[PATCH net-next v13 05/13] net: ethtool: Allow passing a phy index for some commands

[Russell King (Oracle)]

On Sun, Jun 16, 2024 at 05:21:25PM +0200, Andrew Lunn wrote:
> On Sun, Jun 16, 2024 at 06:02:31PM +0200, Maxime Chevallier wrote:
> > Hello Jakub,
> > 
> > On Thu, 13 Jun 2024 18:26:13 -0700
> > Jakub Kicinski <kuba at kernel.org> wrote:
> > 
> > > On Fri,  7 Jun 2024 09:18:18 +0200 Maxime Chevallier wrote:
> > > > +		if (tb[ETHTOOL_A_HEADER_PHY_INDEX]) {
> > > > +			struct nlattr *phy_id;
> > > > +
> > > > +			phy_id = tb[ETHTOOL_A_HEADER_PHY_INDEX];
> > > > +			phydev = phy_link_topo_get_phy(dev,
> > > > +						       nla_get_u32(phy_id));  
> > > 
> > > Sorry for potentially repeating question (please put the answer in the
> > > commit message) - are phys guaranteed not to disappear, even if the
> > > netdev gets closed? this has no rtnl protection
> > 
> > I'll answer here so that people can correct me if I'm wrong, but I'll
> > also add it in the commit logs as well (and possibly with some fixes
> > depending on how this discussion goes)
> > 
> > While a PHY can be attached to/detached from a netdevice at open/close,
> > the phy_device itself will keep on living, as its lifetime is tied to
> > the underlying mdio_device (however phy_attach/detach take a ref on the
> > phy_device, preventing it from vanishing while it's attached to a
> > netdev)
> 
> It gets interesting with copper SFP. They contain a PHY, and that PHY
> can physically disappear at any time. What i don't know is when the
> logical representation of the PHY will disappear after the hotunplug
> event.

On a SFP module unplug, the following upstream device methods will be
called in order:
1. link_down
2. module_stop
3. disconnect_phy

At this point, the PHY device will be removed (phy_device_remove()) and
freed (phy_device_free()), and shortly thereafter, the MDIO bus is
unregistered and thus destroyed.

In response to the above, phylink will, respectively for each method:

1. disable the netdev carrier and call mac_link_down()
2. call phy_stop() on the attached PHY
3. remove the PHY from phylink, and then call phy_disconnect(),
   disconnecting it from the netdev.

Thus, when a SFP PHY is being removed, phylib will see in order the
following calls:

	phy_disconnect()
	phy_device_remove()
	phy_device_free()

Provided the topology linkage is removed on phy_disconnect() which
disassociates the PHY from the netdev, SFP PHYs should be fine.

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