can: add optional DLC element to Classical CAN frame structure

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>

Author: hartkopp

include/uapi/linux/can.h

@@ -84,30 +84,40 @@ 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 */
 #define CANFD_MAX_DLC 15
 #define CANFD_MAX_DLEN 64
 
 /**
- * struct can_frame - basic CAN frame structure
- * @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
- * @__pad:   padding
- * @__res0:  reserved / padding
- * @__res1:  reserved / padding
- * @data:    CAN frame payload (up to 8 byte)
+ * 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
+ * @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
+ * @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) */
-	__u8    __pad;   /* padding */
-	__u8    __res0;  /* reserved / padding */
-	__u8    __res1;  /* reserved / padding */
-	__u8    data[CAN_MAX_DLEN] __attribute__((aligned(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 len8_dlc; /* optional DLC for 8 byte payload length (9 .. 15) */
+	__u8 data[CAN_MAX_DLEN] __attribute__((aligned(8)));
 };
 
 /*

include/uapi/linux/can/netlink.h

@@ -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