linux-kernel - Re: [PATCH] net: fec: defer probe if PHY on external MDIO bus is not available

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Date:   Wed, 03 Nov 2021 12:59:16 +0100
From:   Matthias Schiffer <matthias.schiffer@...tq-group.com>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Joakim Zhang <qiangqing.zhang@....com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net: fec: defer probe if PHY on external MDIO bus is
 not available

On Tue, 2021-10-26 at 13:54 +0200, Matthias Schiffer wrote:
> On Fri, 2021-10-22 at 14:56 +0200, Andrew Lunn wrote:
> > > Hmm, lots of network drivers? I tried to find an example, but all
> > > drivers that generate -EPROBE_DEFER for missing PHYs at all don't have
> > > an internal MDIO bus and thus avoid the circular dependency.
> > 
> > Try drivers/net/dsa.
> > 
> > These often have mdio busses which get registered and then
> > unregistered. There are also IRQ drivers which do the same.
> > 
> > 	Andrew
> 
> 
> All drivers I looked at were careful to register the MDIO bus in the
> last part of the probe function, so that the only errors that could 
> happen after that (and thus require to unregister the bus device again)
> would not be -EPROBE_DEFER.
> 
> The documented infinite loop is easy to reproduce: You just need two
> separate device instances with misbehaving probe() functions that
> return -EPROBE_DEFER after registering and unregistering some
> subdevice. If the missing device that causes the deferral never appears
> (for example because its driver is not available), the two devices will
> be probed ad infinitum.
> 
> I agree with the documentation that a driver should not do this, and
> thus we need another way to deal with the cyclic dependency between an
> Ethernet interface and a PHY on its internal MDIO bus.
> 
> While I think that a generic solution would be theoretically possible
> by ensuring that the probing loop makes some kind of "progress", I
> think this would set a precedent for "expensive" operations happening
> before a -EPROBE_DEFER return, which should be avoided even when no
> infinite loop results.
> 
> Matthias

Does anyone have a suggestion how to proceed with this issue?