netdev - DSA using cpsw and lan9303

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Message-ID: <yw1x8rud4cux.fsf@mansr.com>
Date:   Mon, 14 Feb 2022 16:44:22 +0000
From:   Måns Rullgård <mans@...sr.com>
To:     netdev@...r.kernel.org
Subject: DSA using cpsw and lan9303

The hardware I'm working on has a LAN9303 switch connected to the
Ethernet port of an AM335x (ZCE package).  In trying to make DSA work
with this combination, I have encountered two problems.

Firstly, the cpsw driver configures the hardware to filter out frames
with unknown VLAN tags.  To make it accept the tagged frames coming from
the LAN9303, I had to modify the latter driver like this:

diff --git a/drivers/net/dsa/lan9303-core.c b/drivers/net/dsa/lan9303-core.c
index 2de67708bbd2..460c998c0c33 100644
--- a/drivers/net/dsa/lan9303-core.c
+++ b/drivers/net/dsa/lan9303-core.c
@@ -1078,20 +1079,28 @@ static int lan9303_port_enable(struct dsa_switch *ds, int port,
                               struct phy_device *phy)
 {
        struct lan9303 *chip = ds->priv;
+       struct net_device *master;

        if (!dsa_is_user_port(ds, port))
                return 0;

+       master = dsa_to_port(chip->ds, 0)->master;
+       vlan_vid_add(master, htons(ETH_P_8021Q), port);
+
        return lan9303_enable_processing_port(chip, port);
 }

Secondly, the cpsw driver strips VLAN tags from incoming frames, and
this prevents the DSA parsing from working.  As a dirty workaround, I
did this:

diff --git a/drivers/net/ethernet/ti/cpsw_priv.c b/drivers/net/ethernet/ti/cpsw_priv.c
index 424e644724e4..e15f42ece8bf 100644
--- a/drivers/net/ethernet/ti/cpsw_priv.c
+++ b/drivers/net/ethernet/ti/cpsw_priv.c
@@ -235,6 +235,7 @@ void cpsw_rx_vlan_encap(struct sk_buff *skb)

        /* Remove VLAN header encapsulation word */
        skb_pull(skb, CPSW_RX_VLAN_ENCAP_HDR_SIZE);
+       return;

        pkt_type = (rx_vlan_encap_hdr >>
                    CPSW_RX_VLAN_ENCAP_HDR_PKT_TYPE_SHIFT) &

With these changes, everything seems to work as expected.

Now I'd appreciate if someone could tell me how I should have done this.
Please don't make me send an actual patch.

-- 
Måns Rullgård