Subject: Re: [PATCH RFC net-next v3 2/4] net: dsa: add tag formats for MxL862xx switches

From: Simon Horman <[email-masked]>
Date: Mon, 15 Dec 2025 14:28:17 +0000

On Mon, Dec 15, 2025 at 12:11:43AM +0000, Daniel Golle wrote:

...

> diff --git a/net/dsa/tag_mxl862xx.c b/net/dsa/tag_mxl862xx.c
> new file mode 100644
> index 0000000000000..9c5e5f90dcb63
> --- /dev/null
> +++ b/net/dsa/tag_mxl862xx.c
> @@ -0,0 +1,113 @@
> +// SPDX-License-Identifier: GPL-2.0-or-later
> +/*
> + * DSA Special Tag for MaxLinear 862xx switch chips
> + *
> + * Copyright (C) 2025 Daniel Golle <[email-masked]>
> + * Copyright (C) 2024 MaxLinear Inc.
> + */
> +
> +#include <linux/bitops.h>
> +#include <linux/etherdevice.h>
> +#include <linux/skbuff.h>
> +#include <net/dsa.h>
> +#include "tag.h"
> +
> +#define MXL862_NAME	"mxl862xx"
> +
> +/* To define the outgoing port and to discover the incoming port a special
> + * tag is used by the GSW1xx.
> + *
> + *       Dest MAC       Src MAC    special TAG        EtherType
> + * ...| 1 2 3 4 5 6 | 1 2 3 4 5 6 | 1 2 3 4 5 6 7 8 | 1 2 |...
> + *                                |<--------------->|
> + */
> +
> +#define MXL862_HEADER_LEN 8
> +
> +/* Byte 7 */
> +#define MXL862_IGP_EGP GENMASK(3, 0)
> +
> +static struct sk_buff *mxl862_tag_xmit(struct sk_buff *skb,
> +				       struct net_device *dev)
> +{
> +	struct dsa_port *dp = dsa_user_to_port(dev);
> +	struct dsa_port *cpu_dp = dp->cpu_dp;
> +	unsigned int cpu_port = cpu_dp->index + 1;
> +	unsigned int usr_port = dp->index + 1;
> +	__be16 *mxl862_tag;

Hi Daniel,

Please arrange local variables in reverse xmas tree order.
Even if it means separating declaration and initialisation.

FWIIW, I would probably go for:

	struct dsa_port *dp = dsa_user_to_port(dev);
	struct dsa_port *cpu_dp = dp->cpu_dp;
	unsigned int cpu_port, usr_port;
	__be16 *mxl862_tag;

	cpu_port = cpu_dp->index + 1;
	usr_port = dp->index + 1;

> +
> +	if (!skb)
> +		return skb;
> +
> +	/* provide additional space 'MXL862_HEADER_LEN' bytes */
> +	skb_push(skb, MXL862_HEADER_LEN);
> +
> +	/* shift MAC address to the beginnig of the enlarged buffer,

s/beginnig/beginning/

> +	 * releasing the space required for DSA tag (between MAC address and
> +	 * Ethertype)
> +	 */
> +	dsa_alloc_etype_header(skb, MXL862_HEADER_LEN);
> +
> +	/* special tag ingress */
> +	mxl862_tag = dsa_etype_header_pos_tx(skb);
> +	mxl862_tag[0] = htons(ETH_P_MXLGSW);
> +	mxl862_tag[1] = 0;
> +	mxl862_tag[2] = htons(usr_port + 16 - cpu_port);
> +	mxl862_tag[3] = htons(FIELD_PREP(MXL862_IGP_EGP, cpu_port));
> +
> +	return skb;
> +}

...