net: fec: Fix temporary RMII clock reset on link up

fec_restart() does a hard reset of the MAC module when the link status
changes to up. This temporarily resets the R_CNTRL register which controls
the MII mode of the ENET_OUT clock. In the case of RMII, the clock
frequency momentarily drops from 50MHz to 25MHz until the register is
reconfigured. Some link partners do not tolerate this glitch and
invalidate the link causing failure to establish a stable link when using
PHY polling mode. Since as per IEEE802.3 the criteria for link validity
are PHY-specific, what the partner should tolerate cannot be assumed, so
avoid resetting the MII clock by using software reset instead of hardware
reset when the link is up. This is generally relevant only if the SoC
provides the clock to an external PHY and the PHY is configured for RMII.
Signed-off-by: Laurent Badel <laurentbadel@eaton.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/ethernet/freescale/fec.h

@@ -462,6 +462,11 @@ struct bufdesc_ex {
  */
 #define FEC_QUIRK_CLEAR_SETUP_MII	(1 << 17)
 
+/* Some link partners do not tolerate the momentary reset of the REF_CLK
+ * frequency when the RNCTL register is cleared by hardware reset.
+ */
+#define FEC_QUIRK_NO_HARD_RESET		(1 << 18)
+
 struct bufdesc_prop {
 	int qid;
 	/* Address of Rx and Tx buffers */

drivers/net/ethernet/freescale/fec_main.c

@@ -100,7 +100,8 @@ static const struct fec_devinfo fec_imx27_info = {
 static const struct fec_devinfo fec_imx28_info = {
 	.quirks = FEC_QUIRK_ENET_MAC | FEC_QUIRK_SWAP_FRAME |
 		  FEC_QUIRK_SINGLE_MDIO | FEC_QUIRK_HAS_RACC |
-		  FEC_QUIRK_HAS_FRREG | FEC_QUIRK_CLEAR_SETUP_MII,
+		  FEC_QUIRK_HAS_FRREG | FEC_QUIRK_CLEAR_SETUP_MII |
+		  FEC_QUIRK_NO_HARD_RESET,
 };
 
 static const struct fec_devinfo fec_imx6q_info = {
@@ -953,7 +954,8 @@ fec_restart(struct net_device *ndev)
 	 * For i.MX6SX SOC, enet use AXI bus, we use disable MAC
 	 * instead of reset MAC itself.
 	 */
-	if (fep->quirks & FEC_QUIRK_HAS_AVB) {
+	if (fep->quirks & FEC_QUIRK_HAS_AVB ||
+	    ((fep->quirks & FEC_QUIRK_NO_HARD_RESET) && fep->link)) {
 		writel(0, fep->hwp + FEC_ECNTRL);
 	} else {
 		writel(1, fep->hwp + FEC_ECNTRL);