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can: add optional DLC element to Classical CAN frame structure

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ISO 11898-1 Chapter 8.4.2.3 defines a 4 bit data length code (DLC) table which
maps the DLC to the payload length of the CAN frame in bytes:

    DLC      ->  payload length
    0 .. 8   ->  0 .. 8
    9 .. 15  ->  8

Although the DLC values 8 .. 15 in Classical CAN always result in a payload
length of 8 bytes these DLC values are transparently transmitted on the CAN
bus. As the struct can_frame only provides a 'len' element (formerly 'can_dlc')
which contains the plain payload length ( 0 .. 8 ) of the CAN frame, the raw
DLC is not visible to the application programmer, e.g. for testing use-cases.

To access the raw DLC values 9 .. 15 the len8_dlc element is introduced, which
is only valid when the payload length 'len' is 8 and the DLC is greater than 8.

The len8_dlc element is filled by the CAN interface driver and used for CAN
frame creation by the CAN driver when the CAN_CTRLMODE_CC_LEN8_DLC flag is
supported by the driver and enabled via netlink configuration interface.

Reported-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>
Link: https://lore.kernel.org/r/20201110101852.1973-2-socketcan@hartkopp.net
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
This commit is contained in:
Oliver Hartkopp 2020-11-10 11:18:45 +01:00 committed by Marc Kleine-Budde
parent b7d3c0e593
commit ea7800565a
2 changed files with 25 additions and 14 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

24
include/uapi/linux/can.h
View File

@ -84,6 +84,7 @@ typedef __u32 can_err_mask_t;
/* CAN payload length and DLC definitions according to ISO 11898-1 */
#define CAN_MAX_DLC 8
#define CAN_MAX_RAW_DLC 15
#define CAN_MAX_DLEN 8
/* CAN FD payload length and DLC definitions according to ISO 11898-7 */
@ -91,22 +92,31 @@ typedef __u32 can_err_mask_t;
#define CANFD_MAX_DLEN 64
/**
* struct can_frame - basic CAN frame structure
* struct can_frame - Classical CAN frame structure (aka CAN 2.0B)
* @can_id: CAN ID of the frame and CAN_*_FLAG flags, see canid_t definition
* @can_dlc: frame payload length in byte (0 .. 8) aka data length code
* N.B. the DLC field from ISO 11898-1 Chapter 8.4.2.3 has a 1:1
* mapping of the 'data length code' to the real payload length
* @len: CAN frame payload length in byte (0 .. 8)
* @can_dlc: deprecated name for CAN frame payload length in byte (0 .. 8)
* @__pad: padding
* @__res0: reserved / padding
* @__res1: reserved / padding
* @len8_dlc: optional DLC value (9 .. 15) at 8 byte payload length
* len8_dlc contains values from 9 .. 15 when the payload length is
* 8 bytes but the DLC value (see ISO 11898-1) is greater then 8.
* CAN_CTRLMODE_CC_LEN8_DLC flag has to be enabled in CAN driver.
* @data: CAN frame payload (up to 8 byte)
*/
struct can_frame {
canid_t can_id; /* 32 bit CAN_ID + EFF/RTR/ERR flags */
__u8 can_dlc; /* frame payload length in byte (0 .. CAN_MAX_DLEN) */
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[CAN_MAX_DLEN] __attribute__((aligned(8)));
};

1
include/uapi/linux/can/netlink.h
View File

@ -100,6 +100,7 @@ struct can_ctrlmode {
#define CAN_CTRLMODE_FD 0x20 /* CAN FD mode */
#define CAN_CTRLMODE_PRESUME_ACK 0x40 /* Ignore missing CAN ACKs */
#define CAN_CTRLMODE_FD_NON_ISO 0x80 /* CAN FD in non-ISO mode */
#define CAN_CTRLMODE_CC_LEN8_DLC 0x100 /* Classic CAN DLC option */
/*
* CAN device statistics