net: dsa: microchip: ptp: move pdelay_rsp correction field to tail tag

For PDelay_Resp messages we will likely have a negative value in the
correction field. The switch hardware cannot correctly update such
values (produces an off by one error in the UDP checksum), so it must be
moved to the time stamp field in the tail tag. Format of the correction
field is 48 bit ns + 16 bit fractional ns.  After updating the
correction field, clone is no longer required hence it is freed.
Signed-off-by: Christian Eggers <ceggers@arri.de>
Co-developed-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/dsa/microchip/ksz_ptp.c

@@ -267,6 +267,10 @@ void ksz_port_txtstamp(struct dsa_switch *ds, int port, struct sk_buff *skb)
 	switch (ptp_msg_type) {
 	case PTP_MSGTYPE_PDELAY_REQ:
 		break;
+	case PTP_MSGTYPE_PDELAY_RESP:
+		KSZ_SKB_CB(skb)->ptp_type = type;
+		KSZ_SKB_CB(skb)->update_correction = true;
+		return;
 
 	default:
 		return;

include/linux/dsa/ksz_common.h

@@ -36,6 +36,8 @@ struct ksz_tagger_data {
 
 struct ksz_skb_cb {
 	struct sk_buff *clone;
+	unsigned int ptp_type;
+	bool update_correction;
 	u32 tstamp;
 };
 

net/dsa/tag_ksz.c

@@ -7,6 +7,7 @@
 #include <linux/dsa/ksz_common.h>
 #include <linux/etherdevice.h>
 #include <linux/list.h>
+#include <linux/ptp_classify.h>
 #include <net/dsa.h>
 
 #include "tag.h"
@@ -195,13 +196,39 @@ static void ksz_rcv_timestamp(struct sk_buff *skb, u8 *tag)
 static void ksz_xmit_timestamp(struct dsa_port *dp, struct sk_buff *skb)
 {
 	struct ksz_tagger_private *priv;
+	struct ptp_header *ptp_hdr;
+	unsigned int ptp_type;
+	u32 tstamp_raw = 0;
+	s64 correction;
 
 	priv = ksz_tagger_private(dp->ds);
 
 	if (!test_bit(KSZ_HWTS_EN, &priv->state))
 		return;
 
-	put_unaligned_be32(0, skb_put(skb, KSZ_PTP_TAG_LEN));
+	if (!KSZ_SKB_CB(skb)->update_correction)
+		goto output_tag;
+
+	ptp_type = KSZ_SKB_CB(skb)->ptp_type;
+
+	ptp_hdr = ptp_parse_header(skb, ptp_type);
+	if (!ptp_hdr)
+		goto output_tag;
+
+	correction = (s64)get_unaligned_be64(&ptp_hdr->correction);
+
+	if (correction < 0) {
+		struct timespec64 ts;
+
+		ts = ns_to_timespec64(-correction >> 16);
+		tstamp_raw = ((ts.tv_sec & 3) << 30) | ts.tv_nsec;
+
+		/* Set correction field to 0 and update UDP checksum */
+		ptp_header_update_correction(skb, ptp_type, ptp_hdr, 0);
+	}
+
+output_tag:
+	put_unaligned_be32(tstamp_raw, skb_put(skb, KSZ_PTP_TAG_LEN));
 }
 
 /* Defer transmit if waiting for egress time stamp is required.  */