ath79: force SGMII SerDes mode to MAC operation

The mode on the SGMII SerDes on the QCA9563 is 1000 Base-X by default.
This only allows for 1000 Mbit/s links, however when used with an SGMII
PHY in 100 Mbit/s link mode, the link remains dead.

This strictly has nothing to do with the SerDes calibration, however it
is done at the same point in the QCA reference U-Boot which is the
blueprint for everything happening here. As the current state is more or
less a hack, this should be fine.

This fixes the issues outlined above on a TP-Link EAP-225 Outdoor.

Reported-by: Tom Herbers <[email protected]>
Tested-by: Tom Herbers <[email protected]>
Signed-off-by: David Bauer <[email protected]>

target/linux/ath79/files/drivers/net/ethernet/atheros/ag71xx/ag71xx_main.c

@@ -602,6 +602,11 @@ static void ag71xx_sgmii_serdes_init_qca956x(struct device_node *np)
 		goto err_iomap;
 	}
 
+	t = __raw_readl(gmac_base + QCA956X_GMAC_REG_SGMII_CONFIG);
+	t &= ~(QCA956X_SGMII_CONFIG_MODE_CTRL_MASK << QCA956X_SGMII_CONFIG_MODE_CTRL_SHIFT);
+	t |= QCA956X_SGMII_CONFIG_MODE_CTRL_SGMII_MAC;
+	__raw_writel(t, gmac_base + QCA956X_GMAC_REG_SGMII_CONFIG);
+
 	pr_debug("%pOF: fixup SERDES calibration to value %i\n",
 		np_dev, serdes_cal);
 	t = __raw_readl(gmac_base + QCA956X_GMAC_REG_SGMII_SERDES);

target/linux/ath79/patches-5.4/0040-ath79-sgmii-config.patch

@@ -0,0 +1,9 @@
+--- a/arch/mips/include/asm/mach-ath79/ar71xx_regs.h
++++ b/arch/mips/include/asm/mach-ath79/ar71xx_regs.h
+@@ -1376,5 +1376,6 @@
+ 
+ #define QCA956X_SGMII_CONFIG_MODE_CTRL_SHIFT	0
+ #define QCA956X_SGMII_CONFIG_MODE_CTRL_MASK	0x7
++#define QCA956X_SGMII_CONFIG_MODE_CTRL_SGMII_MAC	0x2
+ 
+ #endif /* __ASM_MACH_AR71XX_REGS_H */