Introduce a method to calculate the exact size in bits of a CAN(-FD)
frame with or without dynamic bitsuffing.
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 macro definitions, declare the
can_frame_bits() static inline function. To this extend, do a full
refactoring of the length definitions.
If given integer constant expressions as argument, can_frame_bits()
folds into a compile time constant expression, giving no penalty over
the use of macros.
Also add the can_frame_bytes(). This function 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)
The different frame lengths (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 to be
used in const struct declarations (folding of can_frame_bytes() occurs
too late in the compilation to be used in struct declarations).
In addition to the above:
- Use ISO 11898-1:2015 definitions for the name 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.
[1] commit 85d99c3e2a13 ("can: length: can_skb_get_frame_len(): introduce
function to get data length of frame in data link layer")
Link: https://git.kernel.org/torvalds/c/85d99c3e2a13
[2] RE: [PATCH] can: mcp251xfd: Increase poll timeout
Link: https://lore.kernel.org/linux-can/BL3PR11MB64846C83ACD04E9330B0FE66FB729@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
Signed-off-by: Vincent Mailhol <mailhol.vincent@xxxxxxxxxx>
---
In case you ask, here is the proof that can_frame_bits() folds into
a compile time constant expression.
This file:
#include <linux/can/length.h>
unsigned int canfd_max_len_bitsuffing(void)
{
return can_frame_bytes(true, true, true, CANFD_MAX_DLEN);
}
Produces this assembly code:
0000000000000010 <canfd_max_len_bitsuffing>:
10: f3 0f 1e fa endbr64
14: b8 5c 00 00 00 mov $0x5c,%eax
19: e9 00 00 00 00 jmpq 1e <canfd_max_len_bitsuffing+0xe>
where 0x5c corresponds to the expected value of 92 bytes.
---
drivers/net/can/dev/length.c | 15 +-
include/linux/can/length.h | 326 +++++++++++++++++++++++++----------
2 files changed, 238 insertions(+), 103 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..7518117ee5fc 100644
--- a/include/linux/can/length.h
+++ b/include/linux/can/length.h
@@ -1,132 +1,278 @@
/* 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
+ * ---------------------------------------------------------
+ * Start Of Frame (SOF) 1
+ * Arbitration field:
+ * base ID 11
+ * Remote Transmission Request (RTR) 1
+ * Control field:
+ * IDentifier Extension bit (IDE) 1
+ * FD Format indicatior (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
+ *
+ * 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 indicatior (FDF) 1
+ * Reserved bit (r0) 1
+ * Data length code (DLC) 4
+ *
+ * including all fields preceding the data field, ignoring bitstuffing
+ */
+#define CAN_FRAME_HEADER_EFF_BITS 39
+
+/*
+ * 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
+ * ---------------------------------------------------------
+ * 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)
+ *
+ * Name of Field Bits
+ * ---------------------------------------------------------
+ * Stuff Count 4
+ * CRC Sequence (CRC17) 17
+ * CRC Delimiter 1
+ * Fixed stuff bits 6
+ */
+#define CANFD_FRAME_CRC17_FIELD_BITS 28
+
+/*
+ * Size of a CAN-FD CRC21 Field in bits (length: 20..64)
+ *
+ * Name of Field Bits
+ * ---------------------------------------------------------
+ * 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) Frame footer in 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
+ * End Of Frame (EOF) 7
*
- * rounded up and ignoring bitstuffing
+ * including all fields following the CRC field
*/
-#define CAN_FRAME_OVERHEAD_SFF DIV_ROUND_UP(47, 8)
+#define CAN_FRAME_FOOTER_BITS 9
/*
- * Size of a Classical CAN Extended Frame
- *
- * 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
- *
- * rounded up and ignoring bitstuffing
+ * First part of the Inter Frame Space
+ * (a.k.a. IMF - intermission field)
*/
-#define CAN_FRAME_OVERHEAD_EFF DIV_ROUND_UP(67, 8)
+#define CAN_INTERMISSION_BITS 3
/*
- * Size of a CAN-FD Standard Frame
+ * CAN bit stuffing overhead multiplication factor
*
- * 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
+ * The worst bit stuffing case is a sequence in which dominant and
+ * recessive bits alternate every four bits:
*
- * assuming CRC21, rounded up and ignoring dynamic bitstuffing
+ * 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
+ *
+ * Ignoring the first bit, one stuff bit is added every four bits
+ * giving us an overhead of 1/4 = 0.25.
*/
-#define CANFD_FRAME_OVERHEAD_SFF DIV_ROUND_UP(67, 8)
+#define can_bit_stuffing_overhead(stream_len) mult_frac(stream_len, 1, 4)
+
+/**
+ * can_frame_bits() - Calculate the number of bits in on the wire in a
+ * CAN frame
+ * @is_eff: true: Extended Frame; false: Standard Frame.
+ * @bitstuffing: if true, calculate the bitsuffing worst case, if
+ * false, calculated the bitsuffing best case (no dynamic
+ * bitsuffing). Fixed stuff bits always get included.
+ * @intermission: if and only if true, include the inter frame space
+ * assuming no bus idle (i.e. only the intermission gets added).
+ * @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.
+ */
+static inline
+unsigned int can_frame_bits(bool is_fd, bool is_eff,
+ bool bitstuffing, bool intermission,
+ unsigned int data_len)
+{
+ unsigned int bits;
+
+ if (is_fd) {
+ if (is_eff)
+ bits = CANFD_FRAME_HEADER_EFF_BITS;
+ else
+ bits = CANFD_FRAME_HEADER_SFF_BITS;
+ } else {
+ if (is_eff)
+ bits = CAN_FRAME_HEADER_EFF_BITS;
+ else
+ bits = CAN_FRAME_HEADER_SFF_BITS;
+ }
+
+ bits += data_len * BITS_PER_BYTE;
+
+ if (!is_fd) {
+ bits += CAN_FRAME_CRC_FIELD_BITS;
+ /*
+ * In Classical CAN, bit stuffing applies to all field
+ * from SOF to CRC delimiter
+ */
+ if (bitstuffing)
+ bits += can_bit_stuffing_overhead(bits);
+ } else {
+ /*
+ * In CAN-FD, the fields preceding the CRC field have
+ * dynamic bit stuffing; but the CRC field has fixed
+ * bitstuffing
+ */
+ if (bitstuffing)
+ bits += can_bit_stuffing_overhead(bits - 1);
+ if (data_len <= 16)
+ bits += CANFD_FRAME_CRC17_FIELD_BITS;
+ else
+ bits += CANFD_FRAME_CRC21_FIELD_BITS;
+ }
+
+ bits += CAN_FRAME_FOOTER_BITS;
+
+ if (intermission)
+ bits += CAN_INTERMISSION_BITS;
+
+ return bits;
+}
/*
- * Size of a CAN-FD Extended Frame
- *
- * 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
+ * Number of bytes in a CAN frame
+ * (rounded up, including intermission)
*/
-#define CANFD_FRAME_OVERHEAD_EFF DIV_ROUND_UP(86, 8)
+static inline
+unsigned int can_frame_bytes(bool is_fd, bool is_eff, bool bitstuffing,
+ unsigned int data_len)
+{
+ return 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 \
+ DIV_ROUND_UP(CAN_FRAME_HEADER_EFF_BITS + \
+ CAN_MAX_DLEN * BITS_PER_BYTE + \
+ CAN_FRAME_CRC_FIELD_BITS + \
+ CAN_FRAME_FOOTER_BITS + \
+ CAN_INTERMISSION_BITS, \
+ BITS_PER_BYTE)
/*
* Maximum size of a CAN-FD frame
* (rounded up and ignoring bitstuffing)
*/
-#define CANFD_FRAME_LEN_MAX (CANFD_FRAME_OVERHEAD_EFF + CANFD_MAX_DLEN)
+#define CANFD_FRAME_LEN_MAX \
+ DIV_ROUND_UP(CANFD_FRAME_HEADER_EFF_BITS + \
+ CANFD_MAX_DLEN * BITS_PER_BYTE + \
+ CANFD_FRAME_CRC21_FIELD_BITS \
+ CAN_FRAME_FOOTER_BITS + \
+ CAN_INTERMISSION_BITS, \
+ BITS_PER_BYTE)
/*
* can_cc_dlc2len(value) - convert a given data length code (dlc) of a
--
2.25.1
