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From:   Laurent Badel <laurentbadel@eaton.com>
To:     Fugang Duan <fugang.duan@nxp.com>,
        "David S . Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Liam Girdwood <lgirdwood@gmail.com>,
        Mark Brown <broonie@kernel.org>, <netdev@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>
CC:     Laurent Badel <laurentbadel@eaton.com>
Subject: [PATCH v2 net 0/1] net: fec: Fix temporary RMII clock reset on link up
Date:   Mon, 25 Jan 2021 11:07:44 +0100
Message-ID: <20210125100745.5090-1-laurentbadel@eaton.com>
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=EF=BB=BFv2: fixed a compilation warning=20

The FEC drivers performs a "hardware reset" of the MAC module when the
link is reported to be up. This causes a short glitch in the RMII clock=20
due to the hardware reset clearing the receive control register which=20
controls the MII mode. It seems that some link partners do not tolerate=20
this glitch, and invalidate the link, which leads to a never-ending loop
of negotiation-link up-link down events.=20

This was observed with the iMX28 Soc and LAN8720/LAN8742 PHYs, with two=20
Intel adapters I218-LM and X722-DA2 as link partners, though a number of
other link partners do not seem to mind the clock glitch. Changing the=20
hardware reset to a software reset (clearing bit 1 of the ECR register)=20
cured the issue.

Attempts to optimize fec_restart() in order to minimize the duration of=20
the glitch were unsuccessful. Furthermore manually producing the glitch by
setting MII mode and then back to RMII in two consecutive instructions,=20
resulting in a clock glitch <10us in duration, was enough to cause the=20
partner to invalidate the link. This strongly suggests that the root cause
of the link being dropped is indeed the change in clock frequency.

In an effort to minimize changes to driver, the patch proposes to use=20
soft reset only for tested SoCs (iMX28) and only if the link is up. This=20
preserves hardware reset in other situations, which might be required for
proper setup of the MAC.


Laurent Badel (1):
  net: fec: Fix temporary RMII clock reset on link up

 drivers/net/ethernet/freescale/fec.h      | 5 +++++
 drivers/net/ethernet/freescale/fec_main.c | 6 ++++--
 2 files changed, 9 insertions(+), 2 deletions(-)

--=20
2.17.1


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