diff options
| -rw-r--r-- | Documentation/networking/can.txt | 154 |
1 files changed, 146 insertions, 8 deletions
diff --git a/Documentation/networking/can.txt b/Documentation/networking/can.txt index a067418..820f553 100644 --- a/Documentation/networking/can.txt +++ b/Documentation/networking/can.txt @@ -22,7 +22,8 @@ This file contains 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 - 4.1.5 RAW socket returned message flags + 4.1.5 RAW socket option CAN_RAW_FD_FRAMES + 4.1.6 RAW socket returned message flags 4.2 Broadcast Manager protocol sockets (SOCK_DGRAM) 4.3 connected transport protocols (SOCK_SEQPACKET) 4.4 unconnected transport protocols (SOCK_DGRAM) @@ -41,7 +42,8 @@ This file contains 6.5.1 Netlink interface to set/get devices properties 6.5.2 Setting the CAN bit-timing 6.5.3 Starting and stopping the CAN network device - 6.6 supported CAN hardware + 6.6 CAN FD (flexible data rate) driver support + 6.7 supported CAN hardware 7 Socket CAN resources @@ -273,7 +275,7 @@ solution for a couple of reasons: struct can_frame { canid_t can_id; /* 32 bit CAN_ID + EFF/RTR/ERR flags */ - __u8 can_dlc; /* data length code: 0 .. 8 */ + __u8 can_dlc; /* frame payload length in byte (0 .. 8) */ __u8 data[8] __attribute__((aligned(8))); }; @@ -375,6 +377,51 @@ solution for a couple of reasons: nbytes = sendto(s, &frame, sizeof(struct can_frame), 0, (struct sockaddr*)&addr, sizeof(addr)); + 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 (legacy) CAN frames simultaneously (see section 4.1.5). + + 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))); + }; + + The struct canfd_frame and the existing struct can_frame have the can_id, + the payload length and the payload data at the same offset inside their + structures. This allows to handle the different structures very similar. + When the content of a struct can_frame is copied into a struct canfd_frame + all structure elements can be used as-is - only the data[] becomes extended. + + When introducing the struct canfd_frame it turned out that the data length + code (DLC) of the struct can_frame was used as a length information as the + length and the DLC has a 1:1 mapping in the range of 0 .. 8. To preserve + the easy handling of the length information the canfd_frame.len element + contains a plain length value from 0 .. 64. So both canfd_frame.len and + can_frame.can_dlc are equal and contain a length information and no DLC. + For details about the distinction of CAN and CAN FD capable devices and + the mapping to the bus-relevant data length code (DLC), see chapter 6.6. + + The length of the two CAN(FD) frame structures define the maximum transfer + unit (MTU) of the CAN(FD) network interface and skbuff data length. Two + definitions are specified for CAN specific MTUs in include/linux/can.h : + + #define CAN_MTU (sizeof(struct can_frame)) == 16 => 'legacy' CAN frame + #define CANFD_MTU (sizeof(struct canfd_frame)) == 72 => CAN FD frame + 4.1 RAW protocol sockets with can_filters (SOCK_RAW) Using CAN_RAW sockets is extensively comparable to the commonly @@ -472,7 +519,69 @@ solution for a couple of reasons: setsockopt(s, SOL_CAN_RAW, CAN_RAW_RECV_OWN_MSGS, &recv_own_msgs, sizeof(recv_own_msgs)); - 4.1.5 RAW socket returned message flags + 4.1.5 RAW socket option CAN_RAW_FD_FRAMES + + CAN FD support in CAN_RAW sockets can be enabled with a new socket option + CAN_RAW_FD_FRAMES which is off by default. When the new socket option is + not supported by the CAN_RAW socket (e.g. on older kernels), switching the + CAN_RAW_FD_FRAMES option returns the error -ENOPROTOOPT. + + Once CAN_RAW_FD_FRAMES is enabled the application can send both CAN frames + and CAN FD frames. OTOH the application has to handle CAN and CAN FD frames + when reading from the socket. + + CAN_RAW_FD_FRAMES enabled: CAN_MTU and CANFD_MTU are allowed + CAN_RAW_FD_FRAMES disabled: only CAN_MTU is allowed (default) + + Example: + [ remember: CANFD_MTU == sizeof(struct canfd_frame) ] + + struct canfd_frame cfd; + + nbytes = read(s, &cfd, CANFD_MTU); + + if (nbytes == CANFD_MTU) { + printf("got CAN FD frame with length %d\n", cfd.len); + /* cfd.flags contains valid data */ + } else if (nbytes == CAN_MTU) { + printf("got legacy CAN frame with length %d\n", cfd.len); + /* cfd.flags is undefined */ + } else { + fprintf(stderr, "read: invalid CAN(FD) frame\n"); + return 1; + } + + /* the content can be handled independently from the received MTU size */ + + printf("can_id: %X data length: %d data: ", cfd.can_id, cfd.len); + for (i = 0; i < cfd.len; i++) + printf("%02X ", cfd.data[i]); + + When reading with size CANFD_MTU only returns CAN_MTU bytes that have + been received from the socket a legacy CAN frame has been read into the + provided CAN FD structure. Note that the canfd_frame.flags data field is + not specified in the struct can_frame and therefore it is only valid in + CANFD_MTU sized CAN FD frames. + + As long as the payload length is <=8 the received CAN frames from CAN FD + capable CAN devices can be received and read by legacy sockets too. When + user-generated CAN FD frames have a payload length <=8 these can be send + by legacy CAN network interfaces too. Sending CAN FD frames with payload + length > 8 to a legacy CAN network interface returns an -EMSGSIZE error. + + Implementation hint for new CAN applications: + + To build a CAN FD aware application use struct canfd_frame as basic CAN + data structure for CAN_RAW based applications. When the application is + executed on an older Linux kernel and switching the CAN_RAW_FD_FRAMES + socket option returns an error: No problem. You'll get legacy CAN frames + or CAN FD frames and can process them the same way. + + When sending to CAN devices make sure that the device is capable to handle + CAN FD frames by checking if the device maximum transfer unit is CANFD_MTU. + The CAN device MTU can be retrieved e.g. with a SIOCGIFMTU ioctl() syscall. + + 4.1.6 RAW socket returned message flags When using recvmsg() call, the msg->msg_flags may contain following flags: @@ -573,10 +682,13 @@ solution for a couple of reasons: dev->type = ARPHRD_CAN; /* the netdevice hardware type */ dev->flags = IFF_NOARP; /* CAN has no arp */ - dev->mtu = sizeof(struct can_frame); + dev->mtu = CAN_MTU; /* sizeof(struct can_frame) -> legacy CAN interface */ - The struct can_frame is the payload of each socket buffer in the - protocol family PF_CAN. + or alternative, when the controller supports CAN with flexible data rate: + dev->mtu = CANFD_MTU; /* sizeof(struct canfd_frame) -> CAN FD interface */ + + The struct can_frame or struct canfd_frame is the payload of each socket + buffer (skbuff) in the protocol family PF_CAN. 6.2 local loopback of sent frames @@ -792,7 +904,33 @@ solution for a couple of reasons: Note that a restart will also create a CAN error message frame (see also chapter 3.4). - 6.6 Supported CAN hardware + 6.6 CAN FD (flexible data rate) driver support + + CAN FD capable CAN controllers support two different bitrates for the + arbitration phase and the payload phase of the CAN FD frame. Therefore a + second bittiming has to be specified in order to enable the CAN FD bitrate. + + Additionally CAN FD capable CAN controllers support up to 64 bytes of + payload. The representation of this length in can_frame.can_dlc and + canfd_frame.len for userspace applications and inside the Linux network + layer is a plain value from 0 .. 64 instead of the CAN 'data length code'. + The data length code was a 1:1 mapping to the payload length in the legacy + CAN frames anyway. The payload length to the bus-relevant DLC mapping is + only performed inside the CAN drivers, preferably with the helper + functions can_dlc2len() and can_len2dlc(). + + The CAN netdevice driver capabilities can be distinguished by the network + devices maximum transfer unit (MTU): + + MTU = 16 (CAN_MTU) => sizeof(struct can_frame) => 'legacy' CAN device + MTU = 72 (CANFD_MTU) => sizeof(struct canfd_frame) => CAN FD capable device + + The CAN device MTU can be retrieved e.g. with a SIOCGIFMTU ioctl() syscall. + N.B. CAN FD capable devices can also handle and send legacy CAN frames. + + FIXME: Add details about the CAN FD controller configuration when available. + + 6.7 Supported CAN hardware Please check the "Kconfig" file in "drivers/net/can" to get an actual list of the support CAN hardware. On the Socket CAN project website |
