Message-ID: <PAVPR10MB720989527F83A44B6CEEE294B15AA@PAVPR10MB7209.EURPRD10.PROD.OUTLOOK.COM>
Date:   Wed, 14 Jun 2023 20:40:29 +0000
From:   HMS Incident Management <Incidentmanagement@....se>
To:     "mkl@...gutronix.de" <mkl@...gutronix.de>,
        "linux-can@...r.kernel.org" <linux-can@...r.kernel.org>,
        "Thomas.Kopp@...rochip.com" <Thomas.Kopp@...rochip.com>
CC:     "socketcan@...tkopp.net" <socketcan@...tkopp.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "marex@...x.de" <marex@...x.de>,
        "simon.horman@...igine.com" <simon.horman@...igine.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mailhol.vincent@...adoo.fr" <mailhol.vincent@...adoo.fr>
Subject: [PATCH v5 3/3] can: length: refactor frame lengths definition to add
 size in bits

**We apologize for the delay in delivering this email, which was caused by a mail incident that occurred over the weekend on June 10th. This email was originally sent from vincent.mailhol@...il.com on 06/11/2023 02:58:17 

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 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 
Signed-off-by: Vincent Mailhol 
Reviewed-by: Thomas Kopp 
---
 drivers/net/can/dev/length.c |  15 +-
 include/linux/can/length.h   | 302 +++++++++++++++++++++++++----------
 2 files changed, 216 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..abc978b38f79 100644
--- a/include/linux/can/length.h
+++ b/include/linux/can/length.h
@@ -1,132 +1,258 @@
 /* SPDX-License-Identifier: GPL-2.0 */
 /* Copyright (C) 2020 Oliver Hartkopp 
  * Copyright (C) 2020 Marc Kleine-Budde 
- * Copyright (C) 2020 Vincent Mailhol 
+ * Copyright (C) 2020, 2023 Vincent Mailhol 
  */
 
 #ifndef _CAN_LENGTH_H
 #define _CAN_LENGTH_H
 
+#include 
 #include 
 #include 
+#include 
 
 /*
- * 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) len. Should be zero for remote frames. No
+ *	sanitization is done on @data_len and it shall have no side
+ *	effects.
+ *
+ * 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

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