net: ethernet: ti: am65-cpsw: VLAN-aware CPSW only if !DSA

Only configure VLAN-aware CPSW mode if no port is used as DSA CPU port.
VLAN-aware mode interferes with some DSA tagging schemes and makes stacking
DSA switches downstream of CPSW impossible. Previous attempts to address
the issue linked below.

Link: https://lore.kernel.org/netdev/[email protected]/
Link: https://lore.kernel.org/linux-arm-kernel/4699400.vD3TdgH1nR@localhost/
Co-developed-by: Siddharth Vadapalli <[email protected]>
Signed-off-by: Siddharth Vadapalli <[email protected]>
Signed-off-by: Alexander Sverdlin <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: ccpalex

drivers/net/ethernet/ti/am65-cpsw-nuss.c

@@ -32,6 +32,7 @@
 #include <linux/dma/ti-cppi5.h>
 #include <linux/dma/k3-udma-glue.h>
 #include <net/page_pool/helpers.h>
+#include <net/dsa.h>
 #include <net/switchdev.h>
 
 #include "cpsw_ale.h"
@@ -1014,6 +1015,15 @@ static int am65_cpsw_nuss_ndo_slave_open(struct net_device *ndev)
 
 	common->usage_count++;
 
+	/* VLAN aware CPSW mode is incompatible with some DSA tagging schemes.
+	 * Therefore disable VLAN_AWARE mode if any of the ports is a DSA Port.
+	 */
+	if (netdev_uses_dsa(ndev)) {
+		reg = readl(common->cpsw_base + AM65_CPSW_REG_CTL);
+		reg &= ~AM65_CPSW_CTL_VLAN_AWARE;
+		writel(reg, common->cpsw_base + AM65_CPSW_REG_CTL);
+	}
+
 	am65_cpsw_port_set_sl_mac(port, ndev->dev_addr);
 	am65_cpsw_port_enable_dscp_map(port);