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linux/Documentation/networking/can.txt
Oliver Hartkopp 56690c2151 [CAN]: Update documentation of struct sockaddr_can 
The struct sockaddr_can has been simplified in the code review
process.  This patch updates this simplification also in the
associated documentation in can.txt .

Signed-off-by: Oliver Hartkopp <oliver@hartkopp.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
2008-04-15 00:46:38 -07:00

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============================================================================
can.txt
Readme file for the Controller Area Network Protocol Family (aka Socket CAN)
This file contains
1 Overview / What is Socket CAN
2 Motivation / Why using the socket API
3 Socket CAN concept
3.1 receive lists
3.2 local loopback of sent frames
3.3 network security issues (capabilities)
3.4 network problem notifications
4 How to use Socket CAN
4.1 RAW protocol sockets with can_filters (SOCK_RAW)
4.1.1 RAW socket option CAN_RAW_FILTER
4.1.2 RAW socket option CAN_RAW_ERR_FILTER
4.1.3 RAW socket option CAN_RAW_LOOPBACK
4.1.4 RAW socket option CAN_RAW_RECV_OWN_MSGS
4.2 Broadcast Manager protocol sockets (SOCK_DGRAM)
4.3 connected transport protocols (SOCK_SEQPACKET)
4.4 unconnected transport protocols (SOCK_DGRAM)
5 Socket CAN core module
5.1 can.ko module params
5.2 procfs content
5.3 writing own CAN protocol modules
6 CAN network drivers
6.1 general settings
6.2 local loopback of sent frames
6.3 CAN controller hardware filters
6.4 currently supported CAN hardware
6.5 todo
7 Credits
============================================================================
1. Overview / What is Socket CAN
--------------------------------
The socketcan package is an implementation of CAN protocols
(Controller Area Network) for Linux. CAN is a networking technology
which has widespread use in automation, embedded devices, and
automotive fields. While there have been other CAN implementations
for Linux based on character devices, Socket CAN uses the Berkeley
socket API, the Linux network stack and implements the CAN device
drivers as network interfaces. The CAN socket API has been designed
as similar as possible to the TCP/IP protocols to allow programmers,
familiar with network programming, to easily learn how to use CAN
sockets.
2. Motivation / Why using the socket API
----------------------------------------
There have been CAN implementations for Linux before Socket CAN so the
question arises, why we have started another project. Most existing
implementations come as a device driver for some CAN hardware, they
are based on character devices and provide comparatively little
functionality. Usually, there is only a hardware-specific device
driver which provides a character device interface to send and
receive raw CAN frames, directly to/from the controller hardware.
Queueing of frames and higher-level transport protocols like ISO-TP
have to be implemented in user space applications. Also, most
character-device implementations support only one single process to
open the device at a time, similar to a serial interface. Exchanging
the CAN controller requires employment of another device driver and
often the need for adaption of large parts of the application to the
new driver's API.
Socket CAN was designed to overcome all of these limitations. A new
protocol family has been implemented which provides a socket interface
to user space applications and which builds upon the Linux network
layer, so to use all of the provided queueing functionality. A device
driver for CAN controller hardware registers itself with the Linux
network layer as a network device, so that CAN frames from the
controller can be passed up to the network layer and on to the CAN
protocol family module and also vice-versa. Also, the protocol family
module provides an API for transport protocol modules to register, so
that any number of transport protocols can be loaded or unloaded
dynamically. In fact, the can core module alone does not provide any
protocol and cannot be used without loading at least one additional
protocol module. Multiple sockets can be opened at the same time,
on different or the same protocol module and they can listen/send
frames on different or the same CAN IDs. Several sockets listening on
the same interface for frames with the same CAN ID are all passed the
same received matching CAN frames. An application wishing to
communicate using a specific transport protocol, e.g. ISO-TP, just
selects that protocol when opening the socket, and then can read and
write application data byte streams, without having to deal with
CAN-IDs, frames, etc.
Similar functionality visible from user-space could be provided by a
character device, too, but this would lead to a technically inelegant
solution for a couple of reasons:
* Intricate usage. Instead of passing a protocol argument to
socket(2) and using bind(2) to select a CAN interface and CAN ID, an
application would have to do all these operations using ioctl(2)s.
* Code duplication. A character device cannot make use of the Linux
network queueing code, so all that code would have to be duplicated
for CAN networking.
* Abstraction. In most existing character-device implementations, the
hardware-specific device driver for a CAN controller directly
provides the character device for the application to work with.
This is at least very unusual in Unix systems for both, char and
block devices. For example you don't have a character device for a
certain UART of a serial interface, a certain sound chip in your
computer, a SCSI or IDE controller providing access to your hard
disk or tape streamer device. Instead, you have abstraction layers
which provide a unified character or block device interface to the
application on the one hand, and a interface for hardware-specific
device drivers on the other hand. These abstractions are provided
by subsystems like the tty layer, the audio subsystem or the SCSI
and IDE subsystems for the devices mentioned above.
The easiest way to implement a CAN device driver is as a character
device without such a (complete) abstraction layer, as is done by most
existing drivers. The right way, however, would be to add such a
layer with all the functionality like registering for certain CAN
IDs, supporting several open file descriptors and (de)multiplexing
CAN frames between them, (sophisticated) queueing of CAN frames, and
providing an API for device drivers to register with. However, then
it would be no more difficult, or may be even easier, to use the
networking framework provided by the Linux kernel, and this is what
Socket CAN does.
The use of the networking framework of the Linux kernel is just the
natural and most appropriate way to implement CAN for Linux.
3. Socket CAN concept
---------------------
As described in chapter 2 it is the main goal of Socket CAN to
provide a socket interface to user space applications which builds
upon the Linux network layer. In contrast to the commonly known
TCP/IP and ethernet networking, the CAN bus is a broadcast-only(!)
medium that has no MAC-layer addressing like ethernet. The CAN-identifier
(can_id) is used for arbitration on the CAN-bus. Therefore the CAN-IDs
have to be chosen uniquely on the bus. When designing a CAN-ECU
network the CAN-IDs are mapped to be sent by a specific ECU.
For this reason a CAN-ID can be treated best as a kind of source address.
3.1 receive lists
The network transparent access of multiple applications leads to the
problem that different applications may be interested in the same
CAN-IDs from the same CAN network interface. The Socket CAN core
module - which implements the protocol family CAN - provides several
high efficient receive lists for this reason. If e.g. a user space
application opens a CAN RAW socket, the raw protocol module itself
requests the (range of) CAN-IDs from the Socket CAN core that are
requested by the user. The subscription and unsubscription of
CAN-IDs can be done for specific CAN interfaces or for all(!) known
CAN interfaces with the can_rx_(un)register() functions provided to
CAN protocol modules by the SocketCAN core (see chapter 5).
To optimize the CPU usage at runtime the receive lists are split up
into several specific lists per device that match the requested
filter complexity for a given use-case.
3.2 local loopback of sent frames
As known from other networking concepts the data exchanging
applications may run on the same or different nodes without any
change (except for the according addressing information):
___ ___ ___ _______ ___
| _ | | _ | | _ | | _ _ | | _ |
||A|| ||B|| ||C|| ||A| |B|| ||C||
|___| |___| |___| |_______| |___|
| | | | |
-----------------(1)- CAN bus -(2)---------------
To ensure that application A receives the same information in the
example (2) as it would receive in example (1) there is need for
some kind of local loopback of the sent CAN frames on the appropriate
node.
The Linux network devices (by default) just can handle the
transmission and reception of media dependent frames. Due to the
arbritration on the CAN bus the transmission of a low prio CAN-ID
may be delayed by the reception of a high prio CAN frame. To
reflect the correct* traffic on the node the loopback of the sent
data has to be performed right after a successful transmission. If
the CAN network interface is not capable of performing the loopback for
some reason the SocketCAN core can do this task as a fallback solution.
See chapter 6.2 for details (recommended).
The loopback functionality is enabled by default to reflect standard
networking behaviour for CAN applications. Due to some requests from
the RT-SocketCAN group the loopback optionally may be disabled for each
separate socket. See sockopts from the CAN RAW sockets in chapter 4.1.
* = you really like to have this when you're running analyser tools
like 'candump' or 'cansniffer' on the (same) node.
3.3 network security issues (capabilities)
The Controller Area Network is a local field bus transmitting only
broadcast messages without any routing and security concepts.
In the majority of cases the user application has to deal with
raw CAN frames. Therefore it might be reasonable NOT to restrict
the CAN access only to the user root, as known from other networks.
Since the currently implemented CAN_RAW and CAN_BCM sockets can only
send and receive frames to/from CAN interfaces it does not affect
security of others networks to allow all users to access the CAN.
To enable non-root users to access CAN_RAW and CAN_BCM protocol
sockets the Kconfig options CAN_RAW_USER and/or CAN_BCM_USER may be
selected at kernel compile time.
3.4 network problem notifications
The use of the CAN bus may lead to several problems on the physical
and media access control layer. Detecting and logging of these lower
layer problems is a vital requirement for CAN users to identify
hardware issues on the physical transceiver layer as well as
arbitration problems and error frames caused by the different
ECUs. The occurrence of detected errors are important for diagnosis
and have to be logged together with the exact timestamp. For this
reason the CAN interface driver can generate so called Error Frames
that can optionally be passed to the user application in the same
way as other CAN frames. Whenever an error on the physical layer
or the MAC layer is detected (e.g. by the CAN controller) the driver
creates an appropriate error frame. Error frames can be requested by
the user application using the common CAN filter mechanisms. Inside
this filter definition the (interested) type of errors may be
selected. The reception of error frames is disabled by default.
4. How to use Socket CAN
------------------------
Like TCP/IP, you first need to open a socket for communicating over a
CAN network. Since Socket CAN implements a new protocol family, you
need to pass PF_CAN as the first argument to the socket(2) system
call. Currently, there are two CAN protocols to choose from, the raw
socket protocol and the broadcast manager (BCM). So to open a socket,
you would write
s = socket(PF_CAN, SOCK_RAW, CAN_RAW);
and
s = socket(PF_CAN, SOCK_DGRAM, CAN_BCM);
respectively. After the successful creation of the socket, you would
normally use the bind(2) system call to bind the socket to a CAN
interface (which is different from TCP/IP due to different addressing
- see chapter 3). After binding (CAN_RAW) or connecting (CAN_BCM)
the socket, you can read(2) and write(2) from/to the socket or use
send(2), sendto(2), sendmsg(2) and the recv* counterpart operations
on the socket as usual. There are also CAN specific socket options
described below.
The basic CAN frame structure and the sockaddr structure are defined
in include/linux/can.h:
struct can_frame {
canid_t can_id; /* 32 bit CAN_ID + EFF/RTR/ERR flags */
__u8 can_dlc; /* data length code: 0 .. 8 */
__u8 data[8] __attribute__((aligned(8)));
};
The alignment of the (linear) payload data[] to a 64bit boundary
allows the user to define own structs and unions to easily access the
CAN payload. There is no given byteorder on the CAN bus by
default. A read(2) system call on a CAN_RAW socket transfers a
struct can_frame to the user space.
The sockaddr_can structure has an interface index like the
PF_PACKET socket, that also binds to a specific interface:
struct sockaddr_can {
sa_family_t can_family;
int can_ifindex;
union {
/* transport protocol class address info (e.g. ISOTP) */
struct { canid_t rx_id, tx_id; } tp;
/* reserved for future CAN protocols address information */
} can_addr;
};
To determine the interface index an appropriate ioctl() has to
be used (example for CAN_RAW sockets without error checking):
int s;
struct sockaddr_can addr;
struct ifreq ifr;
s = socket(PF_CAN, SOCK_RAW, CAN_RAW);
strcpy(ifr.ifr_name, "can0" );
ioctl(s, SIOCGIFINDEX, &ifr);
addr.can_family = AF_CAN;
addr.can_ifindex = ifr.ifr_ifindex;
bind(s, (struct sockaddr *)&addr, sizeof(addr));
(..)
To bind a socket to all(!) CAN interfaces the interface index must
be 0 (zero). In this case the socket receives CAN frames from every
enabled CAN interface. To determine the originating CAN interface
the system call recvfrom(2) may be used instead of read(2). To send
on a socket that is bound to 'any' interface sendto(2) is needed to
specify the outgoing interface.
Reading CAN frames from a bound CAN_RAW socket (see above) consists
of reading a struct can_frame:
struct can_frame frame;
nbytes = read(s, &frame, sizeof(struct can_frame));
if (nbytes < 0) {
perror("can raw socket read");
return 1;
}
/* paraniod check ... */
if (nbytes < sizeof(struct can_frame)) {
fprintf(stderr, "read: incomplete CAN frame\n");
return 1;
}
/* do something with the received CAN frame */
Writing CAN frames can be done similarly, with the write(2) system call:
nbytes = write(s, &frame, sizeof(struct can_frame));
When the CAN interface is bound to 'any' existing CAN interface
(addr.can_ifindex = 0) it is recommended to use recvfrom(2) if the
information about the originating CAN interface is needed:
struct sockaddr_can addr;
struct ifreq ifr;
socklen_t len = sizeof(addr);
struct can_frame frame;
nbytes = recvfrom(s, &frame, sizeof(struct can_frame),
0, (struct sockaddr*)&addr, &len);
/* get interface name of the received CAN frame */
ifr.ifr_ifindex = addr.can_ifindex;
ioctl(s, SIOCGIFNAME, &ifr);
printf("Received a CAN frame from interface %s", ifr.ifr_name);
To write CAN frames on sockets bound to 'any' CAN interface the
outgoing interface has to be defined certainly.
strcpy(ifr.ifr_name, "can0");
ioctl(s, SIOCGIFINDEX, &ifr);
addr.can_ifindex = ifr.ifr_ifindex;
addr.can_family = AF_CAN;
nbytes = sendto(s, &frame, sizeof(struct can_frame),
0, (struct sockaddr*)&addr, sizeof(addr));
4.1 RAW protocol sockets with can_filters (SOCK_RAW)
Using CAN_RAW sockets is extensively comparable to the commonly
known access to CAN character devices. To meet the new possibilities
provided by the multi user SocketCAN approach, some reasonable
defaults are set at RAW socket binding time:
- The filters are set to exactly one filter receiving everything
- The socket only receives valid data frames (=> no error frames)
- The loopback of sent CAN frames is enabled (see chapter 3.2)
- The socket does not receive its own sent frames (in loopback mode)
These default settings may be changed before or after binding the socket.
To use the referenced definitions of the socket options for CAN_RAW
sockets, include <linux/can/raw.h>.
4.1.1 RAW socket option CAN_RAW_FILTER
The reception of CAN frames using CAN_RAW sockets can be controlled
by defining 0 .. n filters with the CAN_RAW_FILTER socket option.
The CAN filter structure is defined in include/linux/can.h:
struct can_filter {
canid_t can_id;
canid_t can_mask;
};
A filter matches, when
<received_can_id> & mask == can_id & mask
which is analogous to known CAN controllers hardware filter semantics.
The filter can be inverted in this semantic, when the CAN_INV_FILTER
bit is set in can_id element of the can_filter structure. In
contrast to CAN controller hardware filters the user may set 0 .. n
receive filters for each open socket separately:
struct can_filter rfilter[2];
rfilter[0].can_id = 0x123;
rfilter[0].can_mask = CAN_SFF_MASK;
rfilter[1].can_id = 0x200;
rfilter[1].can_mask = 0x700;
setsockopt(s, SOL_CAN_RAW, CAN_RAW_FILTER, &rfilter, sizeof(rfilter));
To disable the reception of CAN frames on the selected CAN_RAW socket:
setsockopt(s, SOL_CAN_RAW, CAN_RAW_FILTER, NULL, 0);
To set the filters to zero filters is quite obsolete as not read
data causes the raw socket to discard the received CAN frames. But
having this 'send only' use-case we may remove the receive list in the
Kernel to save a little (really a very little!) CPU usage.
4.1.2 RAW socket option CAN_RAW_ERR_FILTER
As described in chapter 3.4 the CAN interface driver can generate so
called Error Frames that can optionally be passed to the user
application in the same way as other CAN frames. The possible
errors are divided into different error classes that may be filtered
using the appropriate error mask. To register for every possible
error condition CAN_ERR_MASK can be used as value for the error mask.
The values for the error mask are defined in linux/can/error.h .
can_err_mask_t err_mask = ( CAN_ERR_TX_TIMEOUT | CAN_ERR_BUSOFF );
setsockopt(s, SOL_CAN_RAW, CAN_RAW_ERR_FILTER,
&err_mask, sizeof(err_mask));
4.1.3 RAW socket option CAN_RAW_LOOPBACK
To meet multi user needs the local loopback is enabled by default
(see chapter 3.2 for details). But in some embedded use-cases
(e.g. when only one application uses the CAN bus) this loopback
functionality can be disabled (separately for each socket):
int loopback = 0; /* 0 = disabled, 1 = enabled (default) */
setsockopt(s, SOL_CAN_RAW, CAN_RAW_LOOPBACK, &loopback, sizeof(loopback));
4.1.4 RAW socket option CAN_RAW_RECV_OWN_MSGS
When the local loopback is enabled, all the sent CAN frames are
looped back to the open CAN sockets that registered for the CAN
frames' CAN-ID on this given interface to meet the multi user
needs. The reception of the CAN frames on the same socket that was
sending the CAN frame is assumed to be unwanted and therefore
disabled by default. This default behaviour may be changed on
demand:
int recv_own_msgs = 1; /* 0 = disabled (default), 1 = enabled */
setsockopt(s, SOL_CAN_RAW, CAN_RAW_RECV_OWN_MSGS,
&recv_own_msgs, sizeof(recv_own_msgs));
4.2 Broadcast Manager protocol sockets (SOCK_DGRAM)
4.3 connected transport protocols (SOCK_SEQPACKET)
4.4 unconnected transport protocols (SOCK_DGRAM)
5. Socket CAN core module
-------------------------
The Socket CAN core module implements the protocol family
PF_CAN. CAN protocol modules are loaded by the core module at
runtime. The core module provides an interface for CAN protocol
modules to subscribe needed CAN IDs (see chapter 3.1).
5.1 can.ko module params
- stats_timer: To calculate the Socket CAN core statistics
(e.g. current/maximum frames per second) this 1 second timer is
invoked at can.ko module start time by default. This timer can be
disabled by using stattimer=0 on the module comandline.
- debug: (removed since SocketCAN SVN r546)
5.2 procfs content
As described in chapter 3.1 the Socket CAN core uses several filter
lists to deliver received CAN frames to CAN protocol modules. These
receive lists, their filters and the count of filter matches can be
checked in the appropriate receive list. All entries contain the
device and a protocol module identifier:
foo@bar:~$ cat /proc/net/can/rcvlist_all
receive list 'rx_all':
(vcan3: no entry)
(vcan2: no entry)
(vcan1: no entry)
device can_id can_mask function userdata matches ident
vcan0 000 00000000 f88e6370 f6c6f400 0 raw
(any: no entry)
In this example an application requests any CAN traffic from vcan0.
rcvlist_all - list for unfiltered entries (no filter operations)
rcvlist_eff - list for single extended frame (EFF) entries
rcvlist_err - list for error frames masks
rcvlist_fil - list for mask/value filters
rcvlist_inv - list for mask/value filters (inverse semantic)
rcvlist_sff - list for single standard frame (SFF) entries
Additional procfs files in /proc/net/can
stats - Socket CAN core statistics (rx/tx frames, match ratios, ...)
reset_stats - manual statistic reset
version - prints the Socket CAN core version and the ABI version
5.3 writing own CAN protocol modules
To implement a new protocol in the protocol family PF_CAN a new
protocol has to be defined in include/linux/can.h .
The prototypes and definitions to use the Socket CAN core can be
accessed by including include/linux/can/core.h .
In addition to functions that register the CAN protocol and the
CAN device notifier chain there are functions to subscribe CAN
frames received by CAN interfaces and to send CAN frames:
can_rx_register - subscribe CAN frames from a specific interface
can_rx_unregister - unsubscribe CAN frames from a specific interface
can_send - transmit a CAN frame (optional with local loopback)
For details see the kerneldoc documentation in net/can/af_can.c or
the source code of net/can/raw.c or net/can/bcm.c .
6. CAN network drivers
----------------------
Writing a CAN network device driver is much easier than writing a
CAN character device driver. Similar to other known network device
drivers you mainly have to deal with:
- TX: Put the CAN frame from the socket buffer to the CAN controller.
- RX: Put the CAN frame from the CAN controller to the socket buffer.
See e.g. at Documentation/networking/netdevices.txt . The differences
for writing CAN network device driver are described below:
6.1 general settings
dev->type = ARPHRD_CAN; /* the netdevice hardware type */
dev->flags = IFF_NOARP; /* CAN has no arp */
dev->mtu = sizeof(struct can_frame);
The struct can_frame is the payload of each socket buffer in the
protocol family PF_CAN.
6.2 local loopback of sent frames
As described in chapter 3.2 the CAN network device driver should
support a local loopback functionality similar to the local echo
e.g. of tty devices. In this case the driver flag IFF_ECHO has to be
set to prevent the PF_CAN core from locally echoing sent frames
(aka loopback) as fallback solution:
dev->flags = (IFF_NOARP | IFF_ECHO);
6.3 CAN controller hardware filters
To reduce the interrupt load on deep embedded systems some CAN
controllers support the filtering of CAN IDs or ranges of CAN IDs.
These hardware filter capabilities vary from controller to
controller and have to be identified as not feasible in a multi-user
networking approach. The use of the very controller specific
hardware filters could make sense in a very dedicated use-case, as a
filter on driver level would affect all users in the multi-user
system. The high efficient filter sets inside the PF_CAN core allow
to set different multiple filters for each socket separately.
Therefore the use of hardware filters goes to the category 'handmade
tuning on deep embedded systems'. The author is running a MPC603e
@133MHz with four SJA1000 CAN controllers from 2002 under heavy bus
load without any problems ...
6.4 currently supported CAN hardware (September 2007)
On the project website http://developer.berlios.de/projects/socketcan
there are different drivers available:
vcan: Virtual CAN interface driver (if no real hardware is available)
sja1000: Philips SJA1000 CAN controller (recommended)
i82527: Intel i82527 CAN controller
mscan: Motorola/Freescale CAN controller (e.g. inside SOC MPC5200)
ccan: CCAN controller core (e.g. inside SOC h7202)
slcan: For a bunch of CAN adaptors that are attached via a
serial line ASCII protocol (for serial / USB adaptors)
Additionally the different CAN adaptors (ISA/PCI/PCMCIA/USB/Parport)
from PEAK Systemtechnik support the CAN netdevice driver model
since Linux driver v6.0: http://www.peak-system.com/linux/index.htm
Please check the Mailing Lists on the berlios OSS project website.
6.5 todo (September 2007)
The configuration interface for CAN network drivers is still an open
issue that has not been finalized in the socketcan project. Also the
idea of having a library module (candev.ko) that holds functions
that are needed by all CAN netdevices is not ready to ship.
Your contribution is welcome.
7. Credits
----------
Oliver Hartkopp (PF_CAN core, filters, drivers, bcm)
Urs Thuermann (PF_CAN core, kernel integration, socket interfaces, raw, vcan)
Jan Kizka (RT-SocketCAN core, Socket-API reconciliation)
Wolfgang Grandegger (RT-SocketCAN core & drivers, Raw Socket-API reviews)
Robert Schwebel (design reviews, PTXdist integration)
Marc Kleine-Budde (design reviews, Kernel 2.6 cleanups, drivers)
Benedikt Spranger (reviews)
Thomas Gleixner (LKML reviews, coding style, posting hints)
Andrey Volkov (kernel subtree structure, ioctls, mscan driver)
Matthias Brukner (first SJA1000 CAN netdevice implementation Q2/2003)
Klaus Hitschler (PEAK driver integration)
Uwe Koppe (CAN netdevices with PF_PACKET approach)
Michael Schulze (driver layer loopback requirement, RT CAN drivers review)
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