[net-next 10/13] can: length: can_skb_get_frame_len(): introduce function to get data length of frame in data link layer

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From: Vincent Mailhol <mailhol.vincent@xxxxxxxxxx>

This patch adds the function can_skb_get_frame_len() which returns the length
of a CAN frame on the data link layer, including Start-of-frame, Identifier,
various other bits, the actual data, the CRC, the End-of-frame, the Inter frame
spacing.

Co-developed-by: Arunachalam Santhanam <arunachalam.santhanam@xxxxxxxxxxxx>
Not-Signed-off-by: Arunachalam Santhanam <arunachalam.santhanam@xxxxxxxxxxxx>
Co-developed-by: Vincent Mailhol <mailhol.vincent@xxxxxxxxxx>
Not-Signed-off-by: Vincent Mailhol <mailhol.vincent@xxxxxxxxxx>
Co-developed-by: Marc Kleine-Budde <mkl@xxxxxxxxxxxxxx>
Signed-off-by: Marc Kleine-Budde <mkl@xxxxxxxxxxxxxx>
---
 drivers/net/can/dev/length.c |  50 +++++++++++++++
 include/linux/can/length.h   | 121 +++++++++++++++++++++++++++++++++++
 2 files changed, 171 insertions(+)

diff --git a/drivers/net/can/dev/length.c b/drivers/net/can/dev/length.c
index b90d30858935..5b9fa51a3ca3 100644
--- a/drivers/net/can/dev/length.c
+++ b/drivers/net/can/dev/length.c
@@ -40,3 +40,53 @@ u8 can_fd_len2dlc(u8 len)
 	return len2dlc[len];
 }
 EXPORT_SYMBOL_GPL(can_fd_len2dlc);
+
+/**
+ * can_skb_get_frame_len() - Calculate the CAN Frame length in bytes
+ * 	of a given skb.
+ * @skb: socket buffer of a CAN message.
+ *
+ * Do a rough calculation: bit stuffing is ignored and length in bits
+ * is rounded up to a length in bytes.
+ *
+ * Rationale: this function is to be used for the BQL functions
+ * (netdev_sent_queue() and netdev_completed_queue()) which expect a
+ * value in bytes. Just using skb->len is insufficient because it will
+ * return the constant value of CAN(FD)_MTU. Doing the bit stuffing
+ * calculation would be too expensive in term of computing resources
+ * for no noticeable gain.
+ *
+ * Remarks: The payload of CAN FD frames with BRS flag are sent at a
+ * different bitrate. Currently, the can-utils canbusload tool does
+ * not support CAN-FD yet and so we could not run any benchmark to
+ * measure the impact. There might be possible improvement here.
+ *
+ * Return: length in bytes.
+ */
+unsigned int can_skb_get_frame_len(const struct sk_buff *skb)
+{
+	const struct canfd_frame *cf = (const struct canfd_frame *)skb->data;
+	u8 len;
+
+	if (can_is_canfd_skb(skb))
+		len = canfd_sanitize_len(cf->len);
+	else if (cf->can_id & CAN_RTR_FLAG)
+		len = 0;
+	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;
+}
+EXPORT_SYMBOL_GPL(can_skb_get_frame_len);
diff --git a/include/linux/can/length.h b/include/linux/can/length.h
index 0d796f96266b..7d8affcc3d9f 100644
--- a/include/linux/can/length.h
+++ b/include/linux/can/length.h
@@ -3,11 +3,129 @@
  * Copyright (C) 2006 Andrey Volkov <avolkov@xxxxxxxxxxxx>
  *               Varma Electronics Oy
  * Copyright (C) 2008 Wolfgang Grandegger <wg@xxxxxxxxxxxxxx>
+ * Copyright (C) 2020 Marc Kleine-Budde <kernel@xxxxxxxxxxxxxx>
  */
 
 #ifndef _CAN_LENGTH_H
 #define _CAN_LENGTH_H
 
+/*
+ * Size of a Classical CAN Standard Frame
+ *
+ * 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
+ *
+ * rounded up and ignoring bitstuffing
+ */
+#define CAN_FRAME_OVERHEAD_SFF DIV_ROUND_UP(47, 8)
+
+/*
+ * 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
+ */
+#define CAN_FRAME_OVERHEAD_EFF DIV_ROUND_UP(67, 8)
+
+/*
+ * Size of a CAN-FD Standard Frame
+ *
+ * Name of Field			Bits
+ * ---------------------------------------------------------
+ * Start-of-frame			1
+ * Identifier				11
+ * Reserved bit (r1)			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)		0...16: 4 20...64:5
+ * CRC					0...16: 17 20...64:21
+ * CRC delimiter (CD)			1
+ * ACK slot (AS)			1
+ * ACK delimiter (AD)			1
+ * End-of-frame (EOF)			7
+ * Inter frame spacing			3
+ *
+ * assuming CRC21, rounded up and ignoring bitstuffing
+ */
+#define CANFD_FRAME_OVERHEAD_SFF DIV_ROUND_UP(61, 8)
+
+/*
+ * 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
+ * Reserved bit (r1)			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)		0...16: 4 20...64:5
+ * CRC					0...16: 17 20...64:21
+ * CRC delimiter (CD)			1
+ * ACK slot (AS)			1
+ * ACK delimiter (AD)			1
+ * End-of-frame (EOF)			7
+ * Inter frame spacing			3
+ *
+ * assuming CRC21, rounded up and ignoring bitstuffing
+ */
+#define CANFD_FRAME_OVERHEAD_EFF DIV_ROUND_UP(80, 8)
+
+/*
+ * Maximum size of a Classical CAN frame
+ * (rounded up and ignoring bitstuffing)
+ */
+#define CAN_FRAME_LEN_MAX (CAN_FRAME_OVERHEAD_EFF + CAN_MAX_DLEN)
+
+/*
+ * Maximum size of a CAN-FD frame (rough estimation because
+ * ES58X_SFF_BYTES() and ES58X_EFF_BYTES() macros are using the
+ * constant values for Classical CAN, not CAN-FD).
+ */
+#define CANFD_FRAME_LEN_MAX (CANFD_FRAME_OVERHEAD_EFF + CANFD_MAX_DLEN)
+
 /*
  * can_cc_dlc2len(value) - convert a given data length code (dlc) of a
  * Classical CAN frame into a valid data length of max. 8 bytes.
@@ -48,6 +166,9 @@ u8 can_fd_dlc2len(u8 dlc);
 /* map the sanitized data length to an appropriate data length code */
 u8 can_fd_len2dlc(u8 len);
 
+/* calculate the CAN Frame length in bytes of a given skb */
+unsigned int can_skb_get_frame_len(const struct sk_buff *skb);
+
 /* map the data length to an appropriate data link layer length */
 static inline u8 canfd_sanitize_len(u8 len)
 {
-- 
2.29.2





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