Re: [PATCH net] net: phy: fix phy_uses_state_machine()

[Vladimir Oltean <vladimir.oltean@nxp.com>]

On Sun, Sep 07, 2025 at 09:44:01PM +0100, Russell King (Oracle) wrote:
> The blamed commit changed the conditions which phylib uses to stop
> and start the state machine in the suspend and resume paths, and
> while improving it, has caused two issues.
> 
> The original code used this test:
> 
> 	phydev->attached_dev && phydev->adjust_link
> 
> and if true, the paths would handle the PHY state machine. This test
> evaluates true for normal drivers that are using phylib directly
> while the PHY is attached to the network device, but false in all
> other cases, which include the following cases:
> 
> - when the PHY has never been attached to a network device.
> - when the PHY has been detached from a network device (as phy_detach()
>    sets phydev->attached_dev to NULL, phy_disconnect() calls
>    phy_detach() and additionally sets phydev->adjust_link NULL.)
> - when phylink is using the driver (as phydev->adjust_link is NULL.)
> 
> Only the third case was incorrect, and the blamed commit attempted to
> fix this by changing this test to (simplified for brevity, see
> phy_uses_state_machine()):
> 
> 	phydev->phy_link_change == phy_link_change ?
> 		phydev->attached_dev && phydev->adjust_link : true
> 
> However, this also incorrectly evaluates true in the first two cases.
> 
> Fix the first case by ensuring that phy_uses_state_machine() returns
> false when phydev->phy_link_change is NULL.
> 
> Fix the second case by ensuring that phydev->phy_link_change is set to
> NULL when phy_detach() is called.
> 
> Reported-by: Xu Yang <xu.yang_2@nxp.com>
> Link: https://lore.kernel.org/r/20250806082931.3289134-1-xu.yang_2@nxp.com
> Fixes: fc75ea20ffb4 ("net: phy: allow MDIO bus PM ops to start/stop state machine for phylink-controlled PHY")
> Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
> ---
> 
> v2: updated commit description

..and made an addition to phy_detach() which fixes the second case.

> 
>  drivers/net/phy/phy_device.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)

Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>