Re: [PATCH net-next v3 11/12] net: dsa: add tagging driver for MaxLinear GSW1xx switch family

[Vladimir Oltean <olteanv@gmail.com>]

On Sun, Oct 26, 2025 at 11:48:23PM +0000, Daniel Golle wrote:
> Add support for a new DSA tagging protocol driver for the MaxLinear
> GSW1xx switch family. The GSW1xx switches use a proprietary 8-byte
> special tag inserted between the source MAC address and the EtherType
> field to indicate the source and destination ports for frames
> traversing the CPU port.
> 
> Implement the tag handling logic to insert the special tag on transmit
> and parse it on receive.
> 
> Signed-off-by: Daniel Golle <daniel@makrotopia.org>
> ---
> since RFC:
>  * use dsa etype header macros instead of open coding them
>  * maintain alphabetic order in Kconfig and Makefile
> 
>  MAINTAINERS              |   3 +-
>  include/net/dsa.h        |   2 +
>  net/dsa/Kconfig          |   8 +++
>  net/dsa/Makefile         |   1 +
>  net/dsa/tag_mxl-gsw1xx.c | 141 +++++++++++++++++++++++++++++++++++++++
>  5 files changed, 154 insertions(+), 1 deletion(-)
>  create mode 100644 net/dsa/tag_mxl-gsw1xx.c
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index d652f4f27756..4ddff0b0a547 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -14038,7 +14038,7 @@ F:	tools/testing/selftests/landlock/
>  K:	landlock
>  K:	LANDLOCK
>  
> -LANTIQ / INTEL Ethernet drivers
> +LANTIQ / MAXLINEAR / INTEL Ethernet DSA drivers
>  M:	Hauke Mehrtens <hauke@hauke-m.de>
>  L:	netdev@vger.kernel.org
>  S:	Maintained
> @@ -14046,6 +14046,7 @@ F:	Documentation/devicetree/bindings/net/dsa/lantiq,gswip.yaml
>  F:	drivers/net/dsa/lantiq/*
>  F:	drivers/net/ethernet/lantiq_xrx200.c
>  F:	net/dsa/tag_gswip.c
> +F:	net/dsa/tag_mxl-gsw1xx.c
>  
>  LANTIQ MIPS ARCHITECTURE
>  M:	John Crispin <john@phrozen.org>
> diff --git a/include/net/dsa.h b/include/net/dsa.h
> index 67762fdaf3c7..2df2e2ead9a8 100644
> --- a/include/net/dsa.h
> +++ b/include/net/dsa.h
> @@ -56,6 +56,7 @@ struct tc_action;
>  #define DSA_TAG_PROTO_VSC73XX_8021Q_VALUE	28
>  #define DSA_TAG_PROTO_BRCM_LEGACY_FCS_VALUE	29
>  #define DSA_TAG_PROTO_YT921X_VALUE		30
> +#define DSA_TAG_PROTO_MXL_GSW1XX_VALUE		31
>  
>  enum dsa_tag_protocol {
>  	DSA_TAG_PROTO_NONE		= DSA_TAG_PROTO_NONE_VALUE,
> @@ -89,6 +90,7 @@ enum dsa_tag_protocol {
>  	DSA_TAG_PROTO_LAN937X		= DSA_TAG_PROTO_LAN937X_VALUE,
>  	DSA_TAG_PROTO_VSC73XX_8021Q	= DSA_TAG_PROTO_VSC73XX_8021Q_VALUE,
>  	DSA_TAG_PROTO_YT921X		= DSA_TAG_PROTO_YT921X_VALUE,
> +	DSA_TAG_PROTO_MXL_GSW1XX	= DSA_TAG_PROTO_MXL_GSW1XX_VALUE,
>  };
>  
>  struct dsa_switch;
> diff --git a/net/dsa/Kconfig b/net/dsa/Kconfig
> index 6b94028b1fcc..f86b30742122 100644
> --- a/net/dsa/Kconfig
> +++ b/net/dsa/Kconfig
> @@ -104,6 +104,14 @@ config NET_DSA_TAG_MTK
>  	  Say Y or M if you want to enable support for tagging frames for
>  	  Mediatek switches.
>  
> +config NET_DSA_TAG_MXL_GSW1XX
> +	tristate "Tag driver for MaxLinear GSW1xx switches"
> +	help
> +	  The GSW1xx family of switches supports an 8-byte special tag which
> +	  can be used on the CPU port of the switch.
> +	  Say Y or M if you want to enable support for tagging frames for
> +	  MaxLinear GSW1xx switches.
> +
>  config NET_DSA_TAG_KSZ
>  	tristate "Tag driver for Microchip 8795/937x/9477/9893 families of switches"
>  	help
> diff --git a/net/dsa/Makefile b/net/dsa/Makefile
> index 4b011a1d5c87..42d173f5a701 100644
> --- a/net/dsa/Makefile
> +++ b/net/dsa/Makefile
> @@ -28,6 +28,7 @@ obj-$(CONFIG_NET_DSA_TAG_HELLCREEK) += tag_hellcreek.o
>  obj-$(CONFIG_NET_DSA_TAG_KSZ) += tag_ksz.o
>  obj-$(CONFIG_NET_DSA_TAG_LAN9303) += tag_lan9303.o
>  obj-$(CONFIG_NET_DSA_TAG_MTK) += tag_mtk.o
> +obj-$(CONFIG_NET_DSA_TAG_MXL_GSW1XX) += tag_mxl-gsw1xx.o
>  obj-$(CONFIG_NET_DSA_TAG_NONE) += tag_none.o
>  obj-$(CONFIG_NET_DSA_TAG_OCELOT) += tag_ocelot.o
>  obj-$(CONFIG_NET_DSA_TAG_OCELOT_8021Q) += tag_ocelot_8021q.o
> diff --git a/net/dsa/tag_mxl-gsw1xx.c b/net/dsa/tag_mxl-gsw1xx.c
> new file mode 100644
> index 000000000000..9efec6deb494
> --- /dev/null
> +++ b/net/dsa/tag_mxl-gsw1xx.c
> @@ -0,0 +1,141 @@
> +// SPDX-License-Identifier: GPL-2.0+
> +/*
> + * DSA driver Special Tag support for MaxLinear GSW1xx switch chips
> + *
> + * Copyright (C) 2025 Daniel Golle <daniel@makrotopia.org>
> + * Copyright (C) 2023 - 2024 MaxLinear Inc.
> + */
> +
> +#include <linux/bitops.h>
> +#include <linux/etherdevice.h>
> +#include <linux/skbuff.h>
> +#include <net/dsa.h>
> +
> +#include "tag.h"
> +
> +/* 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 GSW1XX_TAG_NAME		"gsw1xx"
> +
> +/* special tag in TX path header */
> +#define GSW1XX_TX_HEADER_LEN	8
> +
> +/* Byte 0 = Ethertype byte 1 -> 0x88 */
> +/* Byte 1 = Ethertype byte 2 -> 0xC3*/
> +
> +/* Byte 2 */
> +#define GSW1XX_TX_PORT_MAP_EN		BIT(7)
> +#define GSW1XX_TX_CLASS_EN		BIT(6)
> +#define GSW1XX_TX_TIME_STAMP_EN		BIT(5)
> +#define GSW1XX_TX_LRN_DIS		BIT(4)
> +#define GSW1XX_TX_CLASS_SHIFT		0
> +#define GSW1XX_TX_CLASS_MASK		GENMASK(3, 0)

Using FIELD_PREP() would eliminate these _SHIFT definitions and _MASK
would also go away from the macro names.

> +
> +/* Byte 3 */
> +#define GSW1XX_TX_PORT_MAP_LOW_SHIFT	0
> +#define GSW1XX_TX_PORT_MAP_LOW_MASK	GENMASK(7, 0)
> +
> +/* Byte 4 */
> +#define GSW1XX_TX_PORT_MAP_HIGH_SHIFT	0
> +#define GSW1XX_TX_PORT_MAP_HIGH_MASK	GENMASK(7, 0)
> +
> +#define GSW1XX_RX_HEADER_LEN		8

Usually you use two separate macros when the lengths are not equal, and
you set .needed_headroom to the largest value.

> +
> +/* special tag in RX path header */
> +/* Byte 4 */
> +#define GSW1XX_RX_PORT_MAP_LOW_SHIFT	0
> +#define GSW1XX_RX_PORT_MAP_LOW_MASK	GENMASK(7, 0)
> +
> +/* Byte 5 */
> +#define GSW1XX_RX_PORT_MAP_HIGH_SHIFT	0
> +#define GSW1XX_RX_PORT_MAP_HIGH_MASK	GENMASK(7, 0)
> +
> +static struct sk_buff *gsw1xx_tag_xmit(struct sk_buff *skb,
> +				       struct net_device *dev)
> +{
> +	struct dsa_port *dp = dsa_user_to_port(dev);
> +	u8 *gsw1xx_tag;
> +
> +	/* provide additional space 'GSW1XX_TX_HEADER_LEN' bytes */
> +	skb_push(skb, GSW1XX_TX_HEADER_LEN);
> +
> +	/* add space between MAC address and Ethertype */
> +	dsa_alloc_etype_header(skb, GSW1XX_TX_HEADER_LEN);
> +
> +	/* special tag ingress */
> +	gsw1xx_tag = dsa_etype_header_pos_tx(skb);
> +	gsw1xx_tag[0] = 0x88;
> +	gsw1xx_tag[1] = 0xc3;

Could you write this as a u16 pointer, to make it obvious to everyone
it's an EtherType, and define the EtherType constant in
include/uapi/linux/if_ether.h, to make it a bit more visible that it's
in use?

> +	gsw1xx_tag[2] = GSW1XX_TX_PORT_MAP_EN | GSW1XX_TX_LRN_DIS;
> +	gsw1xx_tag[3] = BIT(dp->index + GSW1XX_TX_PORT_MAP_LOW_SHIFT) &
> +			GSW1XX_TX_PORT_MAP_LOW_MASK;
> +	gsw1xx_tag[4] = 0;
> +	gsw1xx_tag[5] = 0;
> +	gsw1xx_tag[6] = 0;
> +	gsw1xx_tag[7] = 0;
> +
> +	return skb;
> +}
> +
> +static struct sk_buff *gsw1xx_tag_rcv(struct sk_buff *skb,
> +				      struct net_device *dev)
> +{
> +	int port;
> +	u8 *gsw1xx_tag;
> +
> +	if (unlikely(!pskb_may_pull(skb, GSW1XX_RX_HEADER_LEN))) {
> +		dev_warn_ratelimited(&dev->dev, "Dropping packet, cannot pull SKB\n");
> +		return NULL;
> +	}
> +
> +	gsw1xx_tag = dsa_etype_header_pos_rx(skb);
> +
> +	if (gsw1xx_tag[0] != 0x88 && gsw1xx_tag[1] != 0xc3) {
> +		dev_warn_ratelimited(&dev->dev, "Dropping packet due to invalid special tag\n");
> +		dev_warn_ratelimited(&dev->dev,
> +				     "Tag: 0x%x, 0x%x, 0x%x, 0x%x, 0x%x, 0x%x, 0x%x, 0x%x\n",
> +				     gsw1xx_tag[0], gsw1xx_tag[1], gsw1xx_tag[2], gsw1xx_tag[3],
> +				     gsw1xx_tag[4], gsw1xx_tag[5], gsw1xx_tag[6], gsw1xx_tag[7]);

I think you could print the tag with %*ph, according to
https://elixir.bootlin.com/linux/v6.17.5/source/lib/vsprintf.c#L2453
(needs testing)

> +		return NULL;
> +	}
> +
> +	/* Get source port information */
> +	port = (gsw1xx_tag[2] & GSW1XX_RX_PORT_MAP_LOW_MASK) >> GSW1XX_RX_PORT_MAP_LOW_SHIFT;
> +	skb->dev = dsa_conduit_find_user(dev, 0, port);
> +	if (!skb->dev) {
> +		dev_warn_ratelimited(&dev->dev, "Dropping packet due to invalid source port\n");
> +		dev_warn_ratelimited(&dev->dev,
> +				     "Tag: 0x%x, 0x%x, 0x%x, 0x%x, 0x%x, 0x%x, 0x%x, 0x%x\n",
> +				     gsw1xx_tag[0], gsw1xx_tag[1], gsw1xx_tag[2], gsw1xx_tag[3],
> +				     gsw1xx_tag[4], gsw1xx_tag[5], gsw1xx_tag[6], gsw1xx_tag[7]);
> +		return NULL;
> +	}
> +
> +	/* remove the GSW1xx special tag between MAC addresses and the current
> +	 * ethertype field.
> +	 */
> +	skb_pull_rcsum(skb, GSW1XX_RX_HEADER_LEN);
> +	dsa_strip_etype_header(skb, GSW1XX_RX_HEADER_LEN);

You're not setting skb->offload_fwd_mark but you implement
port_bridge_join() so you offload L2 switching. If a packet gets flooded
from port A to the CPU and also to port B, don't you see that the
software bridge also creates a packet copy that it sends to port B a
second time?

> +
> +	return skb;
> +}
> +
> +static const struct dsa_device_ops gsw1xx_netdev_ops = {
> +	.name = GSW1XX_TAG_NAME,
> +	.proto	= DSA_TAG_PROTO_MXL_GSW1XX,
> +	.xmit = gsw1xx_tag_xmit,
> +	.rcv = gsw1xx_tag_rcv,
> +	.needed_headroom = GSW1XX_RX_HEADER_LEN,
> +};
> +
> +MODULE_DESCRIPTION("DSA tag driver for MaxLinear GSW1xx 8 byte protocol");
> +MODULE_LICENSE("GPL");
> +MODULE_ALIAS_DSA_TAG_DRIVER(DSA_TAG_PROTO_MXL_GSW1XX, GSW1XX_TAG_NAME);
> +
> +module_dsa_tag_driver(gsw1xx_netdev_ops);
> -- 
> 2.51.1