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-rw-r--r--Documentation/networking/can.txt186
1 files changed, 162 insertions, 24 deletions
diff --git a/Documentation/networking/can.txt b/Documentation/networking/can.txt
index ac295399f0d..820f55344ed 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
@@ -232,16 +234,16 @@ solution for a couple of reasons:
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
+ 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 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.
- The format of the CAN error frame is briefly described in the Linux
- header file "include/linux/can/error.h".
+ 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/linux/can/error.h".
4. How to use Socket CAN
------------------------
@@ -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
@@ -383,7 +430,7 @@ solution for a couple of reasons:
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 socket only receives valid data frames (=> no error message 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)
@@ -434,7 +481,7 @@ solution for a couple of reasons:
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
+ called Error Message 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
@@ -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:
@@ -527,7 +636,7 @@ solution for a couple of reasons:
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_err - list for error message 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
@@ -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
@@ -784,15 +896,41 @@ solution for a couple of reasons:
$ ip link set canX type can restart-ms 100
Alternatively, the application may realize the "bus-off" condition
- by monitoring CAN error frames and do a restart when appropriate with
- the command:
+ by monitoring CAN error message frames and do a restart when
+ appropriate with the command:
$ ip link set canX type can restart
- Note that a restart will also create a CAN error frame (see also
- chapter 3.4).
+ Note that a restart will also create a CAN error message frame (see
+ also chapter 3.4).
+
+ 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.
- 6.6 Supported CAN hardware
+ 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