistics link show can0
1025	    2: can0: <NOARP,UP,LOWER_UP,ECHO> mtu 16 qdisc pfifo_fast state UP qlen 10
1026	      link/can
1027	      can <TRIPLE-SAMPLING> state ERROR-ACTIVE restart-ms 100
1028	      bitrate 125000 sample_point 0.875
1029	      tq 125 prop-seg 6 phase-seg1 7 phase-seg2 2 sjw 1
1030	      sja1000: tseg1 1..16 tseg2 1..8 sjw 1..4 brp 1..64 brp-inc 1
1031	      clock 8000000
1032	      re-started bus-errors arbit-lost error-warn error-pass bus-off
1033	      41         17457      0          41         42         41
1034	      RX: bytes  packets  errors  dropped overrun mcast
1035	      140859     17608    17457   0       0       0
1036	      TX: bytes  packets  errors  dropped carrier collsns
1037	      861        112      0       41      0       0
1038	
1039	  More info to the above output:
1040	
1041	    "<TRIPLE-SAMPLING>"
1042		Shows the list of selected CAN controller modes: LOOPBACK,
1043		LISTEN-ONLY, or TRIPLE-SAMPLING.
1044	
1045	    "state ERROR-ACTIVE"
1046		The current state of the CAN controller: "ERROR-ACTIVE",
1047		"ERROR-WARNING", "ERROR-PASSIVE", "BUS-OFF" or "STOPPED"
1048	
1049	    "restart-ms 100"
1050		Automatic restart delay time. If set to a non-zero value, a
1051		restart of the CAN controller will be triggered automatically
1052		in case of a bus-off condition after the specified delay time
1053		in milliseconds. By default it's off.
1054	
1055	    "bitrate 125000 sample-point 0.875"
1056		Shows the real bit-rate in bits/sec and the sample-point in the
1057		range 0.000..0.999. If the calculation of bit-timing parameters
1058		is enabled in the kernel (CONFIG_CAN_CALC_BITTIMING=y), the
1059		bit-timing can be defined by setting the "bitrate" argument.
1060		Optionally the "sample-point" can be specified. By default it's
1061		0.000 assuming CIA-recommended sample-points.
1062	
1063	    "tq 125 prop-seg 6 phase-seg1 7 phase-seg2 2 sjw 1"
1064		Shows the time quanta in ns, propagation segment, phase buffer
1065		segment 1 and 2 and the synchronisation jump width in units of
1066		tq. They allow to define the CAN bit-timing in a hardware
1067		independent format as proposed by the Bosch CAN 2.0 spec (see
1068		chapter 8 of http://www.semiconductors.bosch.de/pdf/can2spec.pdf).
1069	
1070	    "sja1000: tseg1 1..16 tseg2 1..8 sjw 1..4 brp 1..64 brp-inc 1
1071	     clock 8000000"
1072		Shows the bit-timing constants of the CAN controller, here the
1073		"sja1000". The minimum and maximum values of the time segment 1
1074		and 2, the synchronisation jump width in units of tq, the
1075		bitrate pre-scaler and the CAN system clock frequency in Hz.
1076		These constants could be used for user-defined (non-standard)
1077		bit-timing calculation algorithms in user-space.
1078	
1079	    "re-started bus-errors arbit-lost error-warn error-pass bus-off"
1080		Shows the number of restarts, bus and arbitration lost errors,
1081		and the state changes to the error-warning, error-passive and
1082		bus-off state. RX overrun errors are listed in the "overrun"
1083		field of the standard network statistics.
1084	
1085	  6.5.2 Setting the CAN bit-timing
1086	
1087	  The CAN bit-timing parameters can always be defined in a hardware
1088	  independent format as proposed in the Bosch CAN 2.0 specification
1089	  specifying the arguments "tq", "prop_seg", "phase_seg1", "phase_seg2"
1090	  and "sjw":
1091	
1092	    $ ip link set canX type can tq 125 prop-seg 6 \
1093					phase-seg1 7 phase-seg2 2 sjw 1
1094	
1095	  If the kernel option CONFIG_CAN_CALC_BITTIMING is enabled, CIA
1096	  recommended CAN bit-timing parameters will be calculated if the bit-
1097	  rate is specified with the argument "bitrate":
1098	
1099	    $ ip link set canX type can bitrate 125000
1100	
1101	  Note that this works fine for the most common CAN controllers with
1102	  standard bit-rates but may *fail* for exotic bit-rates or CAN system
1103	  clock frequencies. Disabling CONFIG_CAN_CALC_BITTIMING saves some
1104	  space and allows user-space tools to solely determine and set the
1105	  bit-timing parameters. The CAN controller specific bit-timing
1106	  constants can be used for that purpose. They are listed by the
1107	  following command:
1108	
1109	    $ ip -details link show can0
1110	    ...
1111	      sja1000: clock 8000000 tseg1 1..16 tseg2 1..8 sjw 1..4 brp 1..64 brp-inc 1
1112	
1113	  6.5.3 Starting and stopping the CAN network device
1114	
1115	  A CAN network device is started or stopped as usual with the command
1116	  "ifconfig canX up/down" or "ip link set canX up/down". Be aware that
1117	  you *must* define proper bit-timing parameters for real CAN devices
1118	  before you can start it to avoid error-prone default settings:
1119	
1120	    $ ip link set canX up type can bitrate 125000
1121	
1122	  A device may enter the "bus-off" state if too many errors occurred on
1123	  the CAN bus. Then no more messages are received or sent. An automatic
1124	  bus-off recovery can be enabled by setting the "restart-ms" to a
1125	  non-zero value, e.g.:
1126	
1127	    $ ip link set canX type can restart-ms 100
1128	
1129	  Alternatively, the application may realize the "bus-off" condition
1130	  by monitoring CAN error message frames and do a restart when
1131	  appropriate with the command:
1132	
1133	    $ ip link set canX type can restart
1134	
1135	  Note that a restart will also create a CAN error message frame (see
1136	  also chapter 3.4).
1137	
1138	  6.6 CAN FD (flexible data rate) driver support
1139	
1140	  CAN FD capable CAN controllers support two different bitrates for the
1141	  arbitration phase and the payload phase of the CAN FD frame. Therefore a
1142	  second bit timing has to be specified in order to enable the CAN FD bitrate.
1143	
1144	  Additionally CAN FD capable CAN controllers support up to 64 bytes of
1145	  payload. The representation of this length in can_frame.can_dlc and
1146	  canfd_frame.len for userspace applications and inside the Linux network
1147	  layer is a plain value from 0 .. 64 instead of the CAN 'data length code'.
1148	  The data length code was a 1:1 mapping to the payload length in the legacy
1149	  CAN frames anyway. The payload length to the bus-relevant DLC mapping is
1150	  only performed inside the CAN drivers, preferably with the helper
1151	  functions can_dlc2len() and can_len2dlc().
1152	
1153	  The CAN netdevice driver capabilities can be distinguished by the network
1154	  devices maximum transfer unit (MTU):
1155	
1156	  MTU = 16 (CAN_MTU)   => sizeof(struct can_frame)   => 'legacy' CAN device
1157	  MTU = 72 (CANFD_MTU) => sizeof(struct canfd_frame) => CAN FD capable device
1158	
1159	  The CAN device MTU can be retrieved e.g. with a SIOCGIFMTU ioctl() syscall.
1160	  N.B. CAN FD capable devices can also handle and send legacy CAN frames.
1161	
1162	  FIXME: Add details about the CAN FD controller configuration when available.
1163	
1164	  6.7 Supported CAN hardware
1165	
1166	  Please check the "Kconfig" file in "drivers/net/can" to get an actual
1167	  list of the support CAN hardware. On the SocketCAN project website
1168	  (see chapter 7) there might be further drivers available, also for
1169	  older kernel versions.
1170	
1171	7. SocketCAN resources
1172	-----------------------
1173	
1174	  The Linux CAN / SocketCAN project ressources (project site / mailing list)
1175	  are referenced in the MAINTAINERS file in the Linux source tree.
1176	  Search for CAN NETWORK [LAYERS|DRIVERS].
1177	
1178	8. Credits
1179	----------
1180	
1181	  Oliver Hartkopp (PF_CAN core, filters, drivers, bcm, SJA1000 driver)
1182	  Urs Thuermann (PF_CAN core, kernel integration, socket interfaces, raw, vcan)
1183	  Jan Kizka (RT-SocketCAN core, Socket-API reconciliation)
1184	  Wolfgang Grandegger (RT-SocketCAN core & drivers, Raw Socket-API reviews,
1185	                       CAN device driver interface, MSCAN driver)
1186	  Robert Schwebel (design reviews, PTXdist integration)
1187	  Marc Kleine-Budde (design reviews, Kernel 2.6 cleanups, drivers)
1188	  Benedikt Spranger (reviews)
1189	  Thomas Gleixner (LKML reviews, coding style, posting hints)
1190	  Andrey Volkov (kernel subtree structure, ioctls, MSCAN driver)
1191	  Matthias Brukner (first SJA1000 CAN netdevice implementation Q2/2003)
1192	  Klaus Hitschler (PEAK driver integration)
1193	  Uwe Koppe (CAN netdevices with PF_PACKET approach)
1194	  Michael Schulze (driver layer loopback requirement, RT CAN drivers review)
1195	  Pavel Pisa (Bit-timing calculation)
1196	  Sascha Hauer (SJA1000 platform driver)
1197	  Sebastian Haas (SJA1000 EMS PCI driver)
1198	  Markus Plessing (SJA1000 EMS PCI driver)
1199	  Per Dalen (SJA1000 Kvaser PCI driver)
1200	  Sam Ravnborg (reviews, coding style, kbuild help)
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