Linux socketCan官方文档

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https://docs.kernel.org/networking/can.html

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Overview / What is SocketCAN

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, SocketCAN 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.

Motivation / Why Using the Socket API

There have been CAN implementations for Linux before SocketCAN 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.

旧式can的实现存在的问题：

1. 各个can的实现提供的功能相对较少。
2. 用户需要在应用层对can数据的排队和解析做工作。
3. 如果更换can设备，又要换对应的驱动，这是一个负担。
4. 很多驱动只支持单进程打开设备。（意思是同一个时间只能有一个can的连接？）

SocketCAN 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, enabling 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.

socketCAN尝试解决上面的问题，在网络层对can设备进行兼容，并提供数据排队功能。

1. can设备作为一个网络设备注册，数据通过网络层来传输
2. 实现一个抽象的，可以动态加载的协议模块。这样我们可以连接多个can设备，然后根据can ID来加载需要的协议模块。

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 SocketCAN does.

The use of the networking framework of the Linux kernel is just the natural and most appropriate way to implement CAN for Linux.

SocketCAN Concept

As described in Motivation / Why Using the Socket API the main goal of SocketCAN is 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.

CAN总线是广播发消息的，并且不像网络一样有MAC地址。
CAN-ID是CAN总线用来做仲裁用的，必须唯一。
CAN-ID 是用来表示can设备的最佳选择

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 SocketCAN 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 SocketCAN 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 SocketCAN Core Module). 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.

通过订阅方式，一个CAN设备可以被多个APP打开？

每个设备可能有几个收队列，以满足过滤等用户级需求。

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 arbitration 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 [1] 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 Local Loopback of Sent Frames 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 RAW Protocol Sockets with can_filters (SOCK_RAW).

[1] you really like to have this when you’re running analyser tools like ‘candump’ or ‘cansniffer’ on the (same) node.

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 Message 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 message frame. Error messages 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 messages is disabled by default. The format of the CAN error message frame is briefly described in the Linux header file “include/uapi/linux/can/error.h”.

How to use SocketCAN¶

Like TCP/IP, you first need to open a socket for communicating over a CAN network. Since SocketCAN 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);

can有两种通信方式：1 字节流形式的 2 广播性质

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 SocketCAN Concept). 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 Classical CAN frame structure (aka CAN 2.0B), the CAN FD 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 */
        union {
                /* CAN frame payload length in byte (0 .. CAN_MAX_DLEN)
                 * was previously named can_dlc so we need to carry that
                 * name for legacy support
                 */
                __u8 len;
                __u8 can_dlc; /* deprecated */
        };
        __u8    __pad;   /* padding */
        __u8    __res0;  /* reserved / padding */
        __u8    len8_dlc; /* optional DLC for 8 byte payload length (9 .. 15) */
        __u8    data[8] __attribute__((aligned(8)));
};

Remark: The len element contains the payload length in bytes and should be used instead of can_dlc. The deprecated can_dlc was misleadingly named as it always contained the plain payload length in bytes and not the so called ‘data length code’ (DLC).

To pass the raw DLC from/to a Classical CAN network device the len8_dlc element can contain values 9 … 15 when the len element is 8 (the real payload length for all DLC values greater or equal to 8).

The alignment of the (linear) payload data[] to a 64bit boundary allows the user to define their 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;

                /* J1939 address information */
                struct {
                        /* 8 byte name when using dynamic addressing */
                        __u64 name;

                        /* pgn:
                         * 8 bit: PS in PDU2 case, else 0
                         * 8 bit: PF
                         * 1 bit: DP
                         * 1 bit: reserved
                         */
                        __u32 pgn;

                        /* 1 byte address */
                        __u8 addr;
                } j1939;

                /* 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;
}

/* paranoid 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));

An accurate timestamp can be obtained with an ioctl(2) call after reading a message from the socket:

struct timeval tv;
ioctl(s, SIOCGSTAMP, &tv);

The timestamp has a resolution of one microsecond and is set automatically at the reception of a CAN frame.

Remark about CAN FD (flexible data rate) support:

Generally the handling of CAN FD is very similar to the formerly described examples. The new CAN FD capable CAN controllers support two different bitrates for the arbitration phase and the payload phase of the CAN FD frame and up to 64 bytes of payload. This extended payload length breaks all the kernel interfaces (ABI) which heavily rely on the CAN frame with fixed eight bytes of payload (struct can_frame) like the CAN_RAW socket. Therefore e.g. the CAN_RAW socket supports a new socket option CAN_RAW_FD_FRAMES that switches the socket into a mode that allows the handling of CAN FD frames and Classical CAN frames simultaneously (see RAW Socket Option CAN_RAW_FD_FRAMES).

The struct canfd_frame is defined in include/linux/can.h:

struct canfd_frame {
        canid_t can_id;  /* 32 bit CAN_ID + EFF/RTR/ERR flags */
        __u8    len;     /* frame payload length in byte (0 .. 64) */
        __u8    flags;   /* additional flags for CAN FD */
        __u8    __res0;  /* reserved / padding */
        __u8    __res1;  /* reserved / padding */
        __u8    data[64] __attribute__((aligned(8)));
};

RAW Protocol Sockets with can_filters (SOCK_RAW)

Broadcast Manager Protocol Sockets (SOCK_DGRAM)

Connected Transport Protocols (SOCK_SEQPACKET)

(to be written)

Unconnected Transport Protocols (SOCK_DGRAM)

(to be written)