[PATCH v4 3/3] can: length: refactor frame lengths definition to add size in bits

From: Vincent Mailhol
Date: Thu Jun 01 2023 - 12:57:36 EST

Introduce a method to calculate the exact size in bits of a CAN(-FD)
frame with or without dynamic bitstuffing.

These are all the possible combinations taken into account:

- Classical CAN or CAN-FD
- Standard or Extended frame format
- CAN-FD CRC17 or CRC21
- Include or not intermission

Instead of doing several individual macro definitions, declare the
can_frame_bits() function-like macro. To this extent, do a full
refactoring of the length definitions.

In addition add the can_frame_bytes(). This function-like macro
replaces the existing macro:

- CAN_FRAME_OVERHEAD_SFF: can_frame_bytes(false, false, 0)
- CAN_FRAME_OVERHEAD_EFF: can_frame_bytes(false, true, 0)
- CANFD_FRAME_OVERHEAD_SFF: can_frame_bytes(true, false, 0)
- CANFD_FRAME_OVERHEAD_EFF: can_frame_bytes(true, true, 0)

Function-like macros were chosen over inline functions because they
can be used to initialize in const struct fields.

The different maximum frame lengths (maximum data length, including
intermission) are as follow:

Frame type bits bytes
-------------------------------------------------------
Classic CAN SFF no bitstuffing 111 14
Classic CAN EFF no bitstuffing 131 17
Classic CAN SFF bitstuffing 135 17
Classic CAN EFF bitstuffing 160 20
CAN-FD SFF no bitstuffing 579 73
CAN-FD EFF no bitstuffing 598 75
CAN-FD SFF bitstuffing 712 89
CAN-FD EFF bitstuffing 736 92

The macro CAN_FRAME_LEN_MAX and CANFD_FRAME_LEN_MAX are kept as an
alias to, respectively, can_frame_bytes(false, true, CAN_MAX_DLEN) and
can_frame_bytes(true, true, CANFD_MAX_DLEN).

In addition to the above:

- Use ISO 11898-1:2015 definitions for the names of the CAN frame
fields.
- Include linux/bits.h for use of BITS_PER_BYTE.
- Include linux/math.h for use of mult_frac() and
DIV_ROUND_UP(). N.B: the use of DIV_ROUND_UP() is not new to this
patch, but the include was previously omitted.
- Add copyright 2023 for myself.

Suggested-by: Thomas Kopp <Thomas.Kopp@xxxxxxxxxxxxx>
Signed-off-by: Vincent Mailhol <mailhol.vincent@xxxxxxxxxx>
Reviewed-by: Thomas Kopp <Thomas.Kopp@xxxxxxxxxxxxx>
---
drivers/net/can/dev/length.c | 15 +-
include/linux/can/length.h | 303 +++++++++++++++++++++++++----------
2 files changed, 217 insertions(+), 101 deletions(-)

diff --git a/drivers/net/can/dev/length.c b/drivers/net/can/dev/length.c
index b48140b1102e..b7f4d76dd444 100644
--- a/drivers/net/can/dev/length.c
+++ b/drivers/net/can/dev/length.c
@@ -78,18 +78,7 @@ unsigned int can_skb_get_frame_len(const struct sk_buff *skb)
else
len = cf->len;

- if (can_is_canfd_skb(skb)) {
- if (cf->can_id & CAN_EFF_FLAG)
- len += CANFD_FRAME_OVERHEAD_EFF;
- else
- len += CANFD_FRAME_OVERHEAD_SFF;
- } else {
- if (cf->can_id & CAN_EFF_FLAG)
- len += CAN_FRAME_OVERHEAD_EFF;
- else
- len += CAN_FRAME_OVERHEAD_SFF;
- }
-
- return len;
+ return can_frame_bytes(can_is_canfd_skb(skb), cf->can_id & CAN_EFF_FLAG,
+ false, len);
}
EXPORT_SYMBOL_GPL(can_skb_get_frame_len);
diff --git a/include/linux/can/length.h b/include/linux/can/length.h
index 521fdbce2d69..637b9322db82 100644
--- a/include/linux/can/length.h
+++ b/include/linux/can/length.h
@@ -1,132 +1,259 @@
/* SPDX-License-Identifier: GPL-2.0 */
/* Copyright (C) 2020 Oliver Hartkopp <socketcan@xxxxxxxxxxxx>
* Copyright (C) 2020 Marc Kleine-Budde <kernel@xxxxxxxxxxxxxx>
- * Copyright (C) 2020 Vincent Mailhol <mailhol.vincent@xxxxxxxxxx>
+ * Copyright (C) 2020, 2023 Vincent Mailhol <mailhol.vincent@xxxxxxxxxx>
*/

#ifndef _CAN_LENGTH_H
#define _CAN_LENGTH_H

+#include <linux/bits.h>
#include <linux/can.h>
#include <linux/can/netlink.h>
+#include <linux/math.h>

/*
- * Size of a Classical CAN Standard Frame
+ * Size of a Classical CAN Standard Frame header in bits
*
- * Name of Field Bits
+ * Name of Field Bits
* ---------------------------------------------------------
- * Start-of-frame 1
- * Identifier 11
- * Remote transmission request (RTR) 1
- * Identifier extension bit (IDE) 1
- * Reserved bit (r0) 1
- * Data length code (DLC) 4
- * Data field 0...64
- * CRC 15
- * CRC delimiter 1
- * ACK slot 1
- * ACK delimiter 1
- * End-of-frame (EOF) 7
- * Inter frame spacing 3
+ * Start Of Frame (SOF) 1
+ * Arbitration field:
+ * base ID 11
+ * Remote Transmission Request (RTR) 1
+ * Control field:
+ * IDentifier Extension bit (IDE) 1
+ * FD Format indicator (FDF) 1
+ * Data Length Code (DLC) 4
+ *
+ * including all fields preceding the data field, ignoring bitstuffing
+ */
+#define CAN_FRAME_HEADER_SFF_BITS 19
+
+/*
+ * Size of a Classical CAN Extended Frame header in bits
*
- * rounded up and ignoring bitstuffing
+ * Name of Field Bits
+ * ---------------------------------------------------------
+ * Start Of Frame (SOF) 1
+ * Arbitration field:
+ * base ID 11
+ * Substitute Remote Request (SRR) 1
+ * IDentifier Extension bit (IDE) 1
+ * ID extension 18
+ * Remote Transmission Request (RTR) 1
+ * Control field:
+ * FD Format indicator (FDF) 1
+ * Reserved bit (r0) 1
+ * Data length code (DLC) 4
+ *
+ * including all fields preceding the data field, ignoring bitstuffing
*/
-#define CAN_FRAME_OVERHEAD_SFF DIV_ROUND_UP(47, 8)
+#define CAN_FRAME_HEADER_EFF_BITS 39

/*
- * Size of a Classical CAN Extended Frame
+ * Size of a CAN-FD Standard Frame in bits
+ *
+ * Name of Field Bits
+ * ---------------------------------------------------------
+ * Start Of Frame (SOF) 1
+ * Arbitration field:
+ * base ID 11
+ * Remote Request Substitution (RRS) 1
+ * Control field:
+ * IDentifier Extension bit (IDE) 1
+ * FD Format indicator (FDF) 1
+ * Reserved bit (res) 1
+ * Bit Rate Switch (BRS) 1
+ * Error Status Indicator (ESI) 1
+ * Data length code (DLC) 4
+ *
+ * including all fields preceding the data field, ignoring bitstuffing
+ */
+#define CANFD_FRAME_HEADER_SFF_BITS 22
+
+/*
+ * Size of a CAN-FD Extended Frame in bits
+ *
+ * Name of Field Bits
+ * ---------------------------------------------------------
+ * Start Of Frame (SOF) 1
+ * Arbitration field:
+ * base ID 11
+ * Substitute Remote Request (SRR) 1
+ * IDentifier Extension bit (IDE) 1
+ * ID extension 18
+ * Remote Request Substitution (RRS) 1
+ * Control field:
+ * FD Format indicator (FDF) 1
+ * Reserved bit (res) 1
+ * Bit Rate Switch (BRS) 1
+ * Error Status Indicator (ESI) 1
+ * Data length code (DLC) 4
+ *
+ * including all fields preceding the data field, ignoring bitstuffing
+ */
+#define CANFD_FRAME_HEADER_EFF_BITS 41
+
+/*
+ * Size of a CAN CRC Field in bits
*
* Name of Field Bits
* ---------------------------------------------------------
- * Start-of-frame 1
- * Identifier A 11
- * Substitute remote request (SRR) 1
- * Identifier extension bit (IDE) 1
- * Identifier B 18
- * Remote transmission request (RTR) 1
- * Reserved bits (r1, r0) 2
- * Data length code (DLC) 4
- * Data field 0...64
- * CRC 15
- * CRC delimiter 1
- * ACK slot 1
- * ACK delimiter 1
- * End-of-frame (EOF) 7
- * Inter frame spacing 3
+ * CRC sequence (CRC15) 15
+ * CRC Delimiter 1
+ *
+ * ignoring bitstuffing
+ */
+#define CAN_FRAME_CRC_FIELD_BITS 16
+
+/*
+ * Size of a CAN-FD CRC17 Field in bits (length: 0..16)
*
- * rounded up and ignoring bitstuffing
+ * Name of Field Bits
+ * ---------------------------------------------------------
+ * Stuff Count 4
+ * CRC Sequence (CRC17) 17
+ * CRC Delimiter 1
+ * Fixed stuff bits 6
*/
-#define CAN_FRAME_OVERHEAD_EFF DIV_ROUND_UP(67, 8)
+#define CANFD_FRAME_CRC17_FIELD_BITS 28

/*
- * Size of a CAN-FD Standard Frame
+ * Size of a CAN-FD CRC21 Field in bits (length: 20..64)
*
* Name of Field Bits
* ---------------------------------------------------------
- * Start-of-frame 1
- * Identifier 11
- * Remote Request Substitution (RRS) 1
- * Identifier extension bit (IDE) 1
- * Flexible data rate format (FDF) 1
- * Reserved bit (r0) 1
- * Bit Rate Switch (BRS) 1
- * Error Status Indicator (ESI) 1
- * Data length code (DLC) 4
- * Data field 0...512
- * Stuff Bit Count (SBC) 4
- * CRC 0...16: 17 20...64:21
- * CRC delimiter (CD) 1
- * Fixed Stuff bits (FSB) 0...16: 6 20...64:7
- * ACK slot (AS) 1
- * ACK delimiter (AD) 1
- * End-of-frame (EOF) 7
- * Inter frame spacing 3
- *
- * assuming CRC21, rounded up and ignoring dynamic bitstuffing
- */
-#define CANFD_FRAME_OVERHEAD_SFF DIV_ROUND_UP(67, 8)
+ * Stuff Count 4
+ * CRC sequence (CRC21) 21
+ * CRC Delimiter 1
+ * Fixed stuff bits 7
+ */
+#define CANFD_FRAME_CRC21_FIELD_BITS 33

/*
- * Size of a CAN-FD Extended Frame
+ * Size of a CAN(-FD) Frame footer in bits
*
* Name of Field Bits
* ---------------------------------------------------------
- * Start-of-frame 1
- * Identifier A 11
- * Substitute remote request (SRR) 1
- * Identifier extension bit (IDE) 1
- * Identifier B 18
- * Remote Request Substitution (RRS) 1
- * Flexible data rate format (FDF) 1
- * Reserved bit (r0) 1
- * Bit Rate Switch (BRS) 1
- * Error Status Indicator (ESI) 1
- * Data length code (DLC) 4
- * Data field 0...512
- * Stuff Bit Count (SBC) 4
- * CRC 0...16: 17 20...64:21
- * CRC delimiter (CD) 1
- * Fixed Stuff bits (FSB) 0...16: 6 20...64:7
- * ACK slot (AS) 1
- * ACK delimiter (AD) 1
- * End-of-frame (EOF) 7
- * Inter frame spacing 3
- *
- * assuming CRC21, rounded up and ignoring dynamic bitstuffing
- */
-#define CANFD_FRAME_OVERHEAD_EFF DIV_ROUND_UP(86, 8)
+ * ACK slot 1
+ * ACK delimiter 1
+ * End Of Frame (EOF) 7
+ *
+ * including all fields following the CRC field
+ */
+#define CAN_FRAME_FOOTER_BITS 9
+
+/*
+ * First part of the Inter Frame Space
+ * (a.k.a. IMF - intermission field)
+ */
+#define CAN_INTERMISSION_BITS 3
+
+/**
+ * can_bitstuffing_len() - Calculate the maximum length with bitstuffing
+ * @destuffed_len: length of a destuffed bit stream
+ *
+ * The worst bit stuffing case is a sequence in which dominant and
+ * recessive bits alternate every four bits:
+ *
+ * Destuffed: 1 1111 0000 1111 0000 1111
+ * Stuffed: 1 1111o 0000i 1111o 0000i 1111o
+ *
+ * Nomenclature
+ *
+ * - "0": dominant bit
+ * - "o": dominant stuff bit
+ * - "1": recessive bit
+ * - "i": recessive stuff bit
+ *
+ * Aside from the first bit, one stuff bit is added every four bits.
+ *
+ * Return: length of the stuffed bit stream in the worst case scenario.
+ */
+#define can_bitstuffing_len(destuffed_len) \
+ (destuffed_len + (destuffed_len - 1) / 4)
+
+#define __can_bitstuffing_len(bitstuffing, destuffed_len) \
+ (bitstuffing ? can_bitstuffing_len(destuffed_len) : \
+ destuffed_len)
+
+#define __can_cc_frame_bits(is_eff, bitstuffing, \
+ intermission, data_len) \
+( \
+ __can_bitstuffing_len(bitstuffing, \
+ (is_eff ? CAN_FRAME_HEADER_EFF_BITS : \
+ CAN_FRAME_HEADER_SFF_BITS) + \
+ data_len * BITS_PER_BYTE + \
+ CAN_FRAME_CRC_FIELD_BITS) + \
+ CAN_FRAME_FOOTER_BITS + \
+ (intermission ? CAN_INTERMISSION_BITS : 0) \
+)
+
+#define __can_fd_frame_bits(is_eff, bitstuffing, \
+ intermission, data_len) \
+( \
+ __can_bitstuffing_len(bitstuffing, \
+ (is_eff ? CANFD_FRAME_HEADER_EFF_BITS : \
+ CANFD_FRAME_HEADER_SFF_BITS) + \
+ data_len * BITS_PER_BYTE) + \
+ (data_len <= 16 ? \
+ CANFD_FRAME_CRC17_FIELD_BITS : \
+ CANFD_FRAME_CRC21_FIELD_BITS) + \
+ CAN_FRAME_FOOTER_BITS + \
+ (intermission ? CAN_INTERMISSION_BITS : 0) \
+)
+
+/**
+ * can_frame_bits() - Calculate the number of bits on the wire in a
+ * CAN frame
+ * @is_fd: true: CAN-FD frame; false: Classical CAN frame.
+ * @is_eff: true: Extended frame; false: Standard frame.
+ * @bitstuffing: true: calculate the bitstuffing worst case; false:
+ * calculate the bitstuffing best case (no dynamic
+ * bitstuffing). CAN-FD's fixed stuff bits are always included.
+ * @intermission: if and only if true, include the inter frame space
+ * assuming no bus idle (i.e. only the intermission). Strictly
+ * speaking, the inter frame space is not part of the
+ * frame. However, it is needed when calculating the delay
+ * between the Start Of Frame of two consecutive frames.
+ * @data_len: length of the data field in bytes. Correspond to
+ * can(fd)_frame->len. Should be zero for remote frames. No
+ * sanitization is done on @data_len.
+ *
+ * Return: the numbers of bits on the wire of a CAN frame.
+ */
+#define can_frame_bits(is_fd, is_eff, bitstuffing, \
+ intermission, data_len) \
+( \
+ is_fd ? __can_fd_frame_bits(is_eff, bitstuffing, \
+ intermission, data_len) : \
+ __can_cc_frame_bits(is_eff, bitstuffing, \
+ intermission, data_len) \
+)
+
+/*
+ * Number of bytes in a CAN frame
+ * (rounded up, including intermission)
+ */
+#define can_frame_bytes(is_fd, is_eff, bitstuffing, data_len) \
+ DIV_ROUND_UP(can_frame_bits(is_fd, is_eff, bitstuffing, \
+ true, data_len), \
+ BITS_PER_BYTE)

/*
* Maximum size of a Classical CAN frame
- * (rounded up and ignoring bitstuffing)
+ * (rounded up, ignoring bitstuffing but including intermission)
*/
-#define CAN_FRAME_LEN_MAX (CAN_FRAME_OVERHEAD_EFF + CAN_MAX_DLEN)
+#define CAN_FRAME_LEN_MAX \
+ can_frame_bytes(false, true, false, CAN_MAX_DLEN)

/*
* Maximum size of a CAN-FD frame
- * (rounded up and ignoring bitstuffing)
+ * (rounded up, ignoring dynamic bitstuffing but including intermission)
*/
-#define CANFD_FRAME_LEN_MAX (CANFD_FRAME_OVERHEAD_EFF + CANFD_MAX_DLEN)
+#define CANFD_FRAME_LEN_MAX \
+ can_frame_bytes(true, true, false, CANFD_MAX_DLEN)

/*
* can_cc_dlc2len(value) - convert a given data length code (dlc) of a
--
2.39.3