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

From: Simon Horman <[email-masked]>
Date: Tue, 20 Jan 2026 17:17:30 +0000

On Thu, Jan 15, 2026 at 04:57:04PM +0100, 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.
> 
> Add a KSZ8463-specifig DSA_TAG driver to handle KSZ8463 timestamps.
> There is no information in the tail tag to distinguish PTP packets from
> others so use the ptp_classify_raw() helper to find the PTP packets and
> extract the timestamp from their PTP headers.
> 
> Signed-off-by: Bastien Curutchet (Schneider Electric) <[email-masked]>

...

> diff --git a/net/dsa/tag_ksz.c b/net/dsa/tag_ksz.c
> index 9170a0148cc43b4213ec4bd8e81d338589671f23..635679402f8a96b29536a91988346a8825bae976 100644
> --- a/net/dsa/tag_ksz.c
> +++ b/net/dsa/tag_ksz.c
> @@ -16,6 +16,7 @@
>  #define KSZ9477_NAME "ksz9477"
>  #define KSZ9893_NAME "ksz9893"
>  #define LAN937X_NAME "lan937x"
> +#define KSZ8463_NAME "ksz8463"
>  
>  /* Typically only one byte is used for tail tag. */
>  #define KSZ_PTP_TAG_LEN			4
> @@ -383,6 +384,108 @@ static const struct dsa_device_ops ksz9893_netdev_ops = {
>  DSA_TAG_DRIVER(ksz9893_netdev_ops);
>  MODULE_ALIAS_DSA_TAG_DRIVER(DSA_TAG_PROTO_KSZ9893, KSZ9893_NAME);
>  
> +#define KSZ8463_TAIL_TAG_PRIO		GENMASK(4, 3)
> +#define KSZ8463_TAIL_TAG_EG_PORT_M	GENMASK(2, 0)
> +
> +static void ksz8463_xmit_timestamp(struct dsa_port *dp, struct sk_buff *skb)
> +{
> +	struct ksz_tagger_private *priv;
> +	struct ptp_header *ptp_hdr;
> +	unsigned int ptp_type;
> +	u32 tstamp_raw = 0;
> +	s64 correction;
> +
> +	priv = ksz_tagger_private(dp->ds);
> +
> +	if (!test_bit(KSZ_HWTS_EN, &priv->state))
> +		return;
> +
> +	if (!KSZ_SKB_CB(skb)->update_correction)
> +		return;
> +
> +	ptp_type = KSZ_SKB_CB(skb)->ptp_type;
> +	ptp_hdr = ptp_parse_header(skb, ptp_type);
> +	if (!ptp_hdr)
> +		return;
> +
> +	correction = (s64)get_unaligned_be64(&ptp_hdr->correction);
> +
> +	if (correction < 0) {
> +		struct timespec64 ts;
> +
> +		ts = ns_to_timespec64(-correction >> 16);
> +		tstamp_raw = ((ts.tv_sec & 3) << 30) | ts.tv_nsec;
> +
> +		ptp_hdr->reserved2 = tstamp_raw;

I think that you need to assign a be32 rather than a u32 to reserved2.

Flagged by Sparse [1].

[1] This particular commit, from Al Viro's tree:
    https://git.kernel.org/pub/scm/linux/kernel/git/viro/sparse.git/commit/?id=2634e39bf02697a18fece057208150362c985992
    To address this mess:
    https://lore.kernel.org/all/[email-masked]/

> +	}
> +}

...