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

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Message-ID: <722dcba9-d9ae-43ba-b1bf-1d577a882bef@bootlin.com>
Date: Thu, 15 Jan 2026 18:05:43 +0100
From: Maxime Chevallier <maxime.chevallier@...tlin.com>
To: "Bastien Curutchet (Schneider Electric)" <bastien.curutchet@...tlin.com>,
 Woojung Huh <woojung.huh@...rochip.com>, UNGLinuxDriver@...rochip.com,
 Andrew Lunn <andrew@...n.ch>, Vladimir Oltean <olteanv@...il.com>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Richard Cochran <richardcochran@...il.com>, Simon Horman <horms@...nel.org>
Cc: Pascal Eberhard <pascal.eberhard@...com>,
 Miquèl Raynal <miquel.raynal@...tlin.com>,
 Thomas Petazzoni <thomas.petazzoni@...tlin.com>, netdev@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 5/8] net: dsa: microchip: Add KSZ8463 tail tag
 handling

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