net: ptp: add helper for one-step P2P clocks

For P2P delay measurement, the ingress time stamp of the PDelay_Req is
required for the correction field of the PDelay_Resp. The application
echoes back the correction field of the PDelay_Req when sending the
PDelay_Resp.

Some hardware (like the ZHAW InES PTP time stamping IP core) subtracts
the ingress timestamp autonomously from the correction field, so that
the hardware only needs to add the egress timestamp on tx. Other
hardware (like the Microchip KSZ9563) reports the ingress time stamp via
an interrupt and requires that the software provides this time stamp via
tail-tag on tx.

In order to avoid introducing a further application interface for this,
the driver can simply emulate the behavior of the InES device and
subtract the ingress time stamp in software from the correction field.

On egress, the correction field can either be kept as it is (and the
time stamp field in the tail-tag is set to zero) or move the value from
the correction field back to the tail-tag.

Changing the correction field requires updating the UDP checksum (if UDP
is used as transport).

Signed-off-by: Christian Eggers <[e-mail]>
Co-developed-by: Arun Ramadoss <[e-mail]>
Signed-off-by: Arun Ramadoss <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/linux/ptp_classify.h

···
 #ifndef _PTP_CLASSIFY_H_
 #define _PTP_CLASSIFY_H_
 
+#include <asm/unaligned.h>
 #include <linux/ip.h>
+#include <linux/ktime.h>
 #include <linux/skbuff.h>
+#include <linux/udp.h>
+#include <net/checksum.h>
 
 #define PTP_CLASS_NONE  0x00 /* not a PTP event message */
 #define PTP_CLASS_V1    0x01 /* protocol version 1 */
···
 }
 
 /**
+ * ptp_check_diff8 - Computes new checksum (when altering a 64-bit field)
+ * @old: old field value
+ * @new: new field value
+ * @oldsum: previous checksum
+ *
+ * This function can be used to calculate a new checksum when only a single
+ * field is changed. Similar as ip_vs_check_diff*() in ip_vs.h.
+ *
+ * Return: Updated checksum
+ */
+static inline __wsum ptp_check_diff8(__be64 old, __be64 new, __wsum oldsum)
+{
+	__be64 diff[2] = { ~old, new };
+
+	return csum_partial(diff, sizeof(diff), oldsum);
+}
+
+/**
+ * ptp_header_update_correction - Update PTP header's correction field
+ * @skb: packet buffer
+ * @type: type of the packet (see ptp_classify_raw())
+ * @hdr: ptp header
+ * @correction: new correction value
+ *
+ * This updates the correction field of a PTP header and updates the UDP
+ * checksum (if UDP is used as transport). It is needed for hardware capable of
+ * one-step P2P that does not already modify the correction field of Pdelay_Req
+ * event messages on ingress.
+ */
+static inline
+void ptp_header_update_correction(struct sk_buff *skb, unsigned int type,
+				  struct ptp_header *hdr, s64 correction)
+{
+	__be64 correction_old;
+	struct udphdr *uhdr;
+
+	/* previous correction value is required for checksum update. */
+	memcpy(&correction_old,  &hdr->correction, sizeof(correction_old));
+
+	/* write new correction value */
+	put_unaligned_be64((u64)correction, &hdr->correction);
+
+	switch (type & PTP_CLASS_PMASK) {
+	case PTP_CLASS_IPV4:
+	case PTP_CLASS_IPV6:
+		/* locate udp header */
+		uhdr = (struct udphdr *)((char *)hdr - sizeof(struct udphdr));
+		break;
+	default:
+		return;
+	}
+
+	/* update checksum */
+	uhdr->check = csum_fold(ptp_check_diff8(correction_old,
+						hdr->correction,
+						~csum_unfold(uhdr->check)));
+	if (!uhdr->check)
+		uhdr->check = CSUM_MANGLED_0;
+
+	skb->ip_summed = CHECKSUM_NONE;
+}
+
+/**
  * ptp_msg_is_sync - Evaluates whether the given skb is a PTP Sync message
  * @skb: packet buffer
  * @type: type of the packet (see ptp_classify_raw())
···
 static inline bool ptp_msg_is_sync(struct sk_buff *skb, unsigned int type)
 {
 	return false;
+}
+
+static inline
+void ptp_header_update_correction(struct sk_buff *skb, unsigned int type,
+				  struct ptp_header *hdr, s64 correction)
+{
 }
 #endif
 #endif /* _PTP_CLASSIFY_H_ */