[PATCH net] net: mii: Fix unknown speed after link up

[Linmao Li]

mii_ethtool_get_link_ksettings() reads BMSR only once. Since
BMSR_LSTATUS is latched low, the first query after link up can
report SPEED_UNKNOWN even though the link is already up.

This is seen with r8152, which detects carrier using a MAC register
without clearing the BMSR latch. NetworkManager can then keep
reporting 0 Mb/s until the next carrier change.

Read BMSR twice to obtain the current link status, as mii_link_ok()
already does.

Tested on an RTL8153B with a 6.6-based kernel. Traces confirmed
LSTATUS=0 followed by LSTATUS=1 in the same query in 20 rebind runs.
NetworkManager reported 1000 Mb/s in all 10 managed-device runs.
With the cable unplugged or the PHY powered down, the speed is still
reported as unknown.

Fixes: f6f2e946aa4d ("net: mii: Fix the Speed display when the network cable is not connected")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Linmao Li <lilinmao@xxxxxxxxxx>
---
Runtime testing used a 6.6-based kernel containing the change identified
by the Fixes tag. The tested drivers/net/mii.c is identical to the
patched version based on net/main.

 drivers/net/mii.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/net/mii.c b/drivers/net/mii.c
index 37bc3131d31a..bd4fdcb55df3 100644
--- a/drivers/net/mii.c
+++ b/drivers/net/mii.c
@@ -163,6 +163,8 @@ void mii_ethtool_get_link_ksettings(struct mii_if_info *mii,
 	advertising = ADVERTISED_TP | ADVERTISED_MII;
 
 	bmcr = mii->mdio_read(dev, mii->phy_id, MII_BMCR);
+	/* BMSR_LSTATUS is latched low, dummy read to get the current state */
+	mii->mdio_read(dev, mii->phy_id, MII_BMSR);
 	bmsr = mii->mdio_read(dev, mii->phy_id, MII_BMSR);
 	if (mii->supports_gmii) {
 		ctrl1000 = mii->mdio_read(dev, mii->phy_id, MII_CTRL1000);
-- 
2.25.1