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Based on kernel version 4.8. Page generated on 2016-10-06 23:18 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 option CAN_RAW_JOIN_FILTERS
26	      4.1.7 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	      4.2.7 Broadcast Manager CAN FD support
35	    4.3 connected transport protocols (SOCK_SEQPACKET)
36	    4.4 unconnected transport protocols (SOCK_DGRAM)
37	
38	  5 SocketCAN core module
39	    5.1 can.ko module params
40	    5.2 procfs content
41	    5.3 writing own CAN protocol modules
42	
43	  6 CAN network drivers
44	    6.1 general settings
45	    6.2 local loopback of sent frames
46	    6.3 CAN controller hardware filters
47	    6.4 The virtual CAN driver (vcan)
48	    6.5 The CAN network device driver interface
49	      6.5.1 Netlink interface to set/get devices properties
50	      6.5.2 Setting the CAN bit-timing
51	      6.5.3 Starting and stopping the CAN network device
52	    6.6 CAN FD (flexible data rate) driver support
53	    6.7 supported CAN hardware
54	
55	  7 SocketCAN resources
56	
57	  8 Credits
58	
59	============================================================================
60	
61	1. Overview / What is SocketCAN
62	--------------------------------
63	
64	The socketcan package is an implementation of CAN protocols
65	(Controller Area Network) for Linux.  CAN is a networking technology
66	which has widespread use in automation, embedded devices, and
67	automotive fields.  While there have been other CAN implementations
68	for Linux based on character devices, SocketCAN uses the Berkeley
69	socket API, the Linux network stack and implements the CAN device
70	drivers as network interfaces.  The CAN socket API has been designed
71	as similar as possible to the TCP/IP protocols to allow programmers,
72	familiar with network programming, to easily learn how to use CAN
73	sockets.
74	
75	2. Motivation / Why using the socket API
76	----------------------------------------
77	
78	There have been CAN implementations for Linux before SocketCAN so the
79	question arises, why we have started another project.  Most existing
80	implementations come as a device driver for some CAN hardware, they
81	are based on character devices and provide comparatively little
82	functionality.  Usually, there is only a hardware-specific device
83	driver which provides a character device interface to send and
84	receive raw CAN frames, directly to/from the controller hardware.
85	Queueing of frames and higher-level transport protocols like ISO-TP
86	have to be implemented in user space applications.  Also, most
87	character-device implementations support only one single process to
88	open the device at a time, similar to a serial interface.  Exchanging
89	the CAN controller requires employment of another device driver and
90	often the need for adaption of large parts of the application to the
91	new driver's API.
92	
93	SocketCAN was designed to overcome all of these limitations.  A new
94	protocol family has been implemented which provides a socket interface
95	to user space applications and which builds upon the Linux network
96	layer, enabling use all of the provided queueing functionality.  A device
97	driver for CAN controller hardware registers itself with the Linux
98	network layer as a network device, so that CAN frames from the
99	controller can be passed up to the network layer and on to the CAN
100	protocol family module and also vice-versa.  Also, the protocol family
101	module provides an API for transport protocol modules to register, so
102	that any number of transport protocols can be loaded or unloaded
103	dynamically.  In fact, the can core module alone does not provide any
104	protocol and cannot be used without loading at least one additional
105	protocol module.  Multiple sockets can be opened at the same time,
106	on different or the same protocol module and they can listen/send
107	frames on different or the same CAN IDs.  Several sockets listening on
108	the same interface for frames with the same CAN ID are all passed the
109	same received matching CAN frames.  An application wishing to
110	communicate using a specific transport protocol, e.g. ISO-TP, just
111	selects that protocol when opening the socket, and then can read and
112	write application data byte streams, without having to deal with
113	CAN-IDs, frames, etc.
114	
115	Similar functionality visible from user-space could be provided by a
116	character device, too, but this would lead to a technically inelegant
117	solution for a couple of reasons:
118	
119	* Intricate usage.  Instead of passing a protocol argument to
120	  socket(2) and using bind(2) to select a CAN interface and CAN ID, an
121	  application would have to do all these operations using ioctl(2)s.
122	
123	* Code duplication.  A character device cannot make use of the Linux
124	  network queueing code, so all that code would have to be duplicated
125	  for CAN networking.
126	
127	* Abstraction.  In most existing character-device implementations, the
128	  hardware-specific device driver for a CAN controller directly
129	  provides the character device for the application to work with.
130	  This is at least very unusual in Unix systems for both, char and
131	  block devices.  For example you don't have a character device for a
132	  certain UART of a serial interface, a certain sound chip in your
133	  computer, a SCSI or IDE controller providing access to your hard
134	  disk or tape streamer device.  Instead, you have abstraction layers
135	  which provide a unified character or block device interface to the
136	  application on the one hand, and a interface for hardware-specific
137	  device drivers on the other hand.  These abstractions are provided
138	  by subsystems like the tty layer, the audio subsystem or the SCSI
139	  and IDE subsystems for the devices mentioned above.
140	
141	  The easiest way to implement a CAN device driver is as a character
142	  device without such a (complete) abstraction layer, as is done by most
143	  existing drivers.  The right way, however, would be to add such a
144	  layer with all the functionality like registering for certain CAN
145	  IDs, supporting several open file descriptors and (de)multiplexing
146	  CAN frames between them, (sophisticated) queueing of CAN frames, and
147	  providing an API for device drivers to register with.  However, then
148	  it would be no more difficult, or may be even easier, to use the
149	  networking framework provided by the Linux kernel, and this is what
150	  SocketCAN does.
151	
152	  The use of the networking framework of the Linux kernel is just the
153	  natural and most appropriate way to implement CAN for Linux.
154	
155	3. SocketCAN concept
156	---------------------
157	
158	  As described in chapter 2 it is the main goal of SocketCAN to
159	  provide a socket interface to user space applications which builds
160	  upon the Linux network layer. In contrast to the commonly known
161	  TCP/IP and ethernet networking, the CAN bus is a broadcast-only(!)
162	  medium that has no MAC-layer addressing like ethernet. The CAN-identifier
163	  (can_id) is used for arbitration on the CAN-bus. Therefore the CAN-IDs
164	  have to be chosen uniquely on the bus. When designing a CAN-ECU
165	  network the CAN-IDs are mapped to be sent by a specific ECU.
166	  For this reason a CAN-ID can be treated best as a kind of source address.
167	
168	  3.1 receive lists
169	
170	  The network transparent access of multiple applications leads to the
171	  problem that different applications may be interested in the same
172	  CAN-IDs from the same CAN network interface. The SocketCAN core
173	  module - which implements the protocol family CAN - provides several
174	  high efficient receive lists for this reason. If e.g. a user space
175	  application opens a CAN RAW socket, the raw protocol module itself
176	  requests the (range of) CAN-IDs from the SocketCAN core that are
177	  requested by the user. The subscription and unsubscription of
178	  CAN-IDs can be done for specific CAN interfaces or for all(!) known
179	  CAN interfaces with the can_rx_(un)register() functions provided to
180	  CAN protocol modules by the SocketCAN core (see chapter 5).
181	  To optimize the CPU usage at runtime the receive lists are split up
182	  into several specific lists per device that match the requested
183	  filter complexity for a given use-case.
184	
185	  3.2 local loopback of sent frames
186	
187	  As known from other networking concepts the data exchanging
188	  applications may run on the same or different nodes without any
189	  change (except for the according addressing information):
190	
191	         ___   ___   ___                   _______   ___
192	        | _ | | _ | | _ |                 | _   _ | | _ |
193	        ||A|| ||B|| ||C||                 ||A| |B|| ||C||
194	        |___| |___| |___|                 |_______| |___|
195	          |     |     |                       |       |
196	        -----------------(1)- CAN bus -(2)---------------
197	
198	  To ensure that application A receives the same information in the
199	  example (2) as it would receive in example (1) there is need for
200	  some kind of local loopback of the sent CAN frames on the appropriate
201	  node.
202	
203	  The Linux network devices (by default) just can handle the
204	  transmission and reception of media dependent frames. Due to the
205	  arbitration on the CAN bus the transmission of a low prio CAN-ID
206	  may be delayed by the reception of a high prio CAN frame. To
207	  reflect the correct* traffic on the node the loopback of the sent
208	  data has to be performed right after a successful transmission. If
209	  the CAN network interface is not capable of performing the loopback for
210	  some reason the SocketCAN core can do this task as a fallback solution.
211	  See chapter 6.2 for details (recommended).
212	
213	  The loopback functionality is enabled by default to reflect standard
214	  networking behaviour for CAN applications. Due to some requests from
215	  the RT-SocketCAN group the loopback optionally may be disabled for each
216	  separate socket. See sockopts from the CAN RAW sockets in chapter 4.1.
217	
218	  * = you really like to have this when you're running analyser tools
219	      like 'candump' or 'cansniffer' on the (same) node.
220	
221	  3.3 network problem notifications
222	
223	  The use of the CAN bus may lead to several problems on the physical
224	  and media access control layer. Detecting and logging of these lower
225	  layer problems is a vital requirement for CAN users to identify
226	  hardware issues on the physical transceiver layer as well as
227	  arbitration problems and error frames caused by the different
228	  ECUs. The occurrence of detected errors are important for diagnosis
229	  and have to be logged together with the exact timestamp. For this
230	  reason the CAN interface driver can generate so called Error Message
231	  Frames that can optionally be passed to the user application in the
232	  same way as other CAN frames. Whenever an error on the physical layer
233	  or the MAC layer is detected (e.g. by the CAN controller) the driver
234	  creates an appropriate error message frame. Error messages frames can
235	  be requested by the user application using the common CAN filter
236	  mechanisms. Inside this filter definition the (interested) type of
237	  errors may be selected. The reception of error messages is disabled
238	  by default. The format of the CAN error message frame is briefly
239	  described in the Linux header file "include/uapi/linux/can/error.h".
240	
241	4. How to use SocketCAN
242	------------------------
243	
244	  Like TCP/IP, you first need to open a socket for communicating over a
245	  CAN network. Since SocketCAN implements a new protocol family, you
246	  need to pass PF_CAN as the first argument to the socket(2) system
247	  call. Currently, there are two CAN protocols to choose from, the raw
248	  socket protocol and the broadcast manager (BCM). So to open a socket,
249	  you would write
250	
251	    s = socket(PF_CAN, SOCK_RAW, CAN_RAW);
252	
253	  and
254	
255	    s = socket(PF_CAN, SOCK_DGRAM, CAN_BCM);
256	
257	  respectively.  After the successful creation of the socket, you would
258	  normally use the bind(2) system call to bind the socket to a CAN
259	  interface (which is different from TCP/IP due to different addressing
260	  - see chapter 3). After binding (CAN_RAW) or connecting (CAN_BCM)
261	  the socket, you can read(2) and write(2) from/to the socket or use
262	  send(2), sendto(2), sendmsg(2) and the recv* counterpart operations
263	  on the socket as usual. There are also CAN specific socket options
264	  described below.
265	
266	  The basic CAN frame structure and the sockaddr structure are defined
267	  in include/linux/can.h:
268	
269	    struct can_frame {
270	            canid_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */
271	            __u8    can_dlc; /* frame payload length in byte (0 .. 8) */
272	            __u8    __pad;   /* padding */
273	            __u8    __res0;  /* reserved / padding */
274	            __u8    __res1;  /* reserved / padding */
275	            __u8    data[8] __attribute__((aligned(8)));
276	    };
277	
278	  The alignment of the (linear) payload data[] to a 64bit boundary
279	  allows the user to define their own structs and unions to easily access
280	  the CAN payload. There is no given byteorder on the CAN bus by
281	  default. A read(2) system call on a CAN_RAW socket transfers a
282	  struct can_frame to the user space.
283	
284	  The sockaddr_can structure has an interface index like the
285	  PF_PACKET socket, that also binds to a specific interface:
286	
287	    struct sockaddr_can {
288	            sa_family_t can_family;
289	            int         can_ifindex;
290	            union {
291	                    /* transport protocol class address info (e.g. ISOTP) */
292	                    struct { canid_t rx_id, tx_id; } tp;
293	
294	                    /* reserved for future CAN protocols address information */
295	            } can_addr;
296	    };
297	
298	  To determine the interface index an appropriate ioctl() has to
299	  be used (example for CAN_RAW sockets without error checking):
300	
301	    int s;
302	    struct sockaddr_can addr;
303	    struct ifreq ifr;
304	
305	    s = socket(PF_CAN, SOCK_RAW, CAN_RAW);
306	
307	    strcpy(ifr.ifr_name, "can0" );
308	    ioctl(s, SIOCGIFINDEX, &ifr);
309	
310	    addr.can_family = AF_CAN;
311	    addr.can_ifindex = ifr.ifr_ifindex;
312	
313	    bind(s, (struct sockaddr *)&addr, sizeof(addr));
314	
315	    (..)
316	
317	  To bind a socket to all(!) CAN interfaces the interface index must
318	  be 0 (zero). In this case the socket receives CAN frames from every
319	  enabled CAN interface. To determine the originating CAN interface
320	  the system call recvfrom(2) may be used instead of read(2). To send
321	  on a socket that is bound to 'any' interface sendto(2) is needed to
322	  specify the outgoing interface.
323	
324	  Reading CAN frames from a bound CAN_RAW socket (see above) consists
325	  of reading a struct can_frame:
326	
327	    struct can_frame frame;
328	
329	    nbytes = read(s, &frame, sizeof(struct can_frame));
330	
331	    if (nbytes < 0) {
332	            perror("can raw socket read");
333	            return 1;
334	    }
335	
336	    /* paranoid check ... */
337	    if (nbytes < sizeof(struct can_frame)) {
338	            fprintf(stderr, "read: incomplete CAN frame\n");
339	            return 1;
340	    }
341	
342	    /* do something with the received CAN frame */
343	
344	  Writing CAN frames can be done similarly, with the write(2) system call:
345	
346	    nbytes = write(s, &frame, sizeof(struct can_frame));
347	
348	  When the CAN interface is bound to 'any' existing CAN interface
349	  (addr.can_ifindex = 0) it is recommended to use recvfrom(2) if the
350	  information about the originating CAN interface is needed:
351	
352	    struct sockaddr_can addr;
353	    struct ifreq ifr;
354	    socklen_t len = sizeof(addr);
355	    struct can_frame frame;
356	
357	    nbytes = recvfrom(s, &frame, sizeof(struct can_frame),
358	                      0, (struct sockaddr*)&addr, &len);
359	
360	    /* get interface name of the received CAN frame */
361	    ifr.ifr_ifindex = addr.can_ifindex;
362	    ioctl(s, SIOCGIFNAME, &ifr);
363	    printf("Received a CAN frame from interface %s", ifr.ifr_name);
364	
365	  To write CAN frames on sockets bound to 'any' CAN interface the
366	  outgoing interface has to be defined certainly.
367	
368	    strcpy(ifr.ifr_name, "can0");
369	    ioctl(s, SIOCGIFINDEX, &ifr);
370	    addr.can_ifindex = ifr.ifr_ifindex;
371	    addr.can_family  = AF_CAN;
372	
373	    nbytes = sendto(s, &frame, sizeof(struct can_frame),
374	                    0, (struct sockaddr*)&addr, sizeof(addr));
375	
376	  An accurate timestamp can be obtained with an ioctl(2) call after reading
377	  a message from the socket:
378	
379	    struct timeval tv;
380	    ioctl(s, SIOCGSTAMP, &tv);
381	
382	  The timestamp has a resolution of one microsecond and is set automatically
383	  at the reception of a CAN frame.
384	
385	  Remark about CAN FD (flexible data rate) support:
386	
387	  Generally the handling of CAN FD is very similar to the formerly described
388	  examples. The new CAN FD capable CAN controllers support two different
389	  bitrates for the arbitration phase and the payload phase of the CAN FD frame
390	  and up to 64 bytes of payload. This extended payload length breaks all the
391	  kernel interfaces (ABI) which heavily rely on the CAN frame with fixed eight
392	  bytes of payload (struct can_frame) like the CAN_RAW socket. Therefore e.g.
393	  the CAN_RAW socket supports a new socket option CAN_RAW_FD_FRAMES that
394	  switches the socket into a mode that allows the handling of CAN FD frames
395	  and (legacy) CAN frames simultaneously (see section 4.1.5).
396	
397	  The struct canfd_frame is defined in include/linux/can.h:
398	
399	    struct canfd_frame {
400	            canid_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */
401	            __u8    len;     /* frame payload length in byte (0 .. 64) */
402	            __u8    flags;   /* additional flags for CAN FD */
403	            __u8    __res0;  /* reserved / padding */
404	            __u8    __res1;  /* reserved / padding */
405	            __u8    data[64] __attribute__((aligned(8)));
406	    };
407	
408	  The struct canfd_frame and the existing struct can_frame have the can_id,
409	  the payload length and the payload data at the same offset inside their
410	  structures. This allows to handle the different structures very similar.
411	  When the content of a struct can_frame is copied into a struct canfd_frame
412	  all structure elements can be used as-is - only the data[] becomes extended.
413	
414	  When introducing the struct canfd_frame it turned out that the data length
415	  code (DLC) of the struct can_frame was used as a length information as the
416	  length and the DLC has a 1:1 mapping in the range of 0 .. 8. To preserve
417	  the easy handling of the length information the canfd_frame.len element
418	  contains a plain length value from 0 .. 64. So both canfd_frame.len and
419	  can_frame.can_dlc are equal and contain a length information and no DLC.
420	  For details about the distinction of CAN and CAN FD capable devices and
421	  the mapping to the bus-relevant data length code (DLC), see chapter 6.6.
422	
423	  The length of the two CAN(FD) frame structures define the maximum transfer
424	  unit (MTU) of the CAN(FD) network interface and skbuff data length. Two
425	  definitions are specified for CAN specific MTUs in include/linux/can.h :
426	
427	  #define CAN_MTU   (sizeof(struct can_frame))   == 16  => 'legacy' CAN frame
428	  #define CANFD_MTU (sizeof(struct canfd_frame)) == 72  => CAN FD frame
429	
430	  4.1 RAW protocol sockets with can_filters (SOCK_RAW)
431	
432	  Using CAN_RAW sockets is extensively comparable to the commonly
433	  known access to CAN character devices. To meet the new possibilities
434	  provided by the multi user SocketCAN approach, some reasonable
435	  defaults are set at RAW socket binding time:
436	
437	  - The filters are set to exactly one filter receiving everything
438	  - The socket only receives valid data frames (=> no error message frames)
439	  - The loopback of sent CAN frames is enabled (see chapter 3.2)
440	  - The socket does not receive its own sent frames (in loopback mode)
441	
442	  These default settings may be changed before or after binding the socket.
443	  To use the referenced definitions of the socket options for CAN_RAW
444	  sockets, include <linux/can/raw.h>.
445	
446	  4.1.1 RAW socket option CAN_RAW_FILTER
447	
448	  The reception of CAN frames using CAN_RAW sockets can be controlled
449	  by defining 0 .. n filters with the CAN_RAW_FILTER socket option.
450	
451	  The CAN filter structure is defined in include/linux/can.h:
452	
453	    struct can_filter {
454	            canid_t can_id;
455	            canid_t can_mask;
456	    };
457	
458	  A filter matches, when
459	
460	    <received_can_id> & mask == can_id & mask
461	
462	  which is analogous to known CAN controllers hardware filter semantics.
463	  The filter can be inverted in this semantic, when the CAN_INV_FILTER
464	  bit is set in can_id element of the can_filter structure. In
465	  contrast to CAN controller hardware filters the user may set 0 .. n
466	  receive filters for each open socket separately:
467	
468	    struct can_filter rfilter[2];
469	
470	    rfilter[0].can_id   = 0x123;
471	    rfilter[0].can_mask = CAN_SFF_MASK;
472	    rfilter[1].can_id   = 0x200;
473	    rfilter[1].can_mask = 0x700;
474	
475	    setsockopt(s, SOL_CAN_RAW, CAN_RAW_FILTER, &rfilter, sizeof(rfilter));
476	
477	  To disable the reception of CAN frames on the selected CAN_RAW socket:
478	
479	    setsockopt(s, SOL_CAN_RAW, CAN_RAW_FILTER, NULL, 0);
480	
481	  To set the filters to zero filters is quite obsolete as to not read
482	  data causes the raw socket to discard the received CAN frames. But
483	  having this 'send only' use-case we may remove the receive list in the
484	  Kernel to save a little (really a very little!) CPU usage.
485	
486	  4.1.1.1 CAN filter usage optimisation
487	
488	  The CAN filters are processed in per-device filter lists at CAN frame
489	  reception time. To reduce the number of checks that need to be performed
490	  while walking through the filter lists the CAN core provides an optimized
491	  filter handling when the filter subscription focusses on a single CAN ID.
492	
493	  For the possible 2048 SFF CAN identifiers the identifier is used as an index
494	  to access the corresponding subscription list without any further checks.
495	  For the 2^29 possible EFF CAN identifiers a 10 bit XOR folding is used as
496	  hash function to retrieve the EFF table index.
497	
498	  To benefit from the optimized filters for single CAN identifiers the
499	  CAN_SFF_MASK or CAN_EFF_MASK have to be set into can_filter.mask together
500	  with set CAN_EFF_FLAG and CAN_RTR_FLAG bits. A set CAN_EFF_FLAG bit in the
501	  can_filter.mask makes clear that it matters whether a SFF or EFF CAN ID is
502	  subscribed. E.g. in the example from above
503	
504	    rfilter[0].can_id   = 0x123;
505	    rfilter[0].can_mask = CAN_SFF_MASK;
506	
507	  both SFF frames with CAN ID 0x123 and EFF frames with 0xXXXXX123 can pass.
508	
509	  To filter for only 0x123 (SFF) and 0x12345678 (EFF) CAN identifiers the
510	  filter has to be defined in this way to benefit from the optimized filters:
511	
512	    struct can_filter rfilter[2];
513	
514	    rfilter[0].can_id   = 0x123;
515	    rfilter[0].can_mask = (CAN_EFF_FLAG | CAN_RTR_FLAG | CAN_SFF_MASK);
516	    rfilter[1].can_id   = 0x12345678 | CAN_EFF_FLAG;
517	    rfilter[1].can_mask = (CAN_EFF_FLAG | CAN_RTR_FLAG | CAN_EFF_MASK);
518	
519	    setsockopt(s, SOL_CAN_RAW, CAN_RAW_FILTER, &rfilter, sizeof(rfilter));
520	
521	  4.1.2 RAW socket option CAN_RAW_ERR_FILTER
522	
523	  As described in chapter 3.3 the CAN interface driver can generate so
524	  called Error Message Frames that can optionally be passed to the user
525	  application in the same way as other CAN frames. The possible
526	  errors are divided into different error classes that may be filtered
527	  using the appropriate error mask. To register for every possible
528	  error condition CAN_ERR_MASK can be used as value for the error mask.
529	  The values for the error mask are defined in linux/can/error.h .
530	
531	    can_err_mask_t err_mask = ( CAN_ERR_TX_TIMEOUT | CAN_ERR_BUSOFF );
532	
533	    setsockopt(s, SOL_CAN_RAW, CAN_RAW_ERR_FILTER,
534	               &err_mask, sizeof(err_mask));
535	
536	  4.1.3 RAW socket option CAN_RAW_LOOPBACK
537	
538	  To meet multi user needs the local loopback is enabled by default
539	  (see chapter 3.2 for details). But in some embedded use-cases
540	  (e.g. when only one application uses the CAN bus) this loopback
541	  functionality can be disabled (separately for each socket):
542	
543	    int loopback = 0; /* 0 = disabled, 1 = enabled (default) */
544	
545	    setsockopt(s, SOL_CAN_RAW, CAN_RAW_LOOPBACK, &loopback, sizeof(loopback));
546	
547	  4.1.4 RAW socket option CAN_RAW_RECV_OWN_MSGS
548	
549	  When the local loopback is enabled, all the sent CAN frames are
550	  looped back to the open CAN sockets that registered for the CAN
551	  frames' CAN-ID on this given interface to meet the multi user
552	  needs. The reception of the CAN frames on the same socket that was
553	  sending the CAN frame is assumed to be unwanted and therefore
554	  disabled by default. This default behaviour may be changed on
555	  demand:
556	
557	    int recv_own_msgs = 1; /* 0 = disabled (default), 1 = enabled */
558	
559	    setsockopt(s, SOL_CAN_RAW, CAN_RAW_RECV_OWN_MSGS,
560	               &recv_own_msgs, sizeof(recv_own_msgs));
561	
562	  4.1.5 RAW socket option CAN_RAW_FD_FRAMES
563	
564	  CAN FD support in CAN_RAW sockets can be enabled with a new socket option
565	  CAN_RAW_FD_FRAMES which is off by default. When the new socket option is
566	  not supported by the CAN_RAW socket (e.g. on older kernels), switching the
567	  CAN_RAW_FD_FRAMES option returns the error -ENOPROTOOPT.
568	
569	  Once CAN_RAW_FD_FRAMES is enabled the application can send both CAN frames
570	  and CAN FD frames. OTOH the application has to handle CAN and CAN FD frames
571	  when reading from the socket.
572	
573	    CAN_RAW_FD_FRAMES enabled:  CAN_MTU and CANFD_MTU are allowed
574	    CAN_RAW_FD_FRAMES disabled: only CAN_MTU is allowed (default)
575	
576	  Example:
577	    [ remember: CANFD_MTU == sizeof(struct canfd_frame) ]
578	
579	    struct canfd_frame cfd;
580	
581	    nbytes = read(s, &cfd, CANFD_MTU);
582	
583	    if (nbytes == CANFD_MTU) {
584	            printf("got CAN FD frame with length %d\n", cfd.len);
585		    /* cfd.flags contains valid data */
586	    } else if (nbytes == CAN_MTU) {
587	            printf("got legacy CAN frame with length %d\n", cfd.len);
588		    /* cfd.flags is undefined */
589	    } else {
590	            fprintf(stderr, "read: invalid CAN(FD) frame\n");
591	            return 1;
592	    }
593	
594	    /* the content can be handled independently from the received MTU size */
595	
596	    printf("can_id: %X data length: %d data: ", cfd.can_id, cfd.len);
597	    for (i = 0; i < cfd.len; i++)
598	            printf("%02X ", cfd.data[i]);
599	
600	  When reading with size CANFD_MTU only returns CAN_MTU bytes that have
601	  been received from the socket a legacy CAN frame has been read into the
602	  provided CAN FD structure. Note that the canfd_frame.flags data field is
603	  not specified in the struct can_frame and therefore it is only valid in
604	  CANFD_MTU sized CAN FD frames.
605	
606	  Implementation hint for new CAN applications:
607	
608	  To build a CAN FD aware application use struct canfd_frame as basic CAN
609	  data structure for CAN_RAW based applications. When the application is
610	  executed on an older Linux kernel and switching the CAN_RAW_FD_FRAMES
611	  socket option returns an error: No problem. You'll get legacy CAN frames
612	  or CAN FD frames and can process them the same way.
613	
614	  When sending to CAN devices make sure that the device is capable to handle
615	  CAN FD frames by checking if the device maximum transfer unit is CANFD_MTU.
616	  The CAN device MTU can be retrieved e.g. with a SIOCGIFMTU ioctl() syscall.
617	
618	  4.1.6 RAW socket option CAN_RAW_JOIN_FILTERS
619	
620	  The CAN_RAW socket can set multiple CAN identifier specific filters that
621	  lead to multiple filters in the af_can.c filter processing. These filters
622	  are indenpendent from each other which leads to logical OR'ed filters when
623	  applied (see 4.1.1).
624	
625	  This socket option joines the given CAN filters in the way that only CAN
626	  frames are passed to user space that matched *all* given CAN filters. The
627	  semantic for the applied filters is therefore changed to a logical AND.
628	
629	  This is useful especially when the filterset is a combination of filters
630	  where the CAN_INV_FILTER flag is set in order to notch single CAN IDs or
631	  CAN ID ranges from the incoming traffic.
632	
633	  4.1.7 RAW socket returned message flags
634	
635	  When using recvmsg() call, the msg->msg_flags may contain following flags:
636	
637	    MSG_DONTROUTE: set when the received frame was created on the local host.
638	
639	    MSG_CONFIRM: set when the frame was sent via the socket it is received on.
640	      This flag can be interpreted as a 'transmission confirmation' when the
641	      CAN driver supports the echo of frames on driver level, see 3.2 and 6.2.
642	      In order to receive such messages, CAN_RAW_RECV_OWN_MSGS must be set.
643	
644	  4.2 Broadcast Manager protocol sockets (SOCK_DGRAM)
645	
646	  The Broadcast Manager protocol provides a command based configuration
647	  interface to filter and send (e.g. cyclic) CAN messages in kernel space.
648	
649	  Receive filters can be used to down sample frequent messages; detect events
650	  such as message contents changes, packet length changes, and do time-out
651	  monitoring of received messages.
652	
653	  Periodic transmission tasks of CAN frames or a sequence of CAN frames can be
654	  created and modified at runtime; both the message content and the two
655	  possible transmit intervals can be altered.
656	
657	  A BCM socket is not intended for sending individual CAN frames using the
658	  struct can_frame as known from the CAN_RAW socket. Instead a special BCM
659	  configuration message is defined. The basic BCM configuration message used
660	  to communicate with the broadcast manager and the available operations are
661	  defined in the linux/can/bcm.h include. The BCM message consists of a
662	  message header with a command ('opcode') followed by zero or more CAN frames.
663	  The broadcast manager sends responses to user space in the same form:
664	
665	    struct bcm_msg_head {
666	            __u32 opcode;                   /* command */
667	            __u32 flags;                    /* special flags */
668	            __u32 count;                    /* run 'count' times with ival1 */
669	            struct timeval ival1, ival2;    /* count and subsequent interval */
670	            canid_t can_id;                 /* unique can_id for task */
671	            __u32 nframes;                  /* number of can_frames following */
672	            struct can_frame frames[0];
673	    };
674	
675	  The aligned payload 'frames' uses the same basic CAN frame structure defined
676	  at the beginning of section 4 and in the include/linux/can.h include. All
677	  messages to the broadcast manager from user space have this structure.
678	
679	  Note a CAN_BCM socket must be connected instead of bound after socket
680	  creation (example without error checking):
681	
682	    int s;
683	    struct sockaddr_can addr;
684	    struct ifreq ifr;
685	
686	    s = socket(PF_CAN, SOCK_DGRAM, CAN_BCM);
687	
688	    strcpy(ifr.ifr_name, "can0");
689	    ioctl(s, SIOCGIFINDEX, &ifr);
690	
691	    addr.can_family = AF_CAN;
692	    addr.can_ifindex = ifr.ifr_ifindex;
693	
694	    connect(s, (struct sockaddr *)&addr, sizeof(addr));
695	
696	    (..)
697	
698	  The broadcast manager socket is able to handle any number of in flight
699	  transmissions or receive filters concurrently. The different RX/TX jobs are
700	  distinguished by the unique can_id in each BCM message. However additional
701	  CAN_BCM sockets are recommended to communicate on multiple CAN interfaces.
702	  When the broadcast manager socket is bound to 'any' CAN interface (=> the
703	  interface index is set to zero) the configured receive filters apply to any
704	  CAN interface unless the sendto() syscall is used to overrule the 'any' CAN
705	  interface index. When using recvfrom() instead of read() to retrieve BCM
706	  socket messages the originating CAN interface is provided in can_ifindex.
707	
708	  4.2.1 Broadcast Manager operations
709	
710	  The opcode defines the operation for the broadcast manager to carry out,
711	  or details the broadcast managers response to several events, including
712	  user requests.
713	
714	  Transmit Operations (user space to broadcast manager):
715	
716	    TX_SETUP:   Create (cyclic) transmission task.
717	
718	    TX_DELETE:  Remove (cyclic) transmission task, requires only can_id.
719	
720	    TX_READ:    Read properties of (cyclic) transmission task for can_id.
721	
722	    TX_SEND:    Send one CAN frame.
723	
724	  Transmit Responses (broadcast manager to user space):
725	
726	    TX_STATUS:  Reply to TX_READ request (transmission task configuration).
727	
728	    TX_EXPIRED: Notification when counter finishes sending at initial interval
729	      'ival1'. Requires the TX_COUNTEVT flag to be set at TX_SETUP.
730	
731	  Receive Operations (user space to broadcast manager):
732	
733	    RX_SETUP:   Create RX content filter subscription.
734	
735	    RX_DELETE:  Remove RX content filter subscription, requires only can_id.
736	
737	    RX_READ:    Read properties of RX content filter subscription for can_id.
738	
739	  Receive Responses (broadcast manager to user space):
740	
741	    RX_STATUS:  Reply to RX_READ request (filter task configuration).
742	
743	    RX_TIMEOUT: Cyclic message is detected to be absent (timer ival1 expired).
744	
745	    RX_CHANGED: BCM message with updated CAN frame (detected content change).
746	      Sent on first message received or on receipt of revised CAN messages.
747	
748	  4.2.2 Broadcast Manager message flags
749	
750	  When sending a message to the broadcast manager the 'flags' element may
751	  contain the following flag definitions which influence the behaviour:
752	
753	    SETTIMER:           Set the values of ival1, ival2 and count
754	
755	    STARTTIMER:         Start the timer with the actual values of ival1, ival2
756	      and count. Starting the timer leads simultaneously to emit a CAN frame.
757	
758	    TX_COUNTEVT:        Create the message TX_EXPIRED when count expires
759	
760	    TX_ANNOUNCE:        A change of data by the process is emitted immediately.
761	
762	    TX_CP_CAN_ID:       Copies the can_id from the message header to each
763	      subsequent frame in frames. This is intended as usage simplification. For
764	      TX tasks the unique can_id from the message header may differ from the
765	      can_id(s) stored for transmission in the subsequent struct can_frame(s).
766	
767	    RX_FILTER_ID:       Filter by can_id alone, no frames required (nframes=0).
768	
769	    RX_CHECK_DLC:       A change of the DLC leads to an RX_CHANGED.
770	
771	    RX_NO_AUTOTIMER:    Prevent automatically starting the timeout monitor.
772	
773	    RX_ANNOUNCE_RESUME: If passed at RX_SETUP and a receive timeout occurred, a
774	      RX_CHANGED message will be generated when the (cyclic) receive restarts.
775	
776	    TX_RESET_MULTI_IDX: Reset the index for the multiple frame transmission.
777	
778	    RX_RTR_FRAME:       Send reply for RTR-request (placed in op->frames[0]).
779	
780	  4.2.3 Broadcast Manager transmission timers
781	
782	  Periodic transmission configurations may use up to two interval timers.
783	  In this case the BCM sends a number of messages ('count') at an interval
784	  'ival1', then continuing to send at another given interval 'ival2'. When
785	  only one timer is needed 'count' is set to zero and only 'ival2' is used.
786	  When SET_TIMER and START_TIMER flag were set the timers are activated.
787	  The timer values can be altered at runtime when only SET_TIMER is set.
788	
789	  4.2.4 Broadcast Manager message sequence transmission
790	
791	  Up to 256 CAN frames can be transmitted in a sequence in the case of a cyclic
792	  TX task configuration. The number of CAN frames is provided in the 'nframes'
793	  element of the BCM message head. The defined number of CAN frames are added
794	  as array to the TX_SETUP BCM configuration message.
795	
796	    /* create a struct to set up a sequence of four CAN frames */
797	    struct {
798	            struct bcm_msg_head msg_head;
799	            struct can_frame frame[4];
800	    } mytxmsg;
801	
802	    (..)
803	    mytxmsg.msg_head.nframes = 4;
804	    (..)
805	
806	    write(s, &mytxmsg, sizeof(mytxmsg));
807	
808	  With every transmission the index in the array of CAN frames is increased
809	  and set to zero at index overflow.
810	
811	  4.2.5 Broadcast Manager receive filter timers
812	
813	  The timer values ival1 or ival2 may be set to non-zero values at RX_SETUP.
814	  When the SET_TIMER flag is set the timers are enabled:
815	
816	  ival1: Send RX_TIMEOUT when a received message is not received again within
817	    the given time. When START_TIMER is set at RX_SETUP the timeout detection
818	    is activated directly - even without a former CAN frame reception.
819	
820	  ival2: Throttle the received message rate down to the value of ival2. This
821	    is useful to reduce messages for the application when the signal inside the
822	    CAN frame is stateless as state changes within the ival2 periode may get
823	    lost.
824	
825	  4.2.6 Broadcast Manager multiplex message receive filter
826	
827	  To filter for content changes in multiplex message sequences an array of more
828	  than one CAN frames can be passed in a RX_SETUP configuration message. The
829	  data bytes of the first CAN frame contain the mask of relevant bits that
830	  have to match in the subsequent CAN frames with the received CAN frame.
831	  If one of the subsequent CAN frames is matching the bits in that frame data
832	  mark the relevant content to be compared with the previous received content.
833	  Up to 257 CAN frames (multiplex filter bit mask CAN frame plus 256 CAN
834	  filters) can be added as array to the TX_SETUP BCM configuration message.
835	
836	    /* usually used to clear CAN frame data[] - beware of endian problems! */
837	    #define U64_DATA(p) (*(unsigned long long*)(p)->data)
838	
839	    struct {
840	            struct bcm_msg_head msg_head;
841	            struct can_frame frame[5];
842	    } msg;
843	
844	    msg.msg_head.opcode  = RX_SETUP;
845	    msg.msg_head.can_id  = 0x42;
846	    msg.msg_head.flags   = 0;
847	    msg.msg_head.nframes = 5;
848	    U64_DATA(&msg.frame[0]) = 0xFF00000000000000ULL; /* MUX mask */
849	    U64_DATA(&msg.frame[1]) = 0x01000000000000FFULL; /* data mask (MUX 0x01) */
850	    U64_DATA(&msg.frame[2]) = 0x0200FFFF000000FFULL; /* data mask (MUX 0x02) */
851	    U64_DATA(&msg.frame[3]) = 0x330000FFFFFF0003ULL; /* data mask (MUX 0x33) */
852	    U64_DATA(&msg.frame[4]) = 0x4F07FC0FF0000000ULL; /* data mask (MUX 0x4F) */
853	
854	    write(s, &msg, sizeof(msg));
855	
856	  4.2.7 Broadcast Manager CAN FD support
857	
858	  The programming API of the CAN_BCM depends on struct can_frame which is
859	  given as array directly behind the bcm_msg_head structure. To follow this
860	  schema for the CAN FD frames a new flag 'CAN_FD_FRAME' in the bcm_msg_head
861	  flags indicates that the concatenated CAN frame structures behind the
862	  bcm_msg_head are defined as struct canfd_frame.
863	
864	    struct {
865	            struct bcm_msg_head msg_head;
866	            struct canfd_frame frame[5];
867	    } msg;
868	
869	    msg.msg_head.opcode  = RX_SETUP;
870	    msg.msg_head.can_id  = 0x42;
871	    msg.msg_head.flags   = CAN_FD_FRAME;
872	    msg.msg_head.nframes = 5;
873	    (..)
874	
875	  When using CAN FD frames for multiplex filtering the MUX mask is still
876	  expected in the first 64 bit of the struct canfd_frame data section.
877	
878	  4.3 connected transport protocols (SOCK_SEQPACKET)
879	  4.4 unconnected transport protocols (SOCK_DGRAM)
880	
881	
882	5. SocketCAN core module
883	-------------------------
884	
885	  The SocketCAN core module implements the protocol family
886	  PF_CAN. CAN protocol modules are loaded by the core module at
887	  runtime. The core module provides an interface for CAN protocol
888	  modules to subscribe needed CAN IDs (see chapter 3.1).
889	
890	  5.1 can.ko module params
891	
892	  - stats_timer: To calculate the SocketCAN core statistics
893	    (e.g. current/maximum frames per second) this 1 second timer is
894	    invoked at can.ko module start time by default. This timer can be
895	    disabled by using stattimer=0 on the module commandline.
896	
897	  - debug: (removed since SocketCAN SVN r546)
898	
899	  5.2 procfs content
900	
901	  As described in chapter 3.1 the SocketCAN core uses several filter
902	  lists to deliver received CAN frames to CAN protocol modules. These
903	  receive lists, their filters and the count of filter matches can be
904	  checked in the appropriate receive list. All entries contain the
905	  device and a protocol module identifier:
906	
907	    foo@bar:~$ cat /proc/net/can/rcvlist_all
908	
909	    receive list 'rx_all':
910	      (vcan3: no entry)
911	      (vcan2: no entry)
912	      (vcan1: no entry)
913	      device   can_id   can_mask  function  userdata   matches  ident
914	       vcan0     000    00000000  f88e6370  f6c6f400         0  raw
915	      (any: no entry)
916	
917	  In this example an application requests any CAN traffic from vcan0.
918	
919	    rcvlist_all - list for unfiltered entries (no filter operations)
920	    rcvlist_eff - list for single extended frame (EFF) entries
921	    rcvlist_err - list for error message frames masks
922	    rcvlist_fil - list for mask/value filters
923	    rcvlist_inv - list for mask/value filters (inverse semantic)
924	    rcvlist_sff - list for single standard frame (SFF) entries
925	
926	  Additional procfs files in /proc/net/can
927	
928	    stats       - SocketCAN core statistics (rx/tx frames, match ratios, ...)
929	    reset_stats - manual statistic reset
930	    version     - prints the SocketCAN core version and the ABI version
931	
932	  5.3 writing own CAN protocol modules
933	
934	  To implement a new protocol in the protocol family PF_CAN a new
935	  protocol has to be defined in include/linux/can.h .
936	  The prototypes and definitions to use the SocketCAN core can be
937	  accessed by including include/linux/can/core.h .
938	  In addition to functions that register the CAN protocol and the
939	  CAN device notifier chain there are functions to subscribe CAN
940	  frames received by CAN interfaces and to send CAN frames:
941	
942	    can_rx_register   - subscribe CAN frames from a specific interface
943	    can_rx_unregister - unsubscribe CAN frames from a specific interface
944	    can_send          - transmit a CAN frame (optional with local loopback)
945	
946	  For details see the kerneldoc documentation in net/can/af_can.c or
947	  the source code of net/can/raw.c or net/can/bcm.c .
948	
949	6. CAN network drivers
950	----------------------
951	
952	  Writing a CAN network device driver is much easier than writing a
953	  CAN character device driver. Similar to other known network device
954	  drivers you mainly have to deal with:
955	
956	  - TX: Put the CAN frame from the socket buffer to the CAN controller.
957	  - RX: Put the CAN frame from the CAN controller to the socket buffer.
958	
959	  See e.g. at Documentation/networking/netdevices.txt . The differences
960	  for writing CAN network device driver are described below:
961	
962	  6.1 general settings
963	
964	    dev->type  = ARPHRD_CAN; /* the netdevice hardware type */
965	    dev->flags = IFF_NOARP;  /* CAN has no arp */
966	
967	    dev->mtu = CAN_MTU; /* sizeof(struct can_frame) -> legacy CAN interface */
968	
969	    or alternative, when the controller supports CAN with flexible data rate:
970	    dev->mtu = CANFD_MTU; /* sizeof(struct canfd_frame) -> CAN FD interface */
971	
972	  The struct can_frame or struct canfd_frame is the payload of each socket
973	  buffer (skbuff) in the protocol family PF_CAN.
974	
975	  6.2 local loopback of sent frames
976	
977	  As described in chapter 3.2 the CAN network device driver should
978	  support a local loopback functionality similar to the local echo
979	  e.g. of tty devices. In this case the driver flag IFF_ECHO has to be
980	  set to prevent the PF_CAN core from locally echoing sent frames
981	  (aka loopback) as fallback solution:
982	
983	    dev->flags = (IFF_NOARP | IFF_ECHO);
984	
985	  6.3 CAN controller hardware filters
986	
987	  To reduce the interrupt load on deep embedded systems some CAN
988	  controllers support the filtering of CAN IDs or ranges of CAN IDs.
989	  These hardware filter capabilities vary from controller to
990	  controller and have to be identified as not feasible in a multi-user
991	  networking approach. The use of the very controller specific
992	  hardware filters could make sense in a very dedicated use-case, as a
993	  filter on driver level would affect all users in the multi-user
994	  system. The high efficient filter sets inside the PF_CAN core allow
995	  to set different multiple filters for each socket separately.
996	  Therefore the use of hardware filters goes to the category 'handmade
997	  tuning on deep embedded systems'. The author is running a MPC603e
998	  @133MHz with four SJA1000 CAN controllers from 2002 under heavy bus
999	  load without any problems ...
1000	
1001	  6.4 The virtual CAN driver (vcan)
1002	
1003	  Similar to the network loopback devices, vcan offers a virtual local
1004	  CAN interface. A full qualified address on CAN consists of
1005	
1006	  - a unique CAN Identifier (CAN ID)
1007	  - the CAN bus this CAN ID is transmitted on (e.g. can0)
1008	
1009	  so in common use cases more than one virtual CAN interface is needed.
1010	
1011	  The virtual CAN interfaces allow the transmission and reception of CAN
1012	  frames without real CAN controller hardware. Virtual CAN network
1013	  devices are usually named 'vcanX', like vcan0 vcan1 vcan2 ...
1014	  When compiled as a module the virtual CAN driver module is called vcan.ko
1015	
1016	  Since Linux Kernel version 2.6.24 the vcan driver supports the Kernel
1017	  netlink interface to create vcan network devices. The creation and
1018	  removal of vcan network devices can be managed with the ip(8) tool:
1019	
1020	  - Create a virtual CAN network interface:
1021	       $ ip link add type vcan
1022	
1023	  - Create a virtual CAN network interface with a specific name 'vcan42':
1024	       $ ip link add dev vcan42 type vcan
1025	
1026	  - Remove a (virtual CAN) network interface 'vcan42':
1027	       $ ip link del vcan42
1028	
1029	  6.5 The CAN network device driver interface
1030	
1031	  The CAN network device driver interface provides a generic interface
1032	  to setup, configure and monitor CAN network devices. The user can then
1033	  configure the CAN device, like setting the bit-timing parameters, via
1034	  the netlink interface using the program "ip" from the "IPROUTE2"
1035	  utility suite. The following chapter describes briefly how to use it.
1036	  Furthermore, the interface uses a common data structure and exports a
1037	  set of common functions, which all real CAN network device drivers
1038	  should use. Please have a look to the SJA1000 or MSCAN driver to
1039	  understand how to use them. The name of the module is can-dev.ko.
1040	
1041	  6.5.1 Netlink interface to set/get devices properties
1042	
1043	  The CAN device must be configured via netlink interface. The supported
1044	  netlink message types are defined and briefly described in
1045	  "include/linux/can/netlink.h". CAN link support for the program "ip"
1046	  of the IPROUTE2 utility suite is available and it can be used as shown
1047	  below:
1048	
1049	  - Setting CAN device properties:
1050	
1051	    $ ip link set can0 type can help
1052	    Usage: ip link set DEVICE type can
1053	        [ bitrate BITRATE [ sample-point SAMPLE-POINT] ] |
1054	        [ tq TQ prop-seg PROP_SEG phase-seg1 PHASE-SEG1
1055	          phase-seg2 PHASE-SEG2 [ sjw SJW ] ]
1056	
1057	        [ dbitrate BITRATE [ dsample-point SAMPLE-POINT] ] |
1058	        [ dtq TQ dprop-seg PROP_SEG dphase-seg1 PHASE-SEG1
1059	          dphase-seg2 PHASE-SEG2 [ dsjw SJW ] ]
1060	
1061	        [ loopback { on | off } ]
1062	        [ listen-only { on | off } ]
1063	        [ triple-sampling { on | off } ]
1064	        [ one-shot { on | off } ]
1065	        [ berr-reporting { on | off } ]
1066	        [ fd { on | off } ]
1067	        [ fd-non-iso { on | off } ]
1068	        [ presume-ack { on | off } ]
1069	
1070	        [ restart-ms TIME-MS ]
1071	        [ restart ]
1072	
1073	        Where: BITRATE       := { 1..1000000 }
1074	               SAMPLE-POINT  := { 0.000..0.999 }
1075	               TQ            := { NUMBER }
1076	               PROP-SEG      := { 1..8 }
1077	               PHASE-SEG1    := { 1..8 }
1078	               PHASE-SEG2    := { 1..8 }
1079	               SJW           := { 1..4 }
1080	               RESTART-MS    := { 0 | NUMBER }
1081	
1082	  - Display CAN device details and statistics:
1083	
1084	    $ ip -details -statistics link show can0
1085	    2: can0: <NOARP,UP,LOWER_UP,ECHO> mtu 16 qdisc pfifo_fast state UP qlen 10
1086	      link/can
1087	      can <TRIPLE-SAMPLING> state ERROR-ACTIVE restart-ms 100
1088	      bitrate 125000 sample_point 0.875
1089	      tq 125 prop-seg 6 phase-seg1 7 phase-seg2 2 sjw 1
1090	      sja1000: tseg1 1..16 tseg2 1..8 sjw 1..4 brp 1..64 brp-inc 1
1091	      clock 8000000
1092	      re-started bus-errors arbit-lost error-warn error-pass bus-off
1093	      41         17457      0          41         42         41
1094	      RX: bytes  packets  errors  dropped overrun mcast
1095	      140859     17608    17457   0       0       0
1096	      TX: bytes  packets  errors  dropped carrier collsns
1097	      861        112      0       41      0       0
1098	
1099	  More info to the above output:
1100	
1101	    "<TRIPLE-SAMPLING>"
1102		Shows the list of selected CAN controller modes: LOOPBACK,
1103		LISTEN-ONLY, or TRIPLE-SAMPLING.
1104	
1105	    "state ERROR-ACTIVE"
1106		The current state of the CAN controller: "ERROR-ACTIVE",
1107		"ERROR-WARNING", "ERROR-PASSIVE", "BUS-OFF" or "STOPPED"
1108	
1109	    "restart-ms 100"
1110		Automatic restart delay time. If set to a non-zero value, a
1111		restart of the CAN controller will be triggered automatically
1112		in case of a bus-off condition after the specified delay time
1113		in milliseconds. By default it's off.
1114	
1115	    "bitrate 125000 sample-point 0.875"
1116		Shows the real bit-rate in bits/sec and the sample-point in the
1117		range 0.000..0.999. If the calculation of bit-timing parameters
1118		is enabled in the kernel (CONFIG_CAN_CALC_BITTIMING=y), the
1119		bit-timing can be defined by setting the "bitrate" argument.
1120		Optionally the "sample-point" can be specified. By default it's
1121		0.000 assuming CIA-recommended sample-points.
1122	
1123	    "tq 125 prop-seg 6 phase-seg1 7 phase-seg2 2 sjw 1"
1124		Shows the time quanta in ns, propagation segment, phase buffer
1125		segment 1 and 2 and the synchronisation jump width in units of
1126		tq. They allow to define the CAN bit-timing in a hardware
1127		independent format as proposed by the Bosch CAN 2.0 spec (see
1128		chapter 8 of http://www.semiconductors.bosch.de/pdf/can2spec.pdf).
1129	
1130	    "sja1000: tseg1 1..16 tseg2 1..8 sjw 1..4 brp 1..64 brp-inc 1
1131	     clock 8000000"
1132		Shows the bit-timing constants of the CAN controller, here the
1133		"sja1000". The minimum and maximum values of the time segment 1
1134		and 2, the synchronisation jump width in units of tq, the
1135		bitrate pre-scaler and the CAN system clock frequency in Hz.
1136		These constants could be used for user-defined (non-standard)
1137		bit-timing calculation algorithms in user-space.
1138	
1139	    "re-started bus-errors arbit-lost error-warn error-pass bus-off"
1140		Shows the number of restarts, bus and arbitration lost errors,
1141		and the state changes to the error-warning, error-passive and
1142		bus-off state. RX overrun errors are listed in the "overrun"
1143		field of the standard network statistics.
1144	
1145	  6.5.2 Setting the CAN bit-timing
1146	
1147	  The CAN bit-timing parameters can always be defined in a hardware
1148	  independent format as proposed in the Bosch CAN 2.0 specification
1149	  specifying the arguments "tq", "prop_seg", "phase_seg1", "phase_seg2"
1150	  and "sjw":
1151	
1152	    $ ip link set canX type can tq 125 prop-seg 6 \
1153					phase-seg1 7 phase-seg2 2 sjw 1
1154	
1155	  If the kernel option CONFIG_CAN_CALC_BITTIMING is enabled, CIA
1156	  recommended CAN bit-timing parameters will be calculated if the bit-
1157	  rate is specified with the argument "bitrate":
1158	
1159	    $ ip link set canX type can bitrate 125000
1160	
1161	  Note that this works fine for the most common CAN controllers with
1162	  standard bit-rates but may *fail* for exotic bit-rates or CAN system
1163	  clock frequencies. Disabling CONFIG_CAN_CALC_BITTIMING saves some
1164	  space and allows user-space tools to solely determine and set the
1165	  bit-timing parameters. The CAN controller specific bit-timing
1166	  constants can be used for that purpose. They are listed by the
1167	  following command:
1168	
1169	    $ ip -details link show can0
1170	    ...
1171	      sja1000: clock 8000000 tseg1 1..16 tseg2 1..8 sjw 1..4 brp 1..64 brp-inc 1
1172	
1173	  6.5.3 Starting and stopping the CAN network device
1174	
1175	  A CAN network device is started or stopped as usual with the command
1176	  "ifconfig canX up/down" or "ip link set canX up/down". Be aware that
1177	  you *must* define proper bit-timing parameters for real CAN devices
1178	  before you can start it to avoid error-prone default settings:
1179	
1180	    $ ip link set canX up type can bitrate 125000
1181	
1182	  A device may enter the "bus-off" state if too many errors occurred on
1183	  the CAN bus. Then no more messages are received or sent. An automatic
1184	  bus-off recovery can be enabled by setting the "restart-ms" to a
1185	  non-zero value, e.g.:
1186	
1187	    $ ip link set canX type can restart-ms 100
1188	
1189	  Alternatively, the application may realize the "bus-off" condition
1190	  by monitoring CAN error message frames and do a restart when
1191	  appropriate with the command:
1192	
1193	    $ ip link set canX type can restart
1194	
1195	  Note that a restart will also create a CAN error message frame (see
1196	  also chapter 3.3).
1197	
1198	  6.6 CAN FD (flexible data rate) driver support
1199	
1200	  CAN FD capable CAN controllers support two different bitrates for the
1201	  arbitration phase and the payload phase of the CAN FD frame. Therefore a
1202	  second bit timing has to be specified in order to enable the CAN FD bitrate.
1203	
1204	  Additionally CAN FD capable CAN controllers support up to 64 bytes of
1205	  payload. The representation of this length in can_frame.can_dlc and
1206	  canfd_frame.len for userspace applications and inside the Linux network
1207	  layer is a plain value from 0 .. 64 instead of the CAN 'data length code'.
1208	  The data length code was a 1:1 mapping to the payload length in the legacy
1209	  CAN frames anyway. The payload length to the bus-relevant DLC mapping is
1210	  only performed inside the CAN drivers, preferably with the helper
1211	  functions can_dlc2len() and can_len2dlc().
1212	
1213	  The CAN netdevice driver capabilities can be distinguished by the network
1214	  devices maximum transfer unit (MTU):
1215	
1216	  MTU = 16 (CAN_MTU)   => sizeof(struct can_frame)   => 'legacy' CAN device
1217	  MTU = 72 (CANFD_MTU) => sizeof(struct canfd_frame) => CAN FD capable device
1218	
1219	  The CAN device MTU can be retrieved e.g. with a SIOCGIFMTU ioctl() syscall.
1220	  N.B. CAN FD capable devices can also handle and send legacy CAN frames.
1221	
1222	  When configuring CAN FD capable CAN controllers an additional 'data' bitrate
1223	  has to be set. This bitrate for the data phase of the CAN FD frame has to be
1224	  at least the bitrate which was configured for the arbitration phase. This
1225	  second bitrate is specified analogue to the first bitrate but the bitrate
1226	  setting keywords for the 'data' bitrate start with 'd' e.g. dbitrate,
1227	  dsample-point, dsjw or dtq and similar settings. When a data bitrate is set
1228	  within the configuration process the controller option "fd on" can be
1229	  specified to enable the CAN FD mode in the CAN controller. This controller
1230	  option also switches the device MTU to 72 (CANFD_MTU).
1231	
1232	  The first CAN FD specification presented as whitepaper at the International
1233	  CAN Conference 2012 needed to be improved for data integrity reasons.
1234	  Therefore two CAN FD implementations have to be distinguished today:
1235	
1236	  - ISO compliant:     The ISO 11898-1:2015 CAN FD implementation (default)
1237	  - non-ISO compliant: The CAN FD implementation following the 2012 whitepaper
1238	
1239	  Finally there are three types of CAN FD controllers:
1240	
1241	  1. ISO compliant (fixed)
1242	  2. non-ISO compliant (fixed, like the M_CAN IP core v3.0.1 in m_can.c)
1243	  3. ISO/non-ISO CAN FD controllers (switchable, like the PEAK PCAN-USB FD)
1244	
1245	  The current ISO/non-ISO mode is announced by the CAN controller driver via
1246	  netlink and displayed by the 'ip' tool (controller option FD-NON-ISO).
1247	  The ISO/non-ISO-mode can be altered by setting 'fd-non-iso {on|off}' for
1248	  switchable CAN FD controllers only.
1249	
1250	  Example configuring 500 kbit/s arbitration bitrate and 4 Mbit/s data bitrate:
1251	
1252	    $ ip link set can0 up type can bitrate 500000 sample-point 0.75 \
1253	                                   dbitrate 4000000 dsample-point 0.8 fd on
1254	    $ ip -details link show can0
1255	    5: can0: <NOARP,UP,LOWER_UP,ECHO> mtu 72 qdisc pfifo_fast state UNKNOWN \
1256	             mode DEFAULT group default qlen 10
1257	    link/can  promiscuity 0
1258	    can <FD> state ERROR-ACTIVE (berr-counter tx 0 rx 0) restart-ms 0
1259	          bitrate 500000 sample-point 0.750
1260	          tq 50 prop-seg 14 phase-seg1 15 phase-seg2 10 sjw 1
1261	          pcan_usb_pro_fd: tseg1 1..64 tseg2 1..16 sjw 1..16 brp 1..1024 \
1262	          brp-inc 1
1263	          dbitrate 4000000 dsample-point 0.800
1264	          dtq 12 dprop-seg 7 dphase-seg1 8 dphase-seg2 4 dsjw 1
1265	          pcan_usb_pro_fd: dtseg1 1..16 dtseg2 1..8 dsjw 1..4 dbrp 1..1024 \
1266	          dbrp-inc 1
1267	          clock 80000000
1268	
1269	  Example when 'fd-non-iso on' is added on this switchable CAN FD adapter:
1270	   can <FD,FD-NON-ISO> state ERROR-ACTIVE (berr-counter tx 0 rx 0) restart-ms 0
1271	
1272	  6.7 Supported CAN hardware
1273	
1274	  Please check the "Kconfig" file in "drivers/net/can" to get an actual
1275	  list of the support CAN hardware. On the SocketCAN project website
1276	  (see chapter 7) there might be further drivers available, also for
1277	  older kernel versions.
1278	
1279	7. SocketCAN resources
1280	-----------------------
1281	
1282	  The Linux CAN / SocketCAN project resources (project site / mailing list)
1283	  are referenced in the MAINTAINERS file in the Linux source tree.
1284	  Search for CAN NETWORK [LAYERS|DRIVERS].
1285	
1286	8. Credits
1287	----------
1288	
1289	  Oliver Hartkopp (PF_CAN core, filters, drivers, bcm, SJA1000 driver)
1290	  Urs Thuermann (PF_CAN core, kernel integration, socket interfaces, raw, vcan)
1291	  Jan Kizka (RT-SocketCAN core, Socket-API reconciliation)
1292	  Wolfgang Grandegger (RT-SocketCAN core & drivers, Raw Socket-API reviews,
1293	                       CAN device driver interface, MSCAN driver)
1294	  Robert Schwebel (design reviews, PTXdist integration)
1295	  Marc Kleine-Budde (design reviews, Kernel 2.6 cleanups, drivers)
1296	  Benedikt Spranger (reviews)
1297	  Thomas Gleixner (LKML reviews, coding style, posting hints)
1298	  Andrey Volkov (kernel subtree structure, ioctls, MSCAN driver)
1299	  Matthias Brukner (first SJA1000 CAN netdevice implementation Q2/2003)
1300	  Klaus Hitschler (PEAK driver integration)
1301	  Uwe Koppe (CAN netdevices with PF_PACKET approach)
1302	  Michael Schulze (driver layer loopback requirement, RT CAN drivers review)
1303	  Pavel Pisa (Bit-timing calculation)
1304	  Sascha Hauer (SJA1000 platform driver)
1305	  Sebastian Haas (SJA1000 EMS PCI driver)
1306	  Markus Plessing (SJA1000 EMS PCI driver)
1307	  Per Dalen (SJA1000 Kvaser PCI driver)
1308	  Sam Ravnborg (reviews, coding style, kbuild help)
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