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

[Bastien Curutchet]

Hi Maxime,

On 1/15/26 6:05 PM, Maxime Chevallier wrote:
> 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 ?

It doesn't seem to hurt, at least on my test setup (I'm using ptp4l in 
userspace btw), but I agree with you: it feels safer to zero it first. 
I'll do it in next iteration.


Best regards,
Bastien