[PATCH] can: length: add definitions for frame lengths in bits

[Vincent Mailhol]

When created in [1], frames length definitions were added to implement
byte queue limits (bql). Because bql expects lengths in bytes, bit
length definitions were not considered back then.

Recently, a need to refer to the exact frame length in bits, with CAN
bit stuffing, appeared in [2].

Add 9 frames length definitions:

 - CAN_FRAME_OVERHEAD_SFF_BITS:
 - CAN_FRAME_OVERHEAD_EFF_BITS
 - CANFD_FRAME_OVERHEAD_SFF_BITS
 - CANFD_FRAME_OVERHEAD_EFF_BITS
 - CAN_BIT_STUFFING_OVERHEAD
 - CAN_FRAME_LEN_MAX_BITS_NO_STUFFING
 - CAN_FRAME_LEN_MAX_BITS_STUFFING
 - CANFD_FRAME_LEN_MAX_BITS_NO_STUFFING
 - CANFD_FRAME_LEN_MAX_BITS_STUFFING

CAN_FRAME_LEN_MAX_BITS_STUFFING and CANFD_FRAME_LEN_MAX_BITS_STUFFING
define respectively the maximum number of bits in a classical CAN and
CAN-FD frame including bit stuffing. The other definitions are
intermediate values.

In addition to the above:

 - Include linux/bits.h and then replace the value 8 by BITS_PER_BYTE
   whenever relevant.
 - Include linux/math.h because of DIV_ROUND_UP(). N.B: the use of
   DIV_ROUND_UP() is not new to this patch, but the include was
   previously omitted.
 - Update the existing length definitions to use the newly defined values.
 - Add myself as copyright owner for 2020 (as coauthor of the initial
   version, c.f. [1]) and for 2023 (this patch).

[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>
---
As always, let me know if you have better inspiration than me for the
naming.
---
 include/linux/can/length.h | 84 ++++++++++++++++++++++++++++++++------
 1 file changed, 72 insertions(+), 12 deletions(-)

diff --git a/include/linux/can/length.h b/include/linux/can/length.h
index 6995092b774e..60492fcbe34d 100644
--- a/include/linux/can/length.h
+++ b/include/linux/can/length.h
@@ -1,13 +1,17 @@
 /* SPDX-License-Identifier: GPL-2.0 */
 /* Copyright (C) 2020 Oliver Hartkopp <socketcan@xxxxxxxxxxxx>
  * Copyright (C) 2020 Marc Kleine-Budde <kernel@xxxxxxxxxxxxxx>
+ * Copyright (C) 2020, 2023 Vincent Mailhol <mailhol.vincent@xxxxxxxxxx>
  */
 
 #ifndef _CAN_LENGTH_H
 #define _CAN_LENGTH_H
 
+#include <linux/bits.h>
+#include <linux/math.h>
+
 /*
- * Size of a Classical CAN Standard Frame
+ * Size of a Classical CAN Standard Frame in bits
  *
  * Name of Field			Bits
  * ---------------------------------------------------------
@@ -25,12 +29,19 @@
  * End-of-frame (EOF)			7
  * Inter frame spacing			3
  *
- * rounded up and ignoring bitstuffing
+ * ignoring bitstuffing
  */
-#define CAN_FRAME_OVERHEAD_SFF DIV_ROUND_UP(47, 8)
+#define CAN_FRAME_OVERHEAD_SFF_BITS 47
 
 /*
- * Size of a Classical CAN Extended Frame
+ * Size of a Classical CAN Standard Frame
+ * (rounded up and ignoring bitstuffing)
+ */
+#define CAN_FRAME_OVERHEAD_SFF \
+	DIV_ROUND_UP(CAN_FRAME_OVERHEAD_SFF_BITS, BITS_PER_BYTE)
+
+/*
+ * Size of a Classical CAN Extended Frame in bits
  *
  * Name of Field			Bits
  * ---------------------------------------------------------
@@ -50,12 +61,19 @@
  * End-of-frame (EOF)			7
  * Inter frame spacing			3
  *
- * rounded up and ignoring bitstuffing
+ * ignoring bitstuffing
  */
-#define CAN_FRAME_OVERHEAD_EFF DIV_ROUND_UP(67, 8)
+#define CAN_FRAME_OVERHEAD_EFF_BITS 67
 
 /*
- * Size of a CAN-FD Standard Frame
+ * Size of a Classical CAN Extended Frame
+ * (rounded up and ignoring bitstuffing)
+ */
+#define CAN_FRAME_OVERHEAD_EFF \
+	DIV_ROUND_UP(CAN_FRAME_OVERHEAD_EFF_BITS, BITS_PER_BYTE)
+
+/*
+ * Size of a CAN-FD Standard Frame in bits
  *
  * Name of Field			Bits
  * ---------------------------------------------------------
@@ -77,12 +95,19 @@
  * End-of-frame (EOF)			7
  * Inter frame spacing			3
  *
- * assuming CRC21, rounded up and ignoring bitstuffing
+ * assuming CRC21 and ignoring bitstuffing
  */
-#define CANFD_FRAME_OVERHEAD_SFF DIV_ROUND_UP(61, 8)
+#define CANFD_FRAME_OVERHEAD_SFF_BITS 61
 
 /*
- * Size of a CAN-FD Extended Frame
+ * Size of a CAN-FD Standard Frame
+ * (assuming CRC21, rounded up and ignoring bitstuffing)
+ */
+#define CANFD_FRAME_OVERHEAD_SFF \
+	DIV_ROUND_UP(CANFD_FRAME_OVERHEAD_SFF_BITS, BITS_PER_BYTE)
+
+/*
+ * Size of a CAN-FD Extended Frame in bits
  *
  * Name of Field			Bits
  * ---------------------------------------------------------
@@ -106,9 +131,32 @@
  * End-of-frame (EOF)			7
  * Inter frame spacing			3
  *
- * assuming CRC21, rounded up and ignoring bitstuffing
+ * assuming CRC21 and ignoring bitstuffing
+ */
+#define CANFD_FRAME_OVERHEAD_EFF_BITS 80
+
+/*
+ * Size of a CAN-FD Extended Frame
+ * (assuming CRC21, rounded up and ignoring bitstuffing)
+ */
+#define CANFD_FRAME_OVERHEAD_EFF \
+	DIV_ROUND_UP(CANFD_FRAME_OVERHEAD_EFF_BITS, BITS_PER_BYTE)
+
+/* CAN bit stuffing overhead multiplication factor */
+#define CAN_BIT_STUFFING_OVERHEAD 1.2
+
+/*
+ * Maximum size of a Classical CAN frame in bits, ignoring bitstuffing
  */
-#define CANFD_FRAME_OVERHEAD_EFF DIV_ROUND_UP(80, 8)
+#define CAN_FRAME_LEN_MAX_BITS_NO_STUFFING \
+	(CAN_FRAME_OVERHEAD_EFF_BITS + CAN_MAX_DLEN * BITS_PER_BYTE)
+
+/*
+ * Maximum size of a Classical CAN frame in bits, including bitstuffing
+ */
+#define CAN_FRAME_LEN_MAX_BITS_STUFFING				\
+	((unsigned int)(CAN_FRAME_LEN_MAX_BITS_NO_STUFFING *	\
+			CAN_BIT_STUFFING_OVERHEAD))
 
 /*
  * Maximum size of a Classical CAN frame
@@ -116,6 +164,18 @@
  */
 #define CAN_FRAME_LEN_MAX (CAN_FRAME_OVERHEAD_EFF + CAN_MAX_DLEN)
 
+/*
+ * Maximum size of a CAN-FD frame in bits, ignoring bitstuffing
+ */
+#define CANFD_FRAME_LEN_MAX_BITS_NO_STUFFING \
+	(CANFD_FRAME_OVERHEAD_EFF_BITS + CANFD_MAX_DLEN * BITS_PER_BYTE)
+
+/*
+ * Maximum size of a CAN-FD frame in bits, ignoring bitstuffing
+ */
+#define CANFD_FRAME_LEN_MAX_BITS_STUFFING			\
+	((unsigned int)(CANFD_FRAME_LEN_MAX_BITS_NO_STUFFING *	\
+			CAN_BIT_STUFFING_OVERHEAD))
 /*
  * Maximum size of a CAN-FD frame
  * (rounded up and ignoring bitstuffing)
-- 
2.39.3