net: dsa: lantiq_gswip: Don't set GSWIP_MII_CFG_RMII_CLK

Commit 4b592324 ("net: dsa: lantiq_gswip: Configure all remaining
GSWIP_MII_CFG bits") added all known bits in the GSWIP_MII_CFGp
register. It helped bring this register into a well-defined state so the
driver has to rely less on the bootloader to do things right.
Unfortunately it also sets the GSWIP_MII_CFG_RMII_CLK bit without any
possibility to configure it. Upon further testing it turns out that all
boards which are supported by the GSWIP driver in OpenWrt which use an
RMII PHY have a dedicated oscillator on the board which provides the
50MHz RMII reference clock.

Don't set the GSWIP_MII_CFG_RMII_CLK bit (but keep the code which always
clears it) to fix support for the Fritz!Box 7362 SL in OpenWrt. This is
a board with two Atheros AR8030 RMII PHYs. With the "RMII clock" bit set
the MAC also generates the RMII reference clock whose signal then
conflicts with the signal from the oscillator on the board. This results
in a constant cycle of the PHY detecting link up/down (and as a result
of that: the two ports using the AR8030 PHYs are not working).

At the time of writing this patch there's no known board where the MAC
(GSWIP) has to generate the RMII reference clock. If needed this can be
implemented in future by providing a device-tree flag so the
GSWIP_MII_CFG_RMII_CLK bit can be toggled per port.

Fixes: 4b592324 ("net: dsa: lantiq_gswip: Configure all remaining GSWIP_MII_CFG bits")
Tested-by: Jan Hoffmann <jan@3e8.eu>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Link: https://lore.kernel.org/r/20220425152027.2220750-1-martin.blumenstingl@googlemail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/dsa/lantiq_gswip.c

@@ -1681,9 +1681,6 @@ static void gswip_phylink_mac_config(struct dsa_switch *ds, int port,
 		break;
 	case PHY_INTERFACE_MODE_RMII:
 		miicfg |= GSWIP_MII_CFG_MODE_RMIIM;
-
-		/* Configure the RMII clock as output: */
-		miicfg |= GSWIP_MII_CFG_RMII_CLK;
 		break;
 	case PHY_INTERFACE_MODE_RGMII:
 	case PHY_INTERFACE_MODE_RGMII_ID: