Re: [PATCH net-next v3 1/4] net: dsa: add support for DSA rx offloading via metadata dst

[Vladimir Oltean <olteanv@gmail.com>]

On Thu, Nov 10, 2022 at 10:22:08PM +0100, Felix Fietkau wrote:
> If a metadata dst is present with the type METADATA_HW_PORT_MUX on a dsa cpu
> port netdev, assume that it carries the port number and that there is no DSA
> tag present in the skb data.
> 
> Signed-off-by: Felix Fietkau <nbd@nbd.name>
> ---

Doesn't look bad to me, but...

>  net/core/flow_dissector.c |  4 +++-
>  net/dsa/dsa.c             | 19 ++++++++++++++++++-
>  2 files changed, 21 insertions(+), 2 deletions(-)
> 
> diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
> index 25cd35f5922e..1f476abc25e1 100644
> --- a/net/core/flow_dissector.c
> +++ b/net/core/flow_dissector.c
> @@ -972,11 +972,13 @@ bool __skb_flow_dissect(const struct net *net,
>  		if (unlikely(skb->dev && netdev_uses_dsa(skb->dev) &&
>  			     proto == htons(ETH_P_XDSA))) {
>  			const struct dsa_device_ops *ops;
> +			struct metadata_dst *md_dst = skb_metadata_dst(skb);
>  			int offset = 0;
>  
>  			ops = skb->dev->dsa_ptr->tag_ops;
>  			/* Only DSA header taggers break flow dissection */
> -			if (ops->needed_headroom) {
> +			if (ops->needed_headroom &&
> +			    (!md_dst || md_dst->type != METADATA_HW_PORT_MUX)) {
>  				if (ops->flow_dissect)
>  					ops->flow_dissect(skb, &proto, &offset);
>  				else
> diff --git a/net/dsa/dsa.c b/net/dsa/dsa.c
> index 64b14f655b23..0b67622cf905 100644
> --- a/net/dsa/dsa.c
> +++ b/net/dsa/dsa.c
> @@ -11,6 +11,7 @@
>  #include <linux/netdevice.h>
>  #include <linux/sysfs.h>
>  #include <linux/ptp_classify.h>
> +#include <net/dst_metadata.h>
>  
>  #include "dsa_priv.h"
>  
> @@ -216,6 +217,7 @@ static bool dsa_skb_defer_rx_timestamp(struct dsa_slave_priv *p,
>  static int dsa_switch_rcv(struct sk_buff *skb, struct net_device *dev,
>  			  struct packet_type *pt, struct net_device *unused)
>  {
> +	struct metadata_dst *md_dst = skb_metadata_dst(skb);
>  	struct dsa_port *cpu_dp = dev->dsa_ptr;
>  	struct sk_buff *nskb = NULL;
>  	struct dsa_slave_priv *p;
> @@ -229,7 +231,22 @@ static int dsa_switch_rcv(struct sk_buff *skb, struct net_device *dev,
>  	if (!skb)
>  		return 0;
>  
> -	nskb = cpu_dp->rcv(skb, dev);
> +	if (md_dst && md_dst->type == METADATA_HW_PORT_MUX) {
> +		unsigned int port = md_dst->u.port_info.port_id;
> +
> +		skb_dst_set(skb, NULL);

If you insist on not using the refcounting feature and free your
metadata_dst in the master's remove() function, that's going to
invalidate absolutely any point I'm trying to make. Normally I'd leave
you alone, however I really don't like that this is also forcing DSA to
not use the refcount, and therefore, that it's forcing any other driver
to do the same as mtk_eth_soc. Not sure how that's gonna scale in the
hypothetical future when there will be a DSA master which can offload
RX DSA tags, *and* the switch can change tagging protocols dynamically
(which will force the master to allocate/free its metadata dst's at
runtime too). I guess that will be for me to figure out, which I don't
like.

Jakub, what do you think? Refcounting or no refcounting?

> +		if (!skb_has_extensions(skb))
> +			skb->slow_gro = 0;
> +
> +		skb->dev = dsa_master_find_slave(dev, 0, port);
> +		if (likely(skb->dev)) {
> +			dsa_default_offload_fwd_mark(skb);
> +			nskb = skb;
> +		}
> +	} else {
> +		nskb = cpu_dp->rcv(skb, dev);
> +	}
> +
>  	if (!nskb) {
>  		kfree_skb(skb);
>  		return 0;
> -- 
> 2.38.1
>