netdev - Re: [PATCH net-next v4 01/13] net: phy: Introduce ethernet link topology representation

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Date: Sun, 17 Dec 2023 17:57:10 +0100
From: Andrew Lunn <andrew@...n.ch>
To: Vladimir Oltean <vladimir.oltean@....com>
Cc: Maxime Chevallier <maxime.chevallier@...tlin.com>, davem@...emloft.net,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	thomas.petazzoni@...tlin.com, Jakub Kicinski <kuba@...nel.org>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	Russell King <linux@...linux.org.uk>,
	linux-arm-kernel@...ts.infradead.org,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Herve Codina <herve.codina@...tlin.com>,
	Florian Fainelli <f.fainelli@...il.com>,
	Heiner Kallweit <hkallweit1@...il.com>,
	Köry Maincent <kory.maincent@...tlin.com>,
	Jesse Brandeburg <jesse.brandeburg@...el.com>,
	Jonathan Corbet <corbet@....net>,
	Marek Behún <kabel@...nel.org>,
	Piergiorgio Beruto <piergiorgio.beruto@...il.com>,
	Oleksij Rempel <o.rempel@...gutronix.de>,
	Nicolò Veronese <nicveronese@...il.com>
Subject: Re: [PATCH net-next v4 01/13] net: phy: Introduce ethernet link
 topology representation

On Fri, Dec 15, 2023 at 11:45:23PM +0200, Vladimir Oltean wrote:
> On Fri, Dec 15, 2023 at 06:12:23PM +0100, Maxime Chevallier wrote:
> > diff --git a/drivers/net/phy/phy_device.c b/drivers/net/phy/phy_device.c
> > index d8e9335d415c..89daaccc9276 100644
> > --- a/drivers/net/phy/phy_device.c
> > +++ b/drivers/net/phy/phy_device.c
> > @@ -1491,6 +1500,11 @@ int phy_attach_direct(struct net_device *dev, struct phy_device *phydev,
> >  
> >  		if (phydev->sfp_bus_attached)
> >  			dev->sfp_bus = phydev->sfp_bus;
> > +
> > +		err = phy_link_topo_add_phy(&dev->link_topo, phydev,
> > +					    PHY_UPSTREAM_MAC, dev);
> > +		if (err)
> > +			goto error;
> >  	}
> >  
> >  	/* Some Ethernet drivers try to connect to a PHY device before
> > @@ -1816,6 +1830,7 @@ void phy_detach(struct phy_device *phydev)
> >  	if (dev) {
> >  		phydev->attached_dev->phydev = NULL;
> >  		phydev->attached_dev = NULL;
> > +		phy_link_topo_del_phy(&dev->link_topo, phydev);
> >  	}
> >  	phydev->phylink = NULL;
> >  
> > diff --git a/drivers/net/phy/phy_link_topology.c b/drivers/net/phy/phy_link_topology.c
> > new file mode 100644
> > index 000000000000..22f6372d002c
> > --- /dev/null
> > +++ b/drivers/net/phy/phy_link_topology.c
> > +int phy_link_topo_add_phy(struct phy_link_topology *topo,
> > +			  struct phy_device *phy,
> > +			  enum phy_upstream upt, void *upstream)
> > +{
> > +	struct phy_device_node *pdn;
> > +	int ret;
> > +
> > +	/* Protects phy and upstream */
> > +	ASSERT_RTNL();
> 
> Something to think for the PHY library maintainers. This is probably
> the first time when the rtnl_lock() requirement is asserted at
> phy_attach_direct() time.

There are two use cases here for plain MAC drivers.

1) phy_attach_direct() is called from probe. RTNL is normally not
held, the driver would have to take it before making the call.

2) phy_attach_direct() is called from ndo_open. In that case,
__dev_open() has a ASSERT_RTNL() so we can assume RTNL has been taken.

So i don't think we can assume RTNL is held, but it might be held.

We need a better understanding what is being protected here.

   Andrew