netdev - [question] net: phy: rtl8211f: link speed shows 1000Mb/s but actual link speed in phy is 100Mb/s

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Date:   Tue, 12 May 2020 20:48:21 +0800
From:   Yonglong Liu <liuyonglong@...wei.com>
To:     Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        "David S. Miller" <davem@...emloft.net>
CC:     <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        <linuxarm@...wei.com>, Salil Mehta <salil.mehta@...wei.com>
Subject: [question] net: phy: rtl8211f: link speed shows 1000Mb/s but actual
 link speed in phy is 100Mb/s

I use two devices, both support 1000M speed, they are directly connected
with a network cable. Two devices enable autoneg, and then do the following
test repeatedly:
	ifconfig eth5 down
	ifconfig eth5 up
	sleep $((RANDOM%6))
	ifconfig eth5 down
	ifconfig eth5 up
	sleep 10

With low probability, one device A link up with 100Mb/s, the other B link up with
1000Mb/s(the actual link speed read from phy is 100Mb/s), and the network can
not work.

device A:
Settings for eth5:
        Supported ports: [ TP ]
        Supported link modes:   10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Full
        Supported pause frame use: Symmetric Receive-only
        Supports auto-negotiation: Yes
        Supported FEC modes: Not reported
        Advertised link modes:  10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Full
        Advertised pause frame use: Symmetric
        Advertised auto-negotiation: Yes
        Advertised FEC modes: Not reported
        Link partner advertised link modes:  10baseT/Half 10baseT/Full
                                             100baseT/Half 100baseT/Full
        Link partner advertised pause frame use: Symmetric
        Link partner advertised auto-negotiation: Yes
        Link partner advertised FEC modes: Not reported
        Speed: 100Mb/s
        Duplex: Full
        Port: MII
        PHYAD: 3
        Transceiver: internal
        Auto-negotiation: on
        Current message level: 0x00000036 (54)
                               probe link ifdown ifup
        Link detected: yes

The regs value read from mdio are:
reg 9 = 0x200
reg a = 0

device B:
Settings for eth5:
        Supported ports: [ TP ]
        Supported link modes:   10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Full
        Supported pause frame use: Symmetric Receive-only
        Supports auto-negotiation: Yes
        Supported FEC modes: Not reported
        Advertised link modes:  10baseT/Half 10baseT/Full
                                100baseT/Half 100baseT/Full
                                1000baseT/Full
        Advertised pause frame use: Symmetric
        Advertised auto-negotiation: Yes
        Advertised FEC modes: Not reported
        Link partner advertised link modes:  10baseT/Half 10baseT/Full
                                             100baseT/Half 100baseT/Full
                                             1000baseT/Full
        Link partner advertised pause frame use: Symmetric
        Link partner advertised auto-negotiation: Yes
        Link partner advertised FEC modes: Not reported
        Speed: 1000Mb/s
        Duplex: Full
        Port: MII
        PHYAD: 3
        Transceiver: internal
        Auto-negotiation: on
        Current message level: 0x00000036 (54)
                               probe link ifdown ifup
        Link detected: yes

The regs value read from mdio are:
reg 9 = 0
reg a = 0x800

I had talk to the FAE of rtl8211f, they said if negotiation failed with 1000Mb/s,
rtl8211f will change reg 9 to 0, than try to negotiation with 100Mb/s.

The problem happened as:
ifconfig eth5 up -> phy_start -> phy_start_aneg -> phy_modify_changed(MII_CTRL1000)
(this time both A and B, reg 9 = 0x200) -> wait for link up -> (B: reg 9 changed to 0)
-> link up.

I think this is the bug of the rtl8211f itself, any one have an idea to avoid this bug?

When link up, update phydev->advertising before notify the eth driver, is this method
suitable? (phydev->advertising is config from user, if user just set one speed 1000M,
it's hard to )