netdev - [PATCH net] mctp: serial: Fix starting value for frame check sequence

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Date:   Fri, 16 Dec 2022 11:44:09 +0800
From:   Jeremy Kerr <jk@...econstruct.com.au>
To:     netdev@...r.kernel.org
Cc:     Matt Johnston <matt@...econstruct.com.au>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Harsh Tyagi <harshtya@...gle.com>
Subject: [PATCH net] mctp: serial: Fix starting value for frame check sequence

RFC1662 defines the start state for the crc16 FCS to be 0xffff, but
we're currently starting at zero.

This change uses the correct start state. We're only early in the
adoption for the serial binding, so there aren't yet any other users to
interface to.

Fixes: a0c2ccd9b5ad ("mctp: Add MCTP-over-serial transport binding")

Reported-by: Harsh Tyagi <harshtya@...gle.com>
Tested-by: Harsh Tyagi <harshtya@...gle.com>
Signed-off-by: Jeremy Kerr <jk@...econstruct.com.au>
---
 drivers/net/mctp/mctp-serial.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/net/mctp/mctp-serial.c b/drivers/net/mctp/mctp-serial.c
index 7cd103fd34ef..9f9eaf896047 100644
--- a/drivers/net/mctp/mctp-serial.c
+++ b/drivers/net/mctp/mctp-serial.c
@@ -35,6 +35,8 @@
 #define BYTE_FRAME		0x7e
 #define BYTE_ESC		0x7d
 
+#define FCS_INIT		0xffff
+
 static DEFINE_IDA(mctp_serial_ida);
 
 enum mctp_serial_state {
@@ -123,7 +125,7 @@ static void mctp_serial_tx_work(struct work_struct *work)
 		buf[2] = dev->txlen;
 
 		if (!dev->txpos)
-			dev->txfcs = crc_ccitt(0, buf + 1, 2);
+			dev->txfcs = crc_ccitt(FCS_INIT, buf + 1, 2);
 
 		txlen = write_chunk(dev, buf + dev->txpos, 3 - dev->txpos);
 		if (txlen <= 0) {
@@ -303,7 +305,7 @@ static void mctp_serial_push_header(struct mctp_serial *dev, unsigned char c)
 	case 1:
 		if (c == MCTP_SERIAL_VERSION) {
 			dev->rxpos++;
-			dev->rxfcs = crc_ccitt_byte(0, c);
+			dev->rxfcs = crc_ccitt_byte(FCS_INIT, c);
 		} else {
 			dev->rxstate = STATE_ERR;
 		}
-- 
2.35.1