Re: [PATCH v2] phy state machine: failsafe leave invalid RUNNING state

From: Zefir Kurtisi <redacted>
Date: Fri,  6 Jan 2017 12:14:48 +0100

While in RUNNING state, phy_state_machine() checks for link changes by
comparing phydev->link before and after calling phy_read_status().
This works as long as it is guaranteed that phydev->link is never
changed outside the phy_state_machine().

If in some setups this happens, it causes the state machine to miss
a link loss and remain RUNNING despite phydev->link being 0.

This has been observed running a dsa setup with a process continuously
polling the link states over ethtool each second (SNMPD RFC-1213
agent). Disconnecting the link on a phy followed by a ETHTOOL_GSET
causes dsa_slave_get_settings() / dsa_slave_get_link_ksettings() to
call phy_read_status() and with that modify the link status - and
with that bricking the phy state machine.

This patch adds a fail-safe check while in RUNNING, which causes to
move to CHANGELINK when the link is gone and we are still RUNNING.

Signed-off-by: Zefir Kurtisi <redacted>
---
Changes to v1:
* fix kbuild test robot error: use phydev_err instead of dev_warn
  (adapt to changed struct phy_device after 4.4.21)

Florian and Andrew, please provide some feedback on this.

Thank you.