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canfd: update documentation according to CAN FD extensions 

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
This commit is contained in:
Oliver Hartkopp 2012-06-16 12:01:58 +02:00 committed by Marc Kleine-Budde
parent 41052ef6df
commit ea53fe0c66
1 changed files with 146 additions and 8 deletions
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154
Documentation/networking/can.txt
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@ -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