net: airoha: Fix loopback mode configuration for GDM2 port

Add missing configuration for loopback mode in airhoha_set_gdm2_loopback
routine.

Fixes: 9cd451d ("net: airoha: Add loopback support for GDM2")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20251008-airoha-loopback-mode-fix-v2-1-045694fe7f60@kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: LorenzoBianconi

drivers/net/ethernet/airoha/airoha_eth.c

@@ -1710,7 +1710,9 @@ static void airhoha_set_gdm2_loopback(struct airoha_gdm_port *port)
 	airoha_fe_wr(eth, REG_GDM_RXCHN_EN(2), 0xffff);
 	airoha_fe_rmw(eth, REG_GDM_LPBK_CFG(2),
 		      LPBK_CHAN_MASK | LPBK_MODE_MASK | LPBK_EN_MASK,
-		      FIELD_PREP(LPBK_CHAN_MASK, chan) | LPBK_EN_MASK);
+		      FIELD_PREP(LPBK_CHAN_MASK, chan) |
+		      LBK_GAP_MODE_MASK | LBK_LEN_MODE_MASK |
+		      LBK_CHAN_MODE_MASK | LPBK_EN_MASK);
 	airoha_fe_rmw(eth, REG_GDM_LEN_CFG(2),
 		      GDM_SHORT_LEN_MASK | GDM_LONG_LEN_MASK,
 		      FIELD_PREP(GDM_SHORT_LEN_MASK, 60) |

drivers/net/ethernet/airoha/airoha_regs.h

@@ -151,6 +151,9 @@
 #define LPBK_LEN_MASK			GENMASK(23, 10)
 #define LPBK_CHAN_MASK			GENMASK(8, 4)
 #define LPBK_MODE_MASK			GENMASK(3, 1)
+#define LBK_GAP_MODE_MASK		BIT(3)
+#define LBK_LEN_MODE_MASK		BIT(2)
+#define LBK_CHAN_MODE_MASK		BIT(1)
 #define LPBK_EN_MASK			BIT(0)
 
 #define REG_GDM_TXCHN_EN(_n)		(GDM_BASE(_n) + 0x24)