netdev - Re: racing ndo_open()/phylink*connect() with phy

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Message-ID: <i3w534hh4o2klrehag7cwjshwiqxergidzo4h7zz7oa3prra2k@v6xor5k4dv5x>
Date: Tue, 7 May 2024 09:48:30 -0500
From: Andrew Halaney <ahalaney@...hat.com>
To: Andrew Lunn <andrew@...n.ch>
Cc: Serge Semin <fancer.lancer@...il.com>, 
	"Russell King (Oracle)" <linux@...linux.org.uk>, linux-kernel@...r.kernel.org, 
	linux-arm-kernel@...ts.infradead.org, linux-stm32@...md-mailman.stormreply.com, netdev@...r.kernel.org, 
	alexandre.torgue@...s.st.com, joabreu@...opsys.com, davem@...emloft.net, edumazet@...gle.com, 
	kuba@...nel.org, pabeni@...hat.com, mcoquelin.stm32@...il.com, 
	hkallweit1@...il.com
Subject: Re: racing ndo_open()/phylink*connect() with phy_probe()

On Fri, May 03, 2024 at 03:25:19AM GMT, Andrew Lunn wrote:
> > AFAICS the problem is in the race between the end0 and end1 device
> > probes. Right?
> > If so then can't the order be fixed by adding the links between the
> > OF-devices?  As it's already done for various phandle-based references
> > like "clocks", "gpios", "phys", etc?

Thanks for the pointer here Serge, I had no idea (still don't have much of an
idea) on how this works. I think this makes sense to explore some more.
Hopefully sometime this week I'll poke at this more.

> 
> It gets tricky because an MDIO bus master device is often a sub device
> of an Ethernet MAC driver. Typically how it works is that a MAC driver
> probes. Part way through the probe it creates an MDIO bus driver,
> which enumerates the MDIO bus and creates the PHYs. Later in the MAC
> driver probe, or maybe when the MAC driver is opened, it follows the
> phy-handle to a PHY on its own MDIO bus.
> 
> If you were to say it cannot probe the MAC driver until the MDIO bus
> driver is created and the PHYs created, you never get anywhere,
> because it is the act of probing the MAC driver which creates the PHYs
> which fulfils the phandle.
> 
> You would need to differentiate between a phandle pointing deeper into
> the same branch of a DT tree, or pointing to a different branch of a
> DT tree. If it is deeper within the same branch, you need to probe in
> order to make progress. If it points to a different branch you need to
> defer until that sub-branch has successfully probed. And if you get
> two branches which are mutually dependent on each other, probe and
> hope EPROBE_DEFER solves it.

I'll keep this relationship in mind. IIUC the fw_devlink stuff sort of handles
cycles, but I need to look into how all that works further. At least in
my example device, end0 is in this situation, whereas end1 is in the
other situation, so I have a decent test setup for that.