Re: [PATCH net-next 5/8] net: dsa: microchip: Add KSZ8463 tail tag handling

[Maxime Chevallier <maxime.chevallier@bootlin.com>]

Hi Bastien,

On 15/01/2026 16:57, Bastien Curutchet (Schneider Electric) wrote:
> KSZ8463 uses the KSZ9893 DSA TAG driver. However, the KSZ8463 doesn't
> use the tail tag to convey timestamps to the host as KSZ9893 does. It
> uses the reserved fields in the PTP header instead.

[ ... ]

> +static struct sk_buff *ksz8463_rcv(struct sk_buff *skb, struct net_device *dev)
> +{
> +	unsigned int len = KSZ_EGRESS_TAG_LEN;
> +	struct ptp_header *ptp_hdr;
> +	unsigned int ptp_class;
> +	unsigned int port;
> +	ktime_t tstamp;
> +	u8 *tag;
> +
> +	if (skb_linearize(skb))
> +		return NULL;
> +
> +	/* Tag decoding */
> +	tag = skb_tail_pointer(skb) - KSZ_EGRESS_TAG_LEN;
> +	port = tag[0] & KSZ8463_TAIL_TAG_EG_PORT_M;
> +
> +	__skb_push(skb, ETH_HLEN);
> +	ptp_class = ptp_classify_raw(skb);
> +	__skb_pull(skb, ETH_HLEN);
> +	if (ptp_class == PTP_CLASS_NONE)
> +		goto common_rcv;
> +
> +	ptp_hdr = ptp_parse_header(skb, ptp_class);
> +	if (ptp_hdr) {
> +		tstamp = ksz_decode_tstamp(get_unaligned_be32(&ptp_hdr->reserved2));
> +		KSZ_SKB_CB(skb)->tstamp = tstamp;

As it is using a reserved field, is it OK to leave this field as-is when forwarding
this skb to userspace, or should it be zeroed first ?

Maxime