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

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Date:   Fri, 22 Oct 2021 09:57:54 +0200
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 Thu, 2021-10-21 at 14:50 +0200, Andrew Lunn wrote:
> > I would love to do this, but driver-api/driver-model/driver.rst
> > contains the following warning:
> > 
> >       -EPROBE_DEFER must not be returned if probe() has already created
> >       child devices, even if those child devices are removed again
> >       in a cleanup path. If -EPROBE_DEFER is returned after a child
> >       device has been registered, it may result in an infinite loop of
> >       .probe() calls to the same driver.
> > 
> > My understanding of this is that there is simply no way to return
> > -EPROBE_DEFER after fec_enet_mii_init(pdev).
> 
> It might say that, but lots of network drivers actually do this. I've
> not seen an endless loop.
> 
>     Andrew


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.

Matthias