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Based on kernel version 4.1. Page generated on 2015-06-28 12:13 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	    4.3 connected transport protocols (SOCK_SEQPACKET)
35	    4.4 unconnected transport protocols (SOCK_DGRAM)
36	
37	  5 SocketCAN core module
38	    5.1 can.ko module params
39	    5.2 procfs content
40	    5.3 writing own CAN protocol modules
41	
42	  6 CAN network drivers
43	    6.1 general settings
44	    6.2 local loopback of sent frames
45	    6.3 CAN controller hardware filters
46	    6.4 The virtual CAN driver (vcan)
47	    6.5 The CAN network device driver interface
48	      6.5.1 Netlink interface to set/get devices properties
49	      6.5.2 Setting the CAN bit-timing
50	      6.5.3 Starting and stopping the CAN network device
51	    6.6 CAN FD (flexible data rate) driver support
52	    6.7 supported CAN hardware
53	
54	  7 SocketCAN resources
55	
56	  8 Credits
57	
58	============================================================================
59	
60	1. Overview / What is SocketCAN
61	--------------------------------
62	
63	The socketcan package is an implementation of CAN protocols
64	(Controller Area Network) for Linux.  CAN is a networking technology
65	which has widespread use in automation, embedded devices, and
66	automotive fields.  While there have been other CAN implementations
67	for Linux based on character devices, SocketCAN uses the Berkeley
68	socket API, the Linux network stack and implements the CAN device
69	drivers as network interfaces.  The CAN socket API has been designed
70	as similar as possible to the TCP/IP protocols to allow programmers,
71	familiar with network programming, to easily learn how to use CAN
72	sockets.
73	
74	2. Motivation / Why using the socket API
75	----------------------------------------
76	
77	There have been CAN implementations for Linux before SocketCAN so the
78	question arises, why we have started another project.  Most existing
79	implementations come as a device driver for some CAN hardware, they
80	are based on character devices and provide comparatively little
81	functionality.  Usually, there is only a hardware-specific device
82	driver which provides a character device interface to send and
83	receive raw CAN frames, directly to/from the controller hardware.
84	Queueing of frames and higher-level transport protocols like ISO-TP
85	have to be implemented in user space applications.  Also, most
86	character-device implementations support only one single process to
87	open the device at a time, similar to a serial interface.  Exchanging
88	the CAN controller requires employment of another device driver and
89	often the need for adaption of large parts of the application to the
90	new driver's API.
91	
92	SocketCAN was designed to overcome all of these limitations.  A new
93	protocol family has been implemented which provides a socket interface
94	to user space applications and which builds upon the Linux network
95	layer, enabling use all of the provided queueing functionality.  A device
96	driver for CAN controller hardware registers itself with the Linux
97	network layer as a network device, so that CAN frames from the
98	controller can be passed up to the network layer and on to the CAN
99	protocol family module and also vice-versa.  Also, the protocol family
100	module provides an API for transport protocol modules to register, so
101	that any number of transport protocols can be loaded or unloaded
102	dynamically.  In fact, the can core module alone does not provide any
103	protocol and cannot be used without loading at least one additional
104	protocol module.  Multiple sockets can be opened at the same time,
105	on different or the same protocol module and they can listen/send
106	frames on different or the same CAN IDs.  Several sockets listening on
107	the same interface for frames with the same CAN ID are all passed the
108	same received matching CAN frames.  An application wishing to
109	communicate using a specific transport protocol, e.g. ISO-TP, just
110	selects that protocol when opening the socket, and then can read and
111	write application data byte streams, without having to deal with
112	CAN-IDs, frames, etc.
113	
114	Similar functionality visible from user-space could be provided by a
115	character device, too, but this would lead to a technically inelegant
116	solution for a couple of reasons:
117	
118	* Intricate usage.  Instead of passing a protocol argument to
119	  socket(2) and using bind(2) to select a CAN interface and CAN ID, an
120	  application would have to do all these operations using ioctl(2)s.
121	
122	* Code duplication.  A character device cannot make use of the Linux
123	  network queueing code, so all that code would have to be duplicated
124	  for CAN networking.
125	
126	* Abstraction.  In most existing character-device implementations, the
127	  hardware-specific device driver for a CAN controller directly
128	  provides the character device for the application to work with.
129	  This is at least very unusual in Unix systems for both, char and
130	  block devices.  For example you don't have a character device for a
131	  certain UART of a serial interface, a certain sound chip in your
132	  computer, a SCSI or IDE controller providing access to your hard
133	  disk or tape streamer device.  Instead, you have abstraction layers
134	  which provide a unified character or block device interface to the
135	  application on the one hand, and a interface for hardware-specific
136	  device drivers on the other hand.  These abstractions are provided
137	  by subsystems like the tty layer, the audio subsystem or the SCSI
138	  and IDE subsystems for the devices mentioned above.
139	
140	  The easiest way to implement a CAN device driver is as a character
141	  device without such a (complete) abstraction layer, as is done by most
142	  existing drivers.  The right way, however, would be to add such a
143	  layer with all the functionality like registering for certain CAN
144	  IDs, supporting several open file descriptors and (de)multiplexing
145	  CAN frames between them, (sophisticated) queueing of CAN frames, and
146	  providing an API for device drivers to register with.  However, then
147	  it would be no more difficult, or may be even easier, to use the
148	  networking framework provided by the Linux kernel, and this is what
149	  SocketCAN does.
150	
151	  The use of the networking framework of the Linux kernel is just the
152	  natural and most appropriate way to implement CAN for Linux.
153	
154	3. SocketCAN concept
155	---------------------
156	
157	  As described in chapter 2 it is the main goal of SocketCAN to
158	  provide a socket interface to user space applications which builds
159	  upon the Linux network layer. In contrast to the commonly known
160	  TCP/IP and ethernet networking, the CAN bus is a broadcast-only(!)
161	  medium that has no MAC-layer addressing like ethernet. The CAN-identifier
162	  (can_id) is used for arbitration on the CAN-bus. Therefore the CAN-IDs
163	  have to be chosen uniquely on the bus. When designing a CAN-ECU
164	  network the CAN-IDs are mapped to be sent by a specific ECU.
165	  For this reason a CAN-ID can be treated best as a kind of source address.
166	
167	  3.1 receive lists
168	
169	  The network transparent access of multiple applications leads to the
170	  problem that different applications may be interested in the same
171	  CAN-IDs from the same CAN network interface. The SocketCAN core
172	  module - which implements the protocol family CAN - provides several
173	  high efficient receive lists for this reason. If e.g. a user space
174	  application opens a CAN RAW socket, the raw protocol module itself
175	  requests the (range of) CAN-IDs from the SocketCAN core that are
176	  requested by the user. The subscription and unsubscription of
177	  CAN-IDs can be done for specific CAN interfaces or for all(!) known
178	  CAN interfaces with the can_rx_(un)register() functions provided to
179	  CAN protocol modules by the SocketCAN core (see chapter 5).
180	  To optimize the CPU usage at runtime the receive lists are split up
181	  into several specific lists per device that match the requested
182	  filter complexity for a given use-case.
183	
184	  3.2 local loopback of sent frames
185	
186	  As known from other networking concepts the data exchanging
187	  applications may run on the same or different nodes without any
188	  change (except for the according addressing information):
189	
190	         ___   ___   ___                   _______   ___
191	        | _ | | _ | | _ |                 | _   _ | | _ |
192	        ||A|| ||B|| ||C||                 ||A| |B|| ||C||
193	        |___| |___| |___|                 |_______| |___|
194	          |     |     |                       |       |
195	        -----------------(1)- CAN bus -(2)---------------
196	
197	  To ensure that application A receives the same information in the
198	  example (2) as it would receive in example (1) there is need for
199	  some kind of local loopback of the sent CAN frames on the appropriate
200	  node.
201	
202	  The Linux network devices (by default) just can handle the
203	  transmission and reception of media dependent frames. Due to the
204	  arbitration on the CAN bus the transmission of a low prio CAN-ID
205	  may be delayed by the reception of a high prio CAN frame. To
206	  reflect the correct* traffic on the node the loopback of the sent
207	  data has to be performed right after a successful transmission. If
208	  the CAN network interface is not capable of performing the loopback for
209	  some reason the SocketCAN core can do this task as a fallback solution.
210	  See chapter 6.2 for details (recommended).
211	
212	  The loopback functionality is enabled by default to reflect standard
213	  networking behaviour for CAN applications. Due to some requests from
214	  the RT-SocketCAN group the loopback optionally may be disabled for each
215	  separate socket. See sockopts from the CAN RAW sockets in chapter 4.1.
216	
217	  * = you really like to have this when you're running analyser tools
218	      like 'candump' or 'cansniffer' on the (same) node.
219	
220	  3.3 network problem notifications
221	
222	  The use of the CAN bus may lead to several problems on the physical
223	  and media access control layer. Detecting and logging of these lower
224	  layer problems is a vital requirement for CAN users to identify
225	  hardware issues on the physical transceiver layer as well as
226	  arbitration problems and error frames caused by the different
227	  ECUs. The occurrence of detected errors are important for diagnosis
228	  and have to be logged together with the exact timestamp. For this
229	  reason the CAN interface driver can generate so called Error Message
230	  Frames that can optionally be passed to the user application in the
231	  same way as other CAN frames. Whenever an error on the physical layer
232	  or the MAC layer is detected (e.g. by the CAN controller) the driver
233	  creates an appropriate error message frame. Error messages frames can
234	  be requested by the user application using the common CAN filter
235	  mechanisms. Inside this filter definition the (interested) type of
236	  errors may be selected. The reception of error messages is disabled
237	  by default. The format of the CAN error message frame is briefly
238	  described in the Linux header file "include/uapi/linux/can/error.h".
239	
240	4. How to use SocketCAN
241	------------------------
242	
243	  Like TCP/IP, you first need to open a socket for communicating over a
244	  CAN network. Since SocketCAN implements a new protocol family, you
245	  need to pass PF_CAN as the first argument to the socket(2) system
246	  call. Currently, there are two CAN protocols to choose from, the raw
247	  socket protocol and the broadcast manager (BCM). So to open a socket,
248	  you would write
249	
250	    s = socket(PF_CAN, SOCK_RAW, CAN_RAW);
251	
252	  and
253	
254	    s = socket(PF_CAN, SOCK_DGRAM, CAN_BCM);
255	
256	  respectively.  After the successful creation of the socket, you would
257	  normally use the bind(2) system call to bind the socket to a CAN
258	  interface (which is different from TCP/IP due to different addressing
259	  - see chapter 3). After binding (CAN_RAW) or connecting (CAN_BCM)
260	  the socket, you can read(2) and write(2) from/to the socket or use
261	  send(2), sendto(2), sendmsg(2) and the recv* counterpart operations
262	  on the socket as usual. There are also CAN specific socket options
263	  described below.
264	
265	  The basic CAN frame structure and the sockaddr structure are defined
266	  in include/linux/can.h:
267	
268	    struct can_frame {
269	            canid_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */
270	            __u8    can_dlc; /* frame payload length in byte (0 .. 8) */
271	            __u8    data[8] __attribute__((aligned(8)));
272	    };
273	
274	  The alignment of the (linear) payload data[] to a 64bit boundary
275	  allows the user to define their own structs and unions to easily access
276	  the CAN payload. There is no given byteorder on the CAN bus by
277	  default. A read(2) system call on a CAN_RAW socket transfers a
278	  struct can_frame to the user space.
279	
280	  The sockaddr_can structure has an interface index like the
281	  PF_PACKET socket, that also binds to a specific interface:
282	
283	    struct sockaddr_can {
284	            sa_family_t can_family;
285	            int         can_ifindex;
286	            union {
287	                    /* transport protocol class address info (e.g. ISOTP) */
288	                    struct { canid_t rx_id, tx_id; } tp;
289	
290	                    /* reserved for future CAN protocols address information */
291	            } can_addr;
292	    };
293	
294	  To determine the interface index an appropriate ioctl() has to
295	  be used (example for CAN_RAW sockets without error checking):
296	
297	    int s;
298	    struct sockaddr_can addr;
299	    struct ifreq ifr;
300	
301	    s = socket(PF_CAN, SOCK_RAW, CAN_RAW);
302	
303	    strcpy(ifr.ifr_name, "can0" );
304	    ioctl(s, SIOCGIFINDEX, &ifr);
305	
306	    addr.can_family = AF_CAN;
307	    addr.can_ifindex = ifr.ifr_ifindex;
308	
309	    bind(s, (struct sockaddr *)&addr, sizeof(addr));
310	
311	    (..)
312	
313	  To bind a socket to all(!) CAN interfaces the interface index must
314	  be 0 (zero). In this case the socket receives CAN frames from every
315	  enabled CAN interface. To determine the originating CAN interface
316	  the system call recvfrom(2) may be used instead of read(2). To send
317	  on a socket that is bound to 'any' interface sendto(2) is needed to
318	  specify the outgoing interface.
319	
320	  Reading CAN frames from a bound CAN_RAW socket (see above) consists
321	  of reading a struct can_frame:
322	
323	    struct can_frame frame;
324	
325	    nbytes = read(s, &frame, sizeof(struct can_frame));
326	
327	    if (nbytes < 0) {
328	            perror("can raw socket read");
329	            return 1;
330	    }
331	
332	    /* paranoid check ... */
333	    if (nbytes < sizeof(struct can_frame)) {
334	            fprintf(stderr, "read: incomplete CAN frame\n");
335	            return 1;
336	    }
337	
338	    /* do something with the received CAN frame */
339	
340	  Writing CAN frames can be done similarly, with the write(2) system call:
341	
342	    nbytes = write(s, &frame, sizeof(struct can_frame));
343	
344	  When the CAN interface is bound to 'any' existing CAN interface
345	  (addr.can_ifindex = 0) it is recommended to use recvfrom(2) if the
346	  information about the originating CAN interface is needed:
347	
348	    struct sockaddr_can addr;
349	    struct ifreq ifr;
350	    socklen_t len = sizeof(addr);
351	    struct can_frame frame;
352	
353	    nbytes = recvfrom(s, &frame, sizeof(struct can_frame),
354	                      0, (struct sockaddr*)&addr, &len);
355	
356	    /* get interface name of the received CAN frame */
357	    ifr.ifr_ifindex = addr.can_ifindex;
358	    ioctl(s, SIOCGIFNAME, &ifr);
359	    printf("Received a CAN frame from interface %s", ifr.ifr_name);
360	
361	  To write CAN frames on sockets bound to 'any' CAN interface the
362	  outgoing interface has to be defined certainly.
363	
364	    strcpy(ifr.ifr_name, "can0");
365	    ioctl(s, SIOCGIFINDEX, &ifr);
366	    addr.can_ifindex = ifr.ifr_ifindex;
367	    addr.can_family  = AF_CAN;
368	
369	    nbytes = sendto(s, &frame, sizeof(struct can_frame),
370	                    0, (struct sockaddr*)&addr, sizeof(addr));
371	
372	  Remark about CAN FD (flexible data rate) support:
373	
374	  Generally the handling of CAN FD is very similar to the formerly described
375	  examples. The new CAN FD capable CAN controllers support two different
376	  bitrates for the arbitration phase and the payload phase of the CAN FD frame
377	  and up to 64 bytes of payload. This extended payload length breaks all the
378	  kernel interfaces (ABI) which heavily rely on the CAN frame with fixed eight
379	  bytes of payload (struct can_frame) like the CAN_RAW socket. Therefore e.g.
380	  the CAN_RAW socket supports a new socket option CAN_RAW_FD_FRAMES that
381	  switches the socket into a mode that allows the handling of CAN FD frames
382	  and (legacy) CAN frames simultaneously (see section 4.1.5).
383	
384	  The struct canfd_frame is defined in include/linux/can.h:
385	
386	    struct canfd_frame {
387	            canid_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */
388	            __u8    len;     /* frame payload length in byte (0 .. 64) */
389	            __u8    flags;   /* additional flags for CAN FD */
390	            __u8    __res0;  /* reserved / padding */
391	            __u8    __res1;  /* reserved / padding */
392	            __u8    data[64] __attribute__((aligned(8)));
393	    };
394	
395	  The struct canfd_frame and the existing struct can_frame have the can_id,
396	  the payload length and the payload data at the same offset inside their
397	  structures. This allows to handle the different structures very similar.
398	  When the content of a struct can_frame is copied into a struct canfd_frame
399	  all structure elements can be used as-is - only the data[] becomes extended.
400	
401	  When introducing the struct canfd_frame it turned out that the data length
402	  code (DLC) of the struct can_frame was used as a length information as the
403	  length and the DLC has a 1:1 mapping in the range of 0 .. 8. To preserve
404	  the easy handling of the length information the canfd_frame.len element
405	  contains a plain length value from 0 .. 64. So both canfd_frame.len and
406	  can_frame.can_dlc are equal and contain a length information and no DLC.
407	  For details about the distinction of CAN and CAN FD capable devices and
408	  the mapping to the bus-relevant data length code (DLC), see chapter 6.6.
409	
410	  The length of the two CAN(FD) frame structures define the maximum transfer
411	  unit (MTU) of the CAN(FD) network interface and skbuff data length. Two
412	  definitions are specified for CAN specific MTUs in include/linux/can.h :
413	
414	  #define CAN_MTU   (sizeof(struct can_frame))   == 16  => 'legacy' CAN frame
415	  #define CANFD_MTU (sizeof(struct canfd_frame)) == 72  => CAN FD frame
416	
417	  4.1 RAW protocol sockets with can_filters (SOCK_RAW)
418	
419	  Using CAN_RAW sockets is extensively comparable to the commonly
420	  known access to CAN character devices. To meet the new possibilities
421	  provided by the multi user SocketCAN approach, some reasonable
422	  defaults are set at RAW socket binding time:
423	
424	  - The filters are set to exactly one filter receiving everything
425	  - The socket only receives valid data frames (=> no error message frames)
426	  - The loopback of sent CAN frames is enabled (see chapter 3.2)
427	  - The socket does not receive its own sent frames (in loopback mode)
428	
429	  These default settings may be changed before or after binding the socket.
430	  To use the referenced definitions of the socket options for CAN_RAW
431	  sockets, include <linux/can/raw.h>.
432	
433	  4.1.1 RAW socket option CAN_RAW_FILTER
434	
435	  The reception of CAN frames using CAN_RAW sockets can be controlled
436	  by defining 0 .. n filters with the CAN_RAW_FILTER socket option.
437	
438	  The CAN filter structure is defined in include/linux/can.h:
439	
440	    struct can_filter {
441	            canid_t can_id;
442	            canid_t can_mask;
443	    };
444	
445	  A filter matches, when
446	
447	    <received_can_id> & mask == can_id & mask
448	
449	  which is analogous to known CAN controllers hardware filter semantics.
450	  The filter can be inverted in this semantic, when the CAN_INV_FILTER
451	  bit is set in can_id element of the can_filter structure. In
452	  contrast to CAN controller hardware filters the user may set 0 .. n
453	  receive filters for each open socket separately:
454	
455	    struct can_filter rfilter[2];
456	
457	    rfilter[0].can_id   = 0x123;
458	    rfilter[0].can_mask = CAN_SFF_MASK;
459	    rfilter[1].can_id   = 0x200;
460	    rfilter[1].can_mask = 0x700;
461	
462	    setsockopt(s, SOL_CAN_RAW, CAN_RAW_FILTER, &rfilter, sizeof(rfilter));
463	
464	  To disable the reception of CAN frames on the selected CAN_RAW socket:
465	
466	    setsockopt(s, SOL_CAN_RAW, CAN_RAW_FILTER, NULL, 0);
467	
468	  To set the filters to zero filters is quite obsolete as to not read
469	  data causes the raw socket to discard the received CAN frames. But
470	  having this 'send only' use-case we may remove the receive list in the
471	  Kernel to save a little (really a very little!) CPU usage.
472	
473	  4.1.1.1 CAN filter usage optimisation
474	
475	  The CAN filters are processed in per-device filter lists at CAN frame
476	  reception time. To reduce the number of checks that need to be performed
477	  while walking through the filter lists the CAN core provides an optimized
478	  filter handling when the filter subscription focusses on a single CAN ID.
479	
480	  For the possible 2048 SFF CAN identifiers the identifier is used as an index
481	  to access the corresponding subscription list without any further checks.
482	  For the 2^29 possible EFF CAN identifiers a 10 bit XOR folding is used as
483	  hash function to retrieve the EFF table index.
484	
485	  To benefit from the optimized filters for single CAN identifiers the
486	  CAN_SFF_MASK or CAN_EFF_MASK have to be set into can_filter.mask together
487	  with set CAN_EFF_FLAG and CAN_RTR_FLAG bits. A set CAN_EFF_FLAG bit in the
488	  can_filter.mask makes clear that it matters whether a SFF or EFF CAN ID is
489	  subscribed. E.g. in the example from above
490	
491	    rfilter[0].can_id   = 0x123;
492	    rfilter[0].can_mask = CAN_SFF_MASK;
493	
494	  both SFF frames with CAN ID 0x123 and EFF frames with 0xXXXXX123 can pass.
495	
496	  To filter for only 0x123 (SFF) and 0x12345678 (EFF) CAN identifiers the
497	  filter has to be defined in this way to benefit from the optimized filters:
498	
499	    struct can_filter rfilter[2];
500	
501	    rfilter[0].can_id   = 0x123;
502	    rfilter[0].can_mask = (CAN_EFF_FLAG | CAN_RTR_FLAG | CAN_SFF_MASK);
503	    rfilter[1].can_id   = 0x12345678 | CAN_EFF_FLAG;
504	    rfilter[1].can_mask = (CAN_EFF_FLAG | CAN_RTR_FLAG | CAN_EFF_MASK);
505	
506	    setsockopt(s, SOL_CAN_RAW, CAN_RAW_FILTER, &rfilter, sizeof(rfilter));
507	
508	  4.1.2 RAW socket option CAN_RAW_ERR_FILTER
509	
510	  As described in chapter 3.4 the CAN interface driver can generate so
511	  called Error Message Frames that can optionally be passed to the user
512	  application in the same way as other CAN frames. The possible
513	  errors are divided into different error classes that may be filtered
514	  using the appropriate error mask. To register for every possible
515	  error condition CAN_ERR_MASK can be used as value for the error mask.
516	  The values for the error mask are defined in linux/can/error.h .
517	
518	    can_err_mask_t err_mask = ( CAN_ERR_TX_TIMEOUT | CAN_ERR_BUSOFF );
519	
520	    setsockopt(s, SOL_CAN_RAW, CAN_RAW_ERR_FILTER,
521	               &err_mask, sizeof(err_mask));
522	
523	  4.1.3 RAW socket option CAN_RAW_LOOPBACK
524	
525	  To meet multi user needs the local loopback is enabled by default
526	  (see chapter 3.2 for details). But in some embedded use-cases
527	  (e.g. when only one application uses the CAN bus) this loopback
528	  functionality can be disabled (separately for each socket):
529	
530	    int loopback = 0; /* 0 = disabled, 1 = enabled (default) */
531	
532	    setsockopt(s, SOL_CAN_RAW, CAN_RAW_LOOPBACK, &loopback, sizeof(loopback));
533	
534	  4.1.4 RAW socket option CAN_RAW_RECV_OWN_MSGS
535	
536	  When the local loopback is enabled, all the sent CAN frames are
537	  looped back to the open CAN sockets that registered for the CAN
538	  frames' CAN-ID on this given interface to meet the multi user
539	  needs. The reception of the CAN frames on the same socket that was
540	  sending the CAN frame is assumed to be unwanted and therefore
541	  disabled by default. This default behaviour may be changed on
542	  demand:
543	
544	    int recv_own_msgs = 1; /* 0 = disabled (default), 1 = enabled */
545	
546	    setsockopt(s, SOL_CAN_RAW, CAN_RAW_RECV_OWN_MSGS,
547	               &recv_own_msgs, sizeof(recv_own_msgs));
548	
549	  4.1.5 RAW socket option CAN_RAW_FD_FRAMES
550	
551	  CAN FD support in CAN_RAW sockets can be enabled with a new socket option
552	  CAN_RAW_FD_FRAMES which is off by default. When the new socket option is
553	  not supported by the CAN_RAW socket (e.g. on older kernels), switching the
554	  CAN_RAW_FD_FRAMES option returns the error -ENOPROTOOPT.
555	
556	  Once CAN_RAW_FD_FRAMES is enabled the application can send both CAN frames
557	  and CAN FD frames. OTOH the application has to handle CAN and CAN FD frames
558	  when reading from the socket.
559	
560	    CAN_RAW_FD_FRAMES enabled:  CAN_MTU and CANFD_MTU are allowed
561	    CAN_RAW_FD_FRAMES disabled: only CAN_MTU is allowed (default)
562	
563	  Example:
564	    [ remember: CANFD_MTU == sizeof(struct canfd_frame) ]
565	
566	    struct canfd_frame cfd;
567	
568	    nbytes = read(s, &cfd, CANFD_MTU);
569	
570	    if (nbytes == CANFD_MTU) {
571	            printf("got CAN FD frame with length %d\n", cfd.len);
572		    /* cfd.flags contains valid data */
573	    } else if (nbytes == CAN_MTU) {
574	            printf("got legacy CAN frame with length %d\n", cfd.len);
575		    /* cfd.flags is undefined */
576	    } else {
577	            fprintf(stderr, "read: invalid CAN(FD) frame\n");
578	            return 1;
579	    }
580	
581	    /* the content can be handled independently from the received MTU size */
582	
583	    printf("can_id: %X data length: %d data: ", cfd.can_id, cfd.len);
584	    for (i = 0; i < cfd.len; i++)
585	            printf("%02X ", cfd.data[i]);
586	
587	  When reading with size CANFD_MTU only returns CAN_MTU bytes that have
588	  been received from the socket a legacy CAN frame has been read into the
589	  provided CAN FD structure. Note that the canfd_frame.flags data field is
590	  not specified in the struct can_frame and therefore it is only valid in
591	  CANFD_MTU sized CAN FD frames.
592	
593	  Implementation hint for new CAN applications:
594	
595	  To build a CAN FD aware application use struct canfd_frame as basic CAN
596	  data structure for CAN_RAW based applications. When the application is
597	  executed on an older Linux kernel and switching the CAN_RAW_FD_FRAMES
598	  socket option returns an error: No problem. You'll get legacy CAN frames
599	  or CAN FD frames and can process them the same way.
600	
601	  When sending to CAN devices make sure that the device is capable to handle
602	  CAN FD frames by checking if the device maximum transfer unit is CANFD_MTU.
603	  The CAN device MTU can be retrieved e.g. with a SIOCGIFMTU ioctl() syscall.
604	
605	  4.1.6 RAW socket option CAN_RAW_JOIN_FILTERS
606	
607	  The CAN_RAW socket can set multiple CAN identifier specific filters that
608	  lead to multiple filters in the af_can.c filter processing. These filters
609	  are indenpendent from each other which leads to logical OR'ed filters when
610	  applied (see 4.1.1).
611	
612	  This socket option joines the given CAN filters in the way that only CAN
613	  frames are passed to user space that matched *all* given CAN filters. The
614	  semantic for the applied filters is therefore changed to a logical AND.
615	
616	  This is useful especially when the filterset is a combination of filters
617	  where the CAN_INV_FILTER flag is set in order to notch single CAN IDs or
618	  CAN ID ranges from the incoming traffic.
619	
620	  4.1.7 RAW socket returned message flags
621	
622	  When using recvmsg() call, the msg->msg_flags may contain following flags:
623	
624	    MSG_DONTROUTE: set when the received frame was created on the local host.
625	
626	    MSG_CONFIRM: set when the frame was sent via the socket it is received on.
627	      This flag can be interpreted as a 'transmission confirmation' when the
628	      CAN driver supports the echo of frames on driver level, see 3.2 and 6.2.
629	      In order to receive such messages, CAN_RAW_RECV_OWN_MSGS must be set.
630	
631	  4.2 Broadcast Manager protocol sockets (SOCK_DGRAM)
632	
633	  The Broadcast Manager protocol provides a command based configuration
634	  interface to filter and send (e.g. cyclic) CAN messages in kernel space.
635	
636	  Receive filters can be used to down sample frequent messages; detect events
637	  such as message contents changes, packet length changes, and do time-out
638	  monitoring of received messages.
639	
640	  Periodic transmission tasks of CAN frames or a sequence of CAN frames can be
641	  created and modified at runtime; both the message content and the two
642	  possible transmit intervals can be altered.
643	
644	  A BCM socket is not intended for sending individual CAN frames using the
645	  struct can_frame as known from the CAN_RAW socket. Instead a special BCM
646	  configuration message is defined. The basic BCM configuration message used
647	  to communicate with the broadcast manager and the available operations are
648	  defined in the linux/can/bcm.h include. The BCM message consists of a
649	  message header with a command ('opcode') followed by zero or more CAN frames.
650	  The broadcast manager sends responses to user space in the same form:
651	
652	    struct bcm_msg_head {
653	            __u32 opcode;                   /* command */
654	            __u32 flags;                    /* special flags */
655	            __u32 count;                    /* run 'count' times with ival1 */
656	            struct timeval ival1, ival2;    /* count and subsequent interval */
657	            canid_t can_id;                 /* unique can_id for task */
658	            __u32 nframes;                  /* number of can_frames following */
659	            struct can_frame frames[0];
660	    };
661	
662	  The aligned payload 'frames' uses the same basic CAN frame structure defined
663	  at the beginning of section 4 and in the include/linux/can.h include. All
664	  messages to the broadcast manager from user space have this structure.
665	
666	  Note a CAN_BCM socket must be connected instead of bound after socket
667	  creation (example without error checking):
668	
669	    int s;
670	    struct sockaddr_can addr;
671	    struct ifreq ifr;
672	
673	    s = socket(PF_CAN, SOCK_DGRAM, CAN_BCM);
674	
675	    strcpy(ifr.ifr_name, "can0");
676	    ioctl(s, SIOCGIFINDEX, &ifr);
677	
678	    addr.can_family = AF_CAN;
679	    addr.can_ifindex = ifr.ifr_ifindex;
680	
681	    connect(s, (struct sockaddr *)&addr, sizeof(addr))
682	
683	    (..)
684	
685	  The broadcast manager socket is able to handle any number of in flight
686	  transmissions or receive filters concurrently. The different RX/TX jobs are
687	  distinguished by the unique can_id in each BCM message. However additional
688	  CAN_BCM sockets are recommended to communicate on multiple CAN interfaces.
689	  When the broadcast manager socket is bound to 'any' CAN interface (=> the
690	  interface index is set to zero) the configured receive filters apply to any
691	  CAN interface unless the sendto() syscall is used to overrule the 'any' CAN
692	  interface index. When using recvfrom() instead of read() to retrieve BCM
693	  socket messages the originating CAN interface is provided in can_ifindex.
694	
695	  4.2.1 Broadcast Manager operations
696	
697	  The opcode defines the operation for the broadcast manager to carry out,
698	  or details the broadcast managers response to several events, including
699	  user requests.
700	
701	  Transmit Operations (user space to broadcast manager):
702	
703	    TX_SETUP:   Create (cyclic) transmission task.
704	
705	    TX_DELETE:  Remove (cyclic) transmission task, requires only can_id.
706	
707	    TX_READ:    Read properties of (cyclic) transmission task for can_id.
708	
709	    TX_SEND:    Send one CAN frame.
710	
711	  Transmit Responses (broadcast manager to user space):
712	
713	    TX_STATUS:  Reply to TX_READ request (transmission task configuration).
714	
715	    TX_EXPIRED: Notification when counter finishes sending at initial interval
716	      'ival1'. Requires the TX_COUNTEVT flag to be set at TX_SETUP.
717	
718	  Receive Operations (user space to broadcast manager):
719	
720	    RX_SETUP:   Create RX content filter subscription.
721	
722	    RX_DELETE:  Remove RX content filter subscription, requires only can_id.
723	
724	    RX_READ:    Read properties of RX content filter subscription for can_id.
725	
726	  Receive Responses (broadcast manager to user space):
727	
728	    RX_STATUS:  Reply to RX_READ request (filter task configuration).
729	
730	    RX_TIMEOUT: Cyclic message is detected to be absent (timer ival1 expired).
731	
732	    RX_CHANGED: BCM message with updated CAN frame (detected content change).
733	      Sent on first message received or on receipt of revised CAN messages.
734	
735	  4.2.2 Broadcast Manager message flags
736	
737	  When sending a message to the broadcast manager the 'flags' element may
738	  contain the following flag definitions which influence the behaviour:
739	
740	    SETTIMER:           Set the values of ival1, ival2 and count
741	
742	    STARTTIMER:         Start the timer with the actual values of ival1, ival2
743	      and count. Starting the timer leads simultaneously to emit a CAN frame.
744	
745	    TX_COUNTEVT:        Create the message TX_EXPIRED when count expires
746	
747	    TX_ANNOUNCE:        A change of data by the process is emitted immediately.
748	
749	    TX_CP_CAN_ID:       Copies the can_id from the message header to each
750	      subsequent frame in frames. This is intended as usage simplification. For
751	      TX tasks the unique can_id from the message header may differ from the
752	      can_id(s) stored for transmission in the subsequent struct can_frame(s).
753	
754	    RX_FILTER_ID:       Filter by can_id alone, no frames required (nframes=0).
755	
756	    RX_CHECK_DLC:       A change of the DLC leads to an RX_CHANGED.
757	
758	    RX_NO_AUTOTIMER:    Prevent automatically starting the timeout monitor.
759	
760	    RX_ANNOUNCE_RESUME: If passed at RX_SETUP and a receive timeout occurred, a
761	      RX_CHANGED message will be generated when the (cyclic) receive restarts.
762	
763	    TX_RESET_MULTI_IDX: Reset the index for the multiple frame transmission.
764	
765	    RX_RTR_FRAME:       Send reply for RTR-request (placed in op->frames[0]).
766	
767	  4.2.3 Broadcast Manager transmission timers
768	
769	  Periodic transmission configurations may use up to two interval timers.
770	  In this case the BCM sends a number of messages ('count') at an interval
771	  'ival1', then continuing to send at another given interval 'ival2'. When
772	  only one timer is needed 'count' is set to zero and only 'ival2' is used.
773	  When SET_TIMER and START_TIMER flag were set the timers are activated.
774	  The timer values can be altered at runtime when only SET_TIMER is set.
775	
776	  4.2.4 Broadcast Manager message sequence transmission
777	
778	  Up to 256 CAN frames can be transmitted in a sequence in the case of a cyclic
779	  TX task configuration. The number of CAN frames is provided in the 'nframes'
780	  element of the BCM message head. The defined number of CAN frames are added
781	  as array to the TX_SETUP BCM configuration message.
782	
783	    /* create a struct to set up a sequence of four CAN frames */
784	    struct {
785	            struct bcm_msg_head msg_head;
786	            struct can_frame frame[4];
787	    } mytxmsg;
788	
789	    (..)
790	    mytxmsg.nframes = 4;
791	    (..)
792	
793	    write(s, &mytxmsg, sizeof(mytxmsg));
794	
795	  With every transmission the index in the array of CAN frames is increased
796	  and set to zero at index overflow.
797	
798	  4.2.5 Broadcast Manager receive filter timers
799	
800	  The timer values ival1 or ival2 may be set to non-zero values at RX_SETUP.
801	  When the SET_TIMER flag is set the timers are enabled:
802	
803	  ival1: Send RX_TIMEOUT when a received message is not received again within
804	    the given time. When START_TIMER is set at RX_SETUP the timeout detection
805	    is activated directly - even without a former CAN frame reception.
806	
807	  ival2: Throttle the received message rate down to the value of ival2. This
808	    is useful to reduce messages for the application when the signal inside the
809	    CAN frame is stateless as state changes within the ival2 periode may get
810	    lost.
811	
812	  4.2.6 Broadcast Manager multiplex message receive filter
813	
814	  To filter for content changes in multiplex message sequences an array of more
815	  than one CAN frames can be passed in a RX_SETUP configuration message. The
816	  data bytes of the first CAN frame contain the mask of relevant bits that
817	  have to match in the subsequent CAN frames with the received CAN frame.
818	  If one of the subsequent CAN frames is matching the bits in that frame data
819	  mark the relevant content to be compared with the previous received content.
820	  Up to 257 CAN frames (multiplex filter bit mask CAN frame plus 256 CAN
821	  filters) can be added as array to the TX_SETUP BCM configuration message.
822	
823	    /* usually used to clear CAN frame data[] - beware of endian problems! */
824	    #define U64_DATA(p) (*(unsigned long long*)(p)->data)
825	
826	    struct {
827	            struct bcm_msg_head msg_head;
828	            struct can_frame frame[5];
829	    } msg;
830	
831	    msg.msg_head.opcode  = RX_SETUP;
832	    msg.msg_head.can_id  = 0x42;
833	    msg.msg_head.flags   = 0;
834	    msg.msg_head.nframes = 5;
835	    U64_DATA(&msg.frame[0]) = 0xFF00000000000000ULL; /* MUX mask */
836	    U64_DATA(&msg.frame[1]) = 0x01000000000000FFULL; /* data mask (MUX 0x01) */
837	    U64_DATA(&msg.frame[2]) = 0x0200FFFF000000FFULL; /* data mask (MUX 0x02) */
838	    U64_DATA(&msg.frame[3]) = 0x330000FFFFFF0003ULL; /* data mask (MUX 0x33) */
839	    U64_DATA(&msg.frame[4]) = 0x4F07FC0FF0000000ULL; /* data mask (MUX 0x4F) */
840	
841	    write(s, &msg, sizeof(msg));
842	
843	  4.3 connected transport protocols (SOCK_SEQPACKET)
844	  4.4 unconnected transport protocols (SOCK_DGRAM)
845	
846	
847	5. SocketCAN core module
848	-------------------------
849	
850	  The SocketCAN core module implements the protocol family
851	  PF_CAN. CAN protocol modules are loaded by the core module at
852	  runtime. The core module provides an interface for CAN protocol
853	  modules to subscribe needed CAN IDs (see chapter 3.1).
854	
855	  5.1 can.ko module params
856	
857	  - stats_timer: To calculate the SocketCAN core statistics
858	    (e.g. current/maximum frames per second) this 1 second timer is
859	    invoked at can.ko module start time by default. This timer can be
860	    disabled by using stattimer=0 on the module commandline.
861	
862	  - debug: (removed since SocketCAN SVN r546)
863	
864	  5.2 procfs content
865	
866	  As described in chapter 3.1 the SocketCAN core uses several filter
867	  lists to deliver received CAN frames to CAN protocol modules. These
868	  receive lists, their filters and the count of filter matches can be
869	  checked in the appropriate receive list. All entries contain the
870	  device and a protocol module identifier:
871	
872	    foo@bar:~$ cat /proc/net/can/rcvlist_all
873	
874	    receive list 'rx_all':
875	      (vcan3: no entry)
876	      (vcan2: no entry)
877	      (vcan1: no entry)
878	      device   can_id   can_mask  function  userdata   matches  ident
879	       vcan0     000    00000000  f88e6370  f6c6f400         0  raw
880	      (any: no entry)
881	
882	  In this example an application requests any CAN traffic from vcan0.
883	
884	    rcvlist_all - list for unfiltered entries (no filter operations)
885	    rcvlist_eff - list for single extended frame (EFF) entries
886	    rcvlist_err - list for error message frames masks
887	    rcvlist_fil - list for mask/value filters
888	    rcvlist_inv - list for mask/value filters (inverse semantic)
889	    rcvlist_sff - list for single standard frame (SFF) entries
890	
891	  Additional procfs files in /proc/net/can
892	
893	    stats       - SocketCAN core statistics (rx/tx frames, match ratios, ...)
894	    reset_stats - manual statistic reset
895	    version     - prints the SocketCAN core version and the ABI version
896	
897	  5.3 writing own CAN protocol modules
898	
899	  To implement a new protocol in the protocol family PF_CAN a new
900	  protocol has to be defined in include/linux/can.h .
901	  The prototypes and definitions to use the SocketCAN core can be
902	  accessed by including include/linux/can/core.h .
903	  In addition to functions that register the CAN protocol and the
904	  CAN device notifier chain there are functions to subscribe CAN
905	  frames received by CAN interfaces and to send CAN frames:
906	
907	    can_rx_register   - subscribe CAN frames from a specific interface
908	    can_rx_unregister - unsubscribe CAN frames from a specific interface
909	    can_send          - transmit a CAN frame (optional with local loopback)
910	
911	  For details see the kerneldoc documentation in net/can/af_can.c or
912	  the source code of net/can/raw.c or net/can/bcm.c .
913	
914	6. CAN network drivers
915	----------------------
916	
917	  Writing a CAN network device driver is much easier than writing a
918	  CAN character device driver. Similar to other known network device
919	  drivers you mainly have to deal with:
920	
921	  - TX: Put the CAN frame from the socket buffer to the CAN controller.
922	  - RX: Put the CAN frame from the CAN controller to the socket buffer.
923	
924	  See e.g. at Documentation/networking/netdevices.txt . The differences
925	  for writing CAN network device driver are described below:
926	
927	  6.1 general settings
928	
929	    dev->type  = ARPHRD_CAN; /* the netdevice hardware type */
930	    dev->flags = IFF_NOARP;  /* CAN has no arp */
931	
932	    dev->mtu = CAN_MTU; /* sizeof(struct can_frame) -> legacy CAN interface */
933	
934	    or alternative, when the controller supports CAN with flexible data rate:
935	    dev->mtu = CANFD_MTU; /* sizeof(struct canfd_frame) -> CAN FD interface */
936	
937	  The struct can_frame or struct canfd_frame is the payload of each socket
938	  buffer (skbuff) in the protocol family PF_CAN.
939	
940	  6.2 local loopback of sent frames
941	
942	  As described in chapter 3.2 the CAN network device driver should
943	  support a local loopback functionality similar to the local echo
944	  e.g. of tty devices. In this case the driver flag IFF_ECHO has to be
945	  set to prevent the PF_CAN core from locally echoing sent frames
946	  (aka loopback) as fallback solution:
947	
948	    dev->flags = (IFF_NOARP | IFF_ECHO);
949	
950	  6.3 CAN controller hardware filters
951	
952	  To reduce the interrupt load on deep embedded systems some CAN
953	  controllers support the filtering of CAN IDs or ranges of CAN IDs.
954	  These hardware filter capabilities vary from controller to
955	  controller and have to be identified as not feasible in a multi-user
956	  networking approach. The use of the very controller specific
957	  hardware filters could make sense in a very dedicated use-case, as a
958	  filter on driver level would affect all users in the multi-user
959	  system. The high efficient filter sets inside the PF_CAN core allow
960	  to set different multiple filters for each socket separately.
961	  Therefore the use of hardware filters goes to the category 'handmade
962	  tuning on deep embedded systems'. The author is running a MPC603e
963	  @133MHz with four SJA1000 CAN controllers from 2002 under heavy bus
964	  load without any problems ...
965	
966	  6.4 The virtual CAN driver (vcan)
967	
968	  Similar to the network loopback devices, vcan offers a virtual local
969	  CAN interface. A full qualified address on CAN consists of
970	
971	  - a unique CAN Identifier (CAN ID)
972	  - the CAN bus this CAN ID is transmitted on (e.g. can0)
973	
974	  so in common use cases more than one virtual CAN interface is needed.
975	
976	  The virtual CAN interfaces allow the transmission and reception of CAN
977	  frames without real CAN controller hardware. Virtual CAN network
978	  devices are usually named 'vcanX', like vcan0 vcan1 vcan2 ...
979	  When compiled as a module the virtual CAN driver module is called vcan.ko
980	
981	  Since Linux Kernel version 2.6.24 the vcan driver supports the Kernel
982	  netlink interface to create vcan network devices. The creation and
983	  removal of vcan network devices can be managed with the ip(8) tool:
984	
985	  - Create a virtual CAN network interface:
986	       $ ip link add type vcan
987	
988	  - Create a virtual CAN network interface with a specific name 'vcan42':
989	       $ ip link add dev vcan42 type vcan
990	
991	  - Remove a (virtual CAN) network interface 'vcan42':
992	       $ ip link del vcan42
993	
994	  6.5 The CAN network device driver interface
995	
996	  The CAN network device driver interface provides a generic interface
997	  to setup, configure and monitor CAN network devices. The user can then
998	  configure the CAN device, like setting the bit-timing parameters, via
999	  the netlink interface using the program "ip" from the "IPROUTE2"
1000	  utility suite. The following chapter describes briefly how to use it.
1001	  Furthermore, the interface uses a common data structure and exports a
1002	  set of common functions, which all real CAN network device drivers
1003	  should use. Please have a look to the SJA1000 or MSCAN driver to
1004	  understand how to use them. The name of the module is can-dev.ko.
1005	
1006	  6.5.1 Netlink interface to set/get devices properties
1007	
1008	  The CAN device must be configured via netlink interface. The supported
1009	  netlink message types are defined and briefly described in
1010	  "include/linux/can/netlink.h". CAN link support for the program "ip"
1011	  of the IPROUTE2 utility suite is available and it can be used as shown
1012	  below:
1013	
1014	  - Setting CAN device properties:
1015	
1016	    $ ip link set can0 type can help
1017	    Usage: ip link set DEVICE type can
1018	    	[ bitrate BITRATE [ sample-point SAMPLE-POINT] ] |
1019	    	[ tq TQ prop-seg PROP_SEG phase-seg1 PHASE-SEG1
1020	     	  phase-seg2 PHASE-SEG2 [ sjw SJW ] ]
1021	
1022	    	[ loopback { on | off } ]
1023	    	[ listen-only { on | off } ]
1024	    	[ triple-sampling { on | off } ]
1025	
1026	    	[ restart-ms TIME-MS ]
1027	    	[ restart ]
1028	
1029	    	Where: BITRATE       := { 1..1000000 }
1030	    	       SAMPLE-POINT  := { 0.000..0.999 }
1031	    	       TQ            := { NUMBER }
1032	    	       PROP-SEG      := { 1..8 }
1033	    	       PHASE-SEG1    := { 1..8 }
1034	    	       PHASE-SEG2    := { 1..8 }
1035	    	       SJW           := { 1..4 }
1036	    	       RESTART-MS    := { 0 | NUMBER }
1037	
1038	  - Display CAN device details and statistics:
1039	
1040	    $ ip -details -statistics link show can0
1041	    2: can0: <NOARP,UP,LOWER_UP,ECHO> mtu 16 qdisc pfifo_fast state UP qlen 10
1042	      link/can
1043	      can <TRIPLE-SAMPLING> state ERROR-ACTIVE restart-ms 100
1044	      bitrate 125000 sample_point 0.875
1045	      tq 125 prop-seg 6 phase-seg1 7 phase-seg2 2 sjw 1
1046	      sja1000: tseg1 1..16 tseg2 1..8 sjw 1..4 brp 1..64 brp-inc 1
1047	      clock 8000000
1048	      re-started bus-errors arbit-lost error-warn error-pass bus-off
1049	      41         17457      0          41         42         41
1050	      RX: bytes  packets  errors  dropped overrun mcast
1051	      140859     17608    17457   0       0       0
1052	      TX: bytes  packets  errors  dropped carrier collsns
1053	      861        112      0       41      0       0
1054	
1055	  More info to the above output:
1056	
1057	    "<TRIPLE-SAMPLING>"
1058		Shows the list of selected CAN controller modes: LOOPBACK,
1059		LISTEN-ONLY, or TRIPLE-SAMPLING.
1060	
1061	    "state ERROR-ACTIVE"
1062		The current state of the CAN controller: "ERROR-ACTIVE",
1063		"ERROR-WARNING", "ERROR-PASSIVE", "BUS-OFF" or "STOPPED"
1064	
1065	    "restart-ms 100"
1066		Automatic restart delay time. If set to a non-zero value, a
1067		restart of the CAN controller will be triggered automatically
1068		in case of a bus-off condition after the specified delay time
1069		in milliseconds. By default it's off.
1070	
1071	    "bitrate 125000 sample-point 0.875"
1072		Shows the real bit-rate in bits/sec and the sample-point in the
1073		range 0.000..0.999. If the calculation of bit-timing parameters
1074		is enabled in the kernel (CONFIG_CAN_CALC_BITTIMING=y), the
1075		bit-timing can be defined by setting the "bitrate" argument.
1076		Optionally the "sample-point" can be specified. By default it's
1077		0.000 assuming CIA-recommended sample-points.
1078	
1079	    "tq 125 prop-seg 6 phase-seg1 7 phase-seg2 2 sjw 1"
1080		Shows the time quanta in ns, propagation segment, phase buffer
1081		segment 1 and 2 and the synchronisation jump width in units of
1082		tq. They allow to define the CAN bit-timing in a hardware
1083		independent format as proposed by the Bosch CAN 2.0 spec (see
1084		chapter 8 of http://www.semiconductors.bosch.de/pdf/can2spec.pdf).
1085	
1086	    "sja1000: tseg1 1..16 tseg2 1..8 sjw 1..4 brp 1..64 brp-inc 1
1087	     clock 8000000"
1088		Shows the bit-timing constants of the CAN controller, here the
1089		"sja1000". The minimum and maximum values of the time segment 1
1090		and 2, the synchronisation jump width in units of tq, the
1091		bitrate pre-scaler and the CAN system clock frequency in Hz.
1092		These constants could be used for user-defined (non-standard)
1093		bit-timing calculation algorithms in user-space.
1094	
1095	    "re-started bus-errors arbit-lost error-warn error-pass bus-off"
1096		Shows the number of restarts, bus and arbitration lost errors,
1097		and the state changes to the error-warning, error-passive and
1098		bus-off state. RX overrun errors are listed in the "overrun"
1099		field of the standard network statistics.
1100	
1101	  6.5.2 Setting the CAN bit-timing
1102	
1103	  The CAN bit-timing parameters can always be defined in a hardware
1104	  independent format as proposed in the Bosch CAN 2.0 specification
1105	  specifying the arguments "tq", "prop_seg", "phase_seg1", "phase_seg2"
1106	  and "sjw":
1107	
1108	    $ ip link set canX type can tq 125 prop-seg 6 \
1109					phase-seg1 7 phase-seg2 2 sjw 1
1110	
1111	  If the kernel option CONFIG_CAN_CALC_BITTIMING is enabled, CIA
1112	  recommended CAN bit-timing parameters will be calculated if the bit-
1113	  rate is specified with the argument "bitrate":
1114	
1115	    $ ip link set canX type can bitrate 125000
1116	
1117	  Note that this works fine for the most common CAN controllers with
1118	  standard bit-rates but may *fail* for exotic bit-rates or CAN system
1119	  clock frequencies. Disabling CONFIG_CAN_CALC_BITTIMING saves some
1120	  space and allows user-space tools to solely determine and set the
1121	  bit-timing parameters. The CAN controller specific bit-timing
1122	  constants can be used for that purpose. They are listed by the
1123	  following command:
1124	
1125	    $ ip -details link show can0
1126	    ...
1127	      sja1000: clock 8000000 tseg1 1..16 tseg2 1..8 sjw 1..4 brp 1..64 brp-inc 1
1128	
1129	  6.5.3 Starting and stopping the CAN network device
1130	
1131	  A CAN network device is started or stopped as usual with the command
1132	  "ifconfig canX up/down" or "ip link set canX up/down". Be aware that
1133	  you *must* define proper bit-timing parameters for real CAN devices
1134	  before you can start it to avoid error-prone default settings:
1135	
1136	    $ ip link set canX up type can bitrate 125000
1137	
1138	  A device may enter the "bus-off" state if too many errors occurred on
1139	  the CAN bus. Then no more messages are received or sent. An automatic
1140	  bus-off recovery can be enabled by setting the "restart-ms" to a
1141	  non-zero value, e.g.:
1142	
1143	    $ ip link set canX type can restart-ms 100
1144	
1145	  Alternatively, the application may realize the "bus-off" condition
1146	  by monitoring CAN error message frames and do a restart when
1147	  appropriate with the command:
1148	
1149	    $ ip link set canX type can restart
1150	
1151	  Note that a restart will also create a CAN error message frame (see
1152	  also chapter 3.4).
1153	
1154	  6.6 CAN FD (flexible data rate) driver support
1155	
1156	  CAN FD capable CAN controllers support two different bitrates for the
1157	  arbitration phase and the payload phase of the CAN FD frame. Therefore a
1158	  second bit timing has to be specified in order to enable the CAN FD bitrate.
1159	
1160	  Additionally CAN FD capable CAN controllers support up to 64 bytes of
1161	  payload. The representation of this length in can_frame.can_dlc and
1162	  canfd_frame.len for userspace applications and inside the Linux network
1163	  layer is a plain value from 0 .. 64 instead of the CAN 'data length code'.
1164	  The data length code was a 1:1 mapping to the payload length in the legacy
1165	  CAN frames anyway. The payload length to the bus-relevant DLC mapping is
1166	  only performed inside the CAN drivers, preferably with the helper
1167	  functions can_dlc2len() and can_len2dlc().
1168	
1169	  The CAN netdevice driver capabilities can be distinguished by the network
1170	  devices maximum transfer unit (MTU):
1171	
1172	  MTU = 16 (CAN_MTU)   => sizeof(struct can_frame)   => 'legacy' CAN device
1173	  MTU = 72 (CANFD_MTU) => sizeof(struct canfd_frame) => CAN FD capable device
1174	
1175	  The CAN device MTU can be retrieved e.g. with a SIOCGIFMTU ioctl() syscall.
1176	  N.B. CAN FD capable devices can also handle and send legacy CAN frames.
1177	
1178	  FIXME: Add details about the CAN FD controller configuration when available.
1179	
1180	  6.7 Supported CAN hardware
1181	
1182	  Please check the "Kconfig" file in "drivers/net/can" to get an actual
1183	  list of the support CAN hardware. On the SocketCAN project website
1184	  (see chapter 7) there might be further drivers available, also for
1185	  older kernel versions.
1186	
1187	7. SocketCAN resources
1188	-----------------------
1189	
1190	  The Linux CAN / SocketCAN project ressources (project site / mailing list)
1191	  are referenced in the MAINTAINERS file in the Linux source tree.
1192	  Search for CAN NETWORK [LAYERS|DRIVERS].
1193	
1194	8. Credits
1195	----------
1196	
1197	  Oliver Hartkopp (PF_CAN core, filters, drivers, bcm, SJA1000 driver)
1198	  Urs Thuermann (PF_CAN core, kernel integration, socket interfaces, raw, vcan)
1199	  Jan Kizka (RT-SocketCAN core, Socket-API reconciliation)
1200	  Wolfgang Grandegger (RT-SocketCAN core & drivers, Raw Socket-API reviews,
1201	                       CAN device driver interface, MSCAN driver)
1202	  Robert Schwebel (design reviews, PTXdist integration)
1203	  Marc Kleine-Budde (design reviews, Kernel 2.6 cleanups, drivers)
1204	  Benedikt Spranger (reviews)
1205	  Thomas Gleixner (LKML reviews, coding style, posting hints)
1206	  Andrey Volkov (kernel subtree structure, ioctls, MSCAN driver)
1207	  Matthias Brukner (first SJA1000 CAN netdevice implementation Q2/2003)
1208	  Klaus Hitschler (PEAK driver integration)
1209	  Uwe Koppe (CAN netdevices with PF_PACKET approach)
1210	  Michael Schulze (driver layer loopback requirement, RT CAN drivers review)
1211	  Pavel Pisa (Bit-timing calculation)
1212	  Sascha Hauer (SJA1000 platform driver)
1213	  Sebastian Haas (SJA1000 EMS PCI driver)
1214	  Markus Plessing (SJA1000 EMS PCI driver)
1215	  Per Dalen (SJA1000 Kvaser PCI driver)
1216	  Sam Ravnborg (reviews, coding style, kbuild help)
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