netdev - Re: [PATCH net] net: phy: fix phy_uses_state

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Message-ID: <aLWqWeGvMM350dB2@shell.armlinux.org.uk>
Date: Mon, 1 Sep 2025 15:14:49 +0100
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Vladimir Oltean <vladimir.oltean@....com>
Cc: Andrew Lunn <andrew@...n.ch>, Heiner Kallweit <hkallweit1@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
	Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net] net: phy: fix phy_uses_state_machine()

On Mon, Sep 01, 2025 at 01:36:24PM +0300, Vladimir Oltean wrote:
> On Mon, Sep 01, 2025 at 11:05:54AM +0100, Russell King (Oracle) wrote:
> > Well, having spent considerable time writing and rewriting the damn
> > commit message, this is what I'm now using, which I think covers the
> > problem in sufficient detail.
> > 
> > >>>>>>
> > net: phy: fix phy_uses_state_machine()
> > 
> > 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.
> > <<<<<<
> 
> The new explanation and the placement of the function pointer clearing
> make sense, thanks. Maybe add one last sentence at the end: "This covers
> both the phylink_bringup_phy() error path, as well as the normal
> phylink_disconnect_phy() path."

Is it really necessary to add that level of detail? phy_attach() sets
phydev->phy_link_change, so it's natural that phy_detach() should undo
that action, especially as phy_detach() is phy_attach()'s complement.


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