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    SubjectRe: [PATCH] net: phy: fix auto-negotiation in case of 'down-shift'
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    Am 24.11.2020 um 15:38 schrieb Antonio Borneo:
    > If the auto-negotiation fails to establish a gigabit link, the phy
    > can try to 'down-shift': it resets the bits in MII_CTRL1000 to
    > stop advertising 1Gbps and retries the negotiation at 100Mbps.
    >
    I see that Russell answered already. My 2cts:

    Are you sure all PHY's supporting downshift adjust the
    advertisement bits? IIRC an Aquantia PHY I dealt with does not.
    And if a PHY does so I'd consider this problematic:
    Let's say you have a broken cable and the PHY downshifts to
    100Mbps. If you change the cable then the PHY would still negotiate
    100Mbps only.

    Also I think phydev->advertising reflects what the user wants to
    advertise, as mentioned by Russell before.


    >>From commit 5502b218e001 ("net: phy: use phy_resolve_aneg_linkmode
    > in genphy_read_status") the content of MII_CTRL1000 is not checked
    > anymore at the end of the negotiation, preventing the detection of
    > phy 'down-shift'.
    > In case of 'down-shift' phydev->advertising gets out-of-sync wrt
    > MII_CTRL1000 and still includes modes that the phy have already
    > dropped. The link partner could still advertise higher speeds,
    > while the link is established at one of the common lower speeds.
    > The logic 'and' in phy_resolve_aneg_linkmode() between
    > phydev->advertising and phydev->lp_advertising will report an
    > incorrect mode.
    >
    > Issue detected with a local phy rtl8211f connected with a gigabit
    > capable router through a two-pairs network cable.
    >
    > After auto-negotiation, read back MII_CTRL1000 and mask-out from
    > phydev->advertising the modes that have been eventually discarded
    > due to the 'down-shift'.
    >
    > Fixes: 5502b218e001 ("net: phy: use phy_resolve_aneg_linkmode in genphy_read_status")
    > Cc: stable@vger.kernel.org # v5.1+
    > Signed-off-by: Antonio Borneo <antonio.borneo@st.com>
    > Link: https://lore.kernel.org/r/478f871a-583d-01f1-9cc5-2eea56d8c2a7@huawei.com
    > ---
    > To: Andrew Lunn <andrew@lunn.ch>
    > To: Heiner Kallweit <hkallweit1@gmail.com>
    > To: Russell King <linux@armlinux.org.uk>
    > To: "David S. Miller" <davem@davemloft.net>
    > To: Jakub Kicinski <kuba@kernel.org>
    > To: netdev@vger.kernel.org
    > To: Yonglong Liu <liuyonglong@huawei.com>
    > Cc: linuxarm@huawei.com
    > Cc: Salil Mehta <salil.mehta@huawei.com>
    > Cc: linux-stm32@st-md-mailman.stormreply.com
    > Cc: linux-kernel@vger.kernel.org
    > Cc: Antonio Borneo <antonio.borneo@st.com>
    >
    > drivers/net/phy/phy_device.c | 10 +++++++++-
    > 1 file changed, 9 insertions(+), 1 deletion(-)
    >
    > diff --git a/drivers/net/phy/phy_device.c b/drivers/net/phy/phy_device.c
    > index 5dab6be6fc38..5d1060aa1b25 100644
    > --- a/drivers/net/phy/phy_device.c
    > +++ b/drivers/net/phy/phy_device.c
    > @@ -2331,7 +2331,7 @@ EXPORT_SYMBOL(genphy_read_status_fixed);
    > */
    > int genphy_read_status(struct phy_device *phydev)
    > {
    > - int err, old_link = phydev->link;
    > + int adv, err, old_link = phydev->link;
    >
    > /* Update the link, but return if there was an error */
    > err = genphy_update_link(phydev);
    > @@ -2356,6 +2356,14 @@ int genphy_read_status(struct phy_device *phydev)
    > return err;
    >
    > if (phydev->autoneg == AUTONEG_ENABLE && phydev->autoneg_complete) {
    > + if (phydev->is_gigabit_capable) {
    > + adv = phy_read(phydev, MII_CTRL1000);
    > + if (adv < 0)
    > + return adv;
    > + /* update advertising in case of 'down-shift' */
    > + mii_ctrl1000_mod_linkmode_adv_t(phydev->advertising,
    > + adv);
    > + }
    > phy_resolve_aneg_linkmode(phydev);
    > } else if (phydev->autoneg == AUTONEG_DISABLE) {
    > err = genphy_read_status_fixed(phydev);
    >
    > base-commit: d549699048b4b5c22dd710455bcdb76966e55aa3
    >

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