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diff --git a/Documentation/networking/can.txt b/Documentation/networking/can.txt new file mode 100644 index 000000000000..f1b2de170929 --- /dev/null +++ b/Documentation/networking/can.txt @@ -0,0 +1,629 @@ +============================================================================ + +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 { + struct { canid_t rx_id, tx_id; } tp16; + struct { canid_t rx_id, tx_id; } tp20; + struct { canid_t rx_id, tx_id; } mcnet; + struct { canid_t rx_id, tx_id; } isotp; + } 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) |
