Hi Luke,

On 7/3/26 08:41, Luke Howard wrote:

mv88e6xxx switches can support embedding PTP timestamps directly
in the frame, either as a trailer or at a configurable offset
(typically the reserved bytes in the PTP header).

Add support for this on the 88E6341 and 88E6352 switches, being
those on which I was able to verify this. Other switch chips may
also work. The arrival timestamp offsets are relative to the PTP
common header and will work for both L2 and L3 PTP packets; the
respective headers are skipped by the number of bytes set in the
ETJump and IPJump registers, which are initialized to sensible
defaults and are VLAN tag-aware.

(Note: the 6352 datasheet incorrectly states that ETJump and
IPJump are initialized to zero. They are initialized to 12 and
2 respectively; this is corrected in the 6341 data sheet.)

Signed-off-by: Luke Howard <redacted>

[...]

+static bool parse_embedded_ts(unsigned int arr_ts_mode,
+			      struct sk_buff *skb, u64 *ns)
+{
+	struct ptp_header *hdr;
+
+	*ns = 0;
+
+	/* APPEND means the switch appended the time stamp as a 4-byte trailer
+	 * (not all switches support this). Any other non-zero value is the byte
+	 * offset past the start of the PTP common header at which the switch
+	 * overwrote the time stamp in place (e.g. the reserved header bytes).
+	 */
+	if (arr_ts_mode == MV88E6XXX_PTP_ARR_TS_MODE_APPEND && skb->len >= 4) {
+		*ns = (u64)get_unaligned_be32(&skb->data[skb->len - 4]);

One thing is that you need to make sure you're dealing with a linear skb
when accessing the trailer, which can be done by calling skb_linearize(skb).

Then you should use skb_tail_pointer(skb) to get to the trailer.

You can take as examples the DSA taggers that pull the tag from the tail, such as
the tag_trailer [1], tag_ksz, tag_rtl8_4, etc. 

And finally you may have to use the chksum variants of the skb_trim such as
pskb_trim_rcsum(), like done in the above-mentionned taggers.

[1] : https://elixir.bootlin.com/linux/v7.1.2/source/net/dsa/tag_trailer.c#L28

Thanks,

Maxime