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can: update documentation for DLC usage in Classical CAN

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The extension of struct can_frame with the len8_dlc element and the
can_dlc naming issue required an update of the documentation.

Additionally introduce the term 'Classical CAN' which has been established
by CAN in Automation to separate the original CAN2.0 A/B from CAN FD.

Updated some data structures and flags.

Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Link: https://lore.kernel.org/r/20201110101852.1973-7-socketcan@hartkopp.net
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
This commit is contained in:
Oliver Hartkopp 2020-11-10 11:18:50 +01:00 committed by Marc Kleine-Budde
parent 3ab4ce0d6f
commit 75191707c7
1 changed files with 52 additions and 16 deletions
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68
Documentation/networking/can.rst
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@ -228,20 +228,36 @@ 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:
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:
.. code-block:: C
struct can_frame {
canid_t can_id; /* 32 bit CAN_ID + EFF/RTR/ERR flags */
__u8 can_dlc; /* frame payload length in byte (0 .. 8) */
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 __res1; /* 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
@ -260,6 +276,23 @@ PF_PACKET socket, that also binds to a specific interface:
/* 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;
};
@ -371,7 +404,7 @@ 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 :ref:`socketcan-rawfd`).
and Classical CAN frames simultaneously (see :ref:`socketcan-rawfd`).
The struct canfd_frame is defined in include/linux/can.h:
@ -397,7 +430,7 @@ 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.
can_frame.len 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 :ref:`socketcan-can-fd-driver`.
@ -407,7 +440,7 @@ definitions are specified for CAN specific MTUs in include/linux/can.h:
.. code-block:: C
#define CAN_MTU (sizeof(struct can_frame)) == 16 => 'legacy' CAN frame
#define CAN_MTU (sizeof(struct can_frame)) == 16 => Classical CAN frame
#define CANFD_MTU (sizeof(struct canfd_frame)) == 72 => CAN FD frame
@ -609,7 +642,7 @@ Example:
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);
printf("got Classical CAN frame with length %d\n", cfd.len);
/* cfd.flags is undefined */
} else {
fprintf(stderr, "read: invalid CAN(FD) frame\n");
@ -623,7 +656,7 @@ Example:
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
been received from the socket a Classical 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.
@ -633,7 +666,7 @@ 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
socket option returns an error: No problem. You'll get Classical 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
@ -842,6 +875,8 @@ TX_RESET_MULTI_IDX:
RX_RTR_FRAME:
Send reply for RTR-request (placed in op->frames[0]).
CAN_FD_FRAME:
The CAN frames following the bcm_msg_head are struct canfd_frame's
Broadcast Manager Transmission Timers
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@ -1026,7 +1061,7 @@ Additional procfs files in /proc/net/can::
stats - SocketCAN core statistics (rx/tx frames, match ratios, ...)
reset_stats - manual statistic reset
version - prints the SocketCAN core version and the ABI version
version - prints SocketCAN core and ABI version (removed in Linux 5.10)
Writing Own CAN Protocol Modules
@ -1070,7 +1105,7 @@ General Settings
dev->type = ARPHRD_CAN; /* the netdevice hardware type */
dev->flags = IFF_NOARP; /* CAN has no arp */
dev->mtu = CAN_MTU; /* sizeof(struct can_frame) -> legacy CAN interface */
dev->mtu = CAN_MTU; /* sizeof(struct can_frame) -> Classical CAN interface */
or alternative, when the controller supports CAN with flexible data rate:
dev->mtu = CANFD_MTU; /* sizeof(struct canfd_frame) -> CAN FD interface */
@ -1184,6 +1219,7 @@ Setting CAN device properties::
[ fd { on | off } ]
[ fd-non-iso { on | off } ]
[ presume-ack { on | off } ]
[ cc-len8-dlc { on | off } ]
[ restart-ms TIME-MS ]
[ restart ]
@ -1326,10 +1362,10 @@ arbitration phase and the payload phase of the CAN FD frame. Therefore a
second bit timing 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
payload. The representation of this length in can_frame.len 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
The data length code was a 1:1 mapping to the payload length in the Classical
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_fd_dlc2len() and can_fd_len2dlc().
@ -1337,11 +1373,11 @@ functions can_fd_dlc2len() and can_fd_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 = 16 (CAN_MTU) => sizeof(struct can_frame) => Classical 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.
N.B. CAN FD capable devices can also handle and send Classical CAN frames.
When configuring CAN FD capable CAN controllers an additional 'data' bitrate
has to be set. This bitrate for the data phase of the CAN FD frame has to be