linux-kernel - Re: [RFC PATCH net-next 0/3] Make the PHY library stop being so greedy when binding the generic PHY driver

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Message-ID: <20210902190507.shcdmfi3v55l2zuj@skbuf>
Date:   Thu, 2 Sep 2021 22:05:07 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     "Russell King (Oracle)" <linux@...linux.org.uk>
Cc:     Vladimir Oltean <vladimir.oltean@....com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Andrew Lunn <andrew@...n.ch>,
        Heiner Kallweit <hkallweit1@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Florian Fainelli <f.fainelli@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Alvin Šipraga <alsi@...g-olufsen.dk>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        kernel-team <kernel-team@...roid.com>,
        Len Brown <lenb@...nel.org>
Subject: Re: [RFC PATCH net-next 0/3] Make the PHY library stop being so
 greedy when binding the generic PHY driver

On Thu, Sep 02, 2021 at 06:50:43PM +0100, Russell King (Oracle) wrote:
> On Thu, Sep 02, 2021 at 05:10:34PM +0000, Vladimir Oltean wrote:
> > On Thu, Sep 02, 2021 at 05:31:44PM +0100, Russell King (Oracle) wrote:
> > > On Thu, Sep 02, 2021 at 06:23:42PM +0300, Vladimir Oltean wrote:
> > > > On Thu, Sep 02, 2021 at 02:26:35PM +0100, Russell King (Oracle) wrote:
> > > > > Debian has had support for configuring bridges at boot time via
> > > > > the interfaces file for years. Breaking that is going to upset a
> > > > > lot of people (me included) resulting in busted networks. It
> > > > > would be a sure way to make oneself unpopular.
> > > > >
> > > > > > I expect there to be 2 call paths of phy_attach_direct:
> > > > > > - At probe time. Both the MAC driver and the PHY driver are probing.
> > > > > >   This is what has this patch addresses. There is no issue to return
> > > > > >   -EPROBE_DEFER at that time, since drivers connect to the PHY before
> > > > > >   they register their netdev. So if connecting defers, there is no
> > > > > >   netdev to unregister, and user space knows nothing of this.
> > > > > > - At .ndo_open time. This is where it maybe gets interesting, but not to
> > > > > >   user space. If you open a netdev and it connects to the PHY then, I
> > > > > >   wouldn't expect the PHY to be undergoing a probing process, all of
> > > > > >   that should have been settled by then, should it not? Where it might
> > > > > >   get interesting is with NFS root, and I admit I haven't tested that.
> > > > >
> > > > > I don't think you can make that assumption. Consider the case where
> > > > > systemd is being used, DSA stuff is modular, and we're trying to
> > > > > setup a bridge device on DSA. DSA could be probing while the bridge
> > > > > is being setup.
> > > > >
> > > > > Sadly, this isn't theoretical. I've ended up needing:
> > > > >
> > > > > 	pre-up sleep 1
> > > > >
> > > > > in my bridge configuration to allow time for DSA to finish probing.
> > > > > It's not a pleasant solution, nor a particularly reliable one at
> > > > > that, but it currently works around the problem.
> > > >
> > > > What problem? This is the first time I've heard of this report, and you
> > > > should definitely not need that.
> > >
> > > I found it when upgrading the Clearfog by the DSL modems to v5.13.
> > > When I rebooted it with a previously working kernel (v5.7) it has
> > > never had a problem. With v5.13, it failed to add all the lan ports
> > > into the bridge, because the bridge was still being setup by the
> > > kernel while userspace was trying to configure it. Note that I have
> > > extra debug in my kernels, hence the extra messages:
> >
> > Ok, first you talked about the interfaces file, then systemd. If it's
> > not about systemd's network manager then I don't see how it is relevant.
>
> You're reading in stuff to what I write that I did not write... I said:
>
> "Consider the case where systemd is being used, DSA stuff is modular,
> and we're trying to setup a bridge device on DSA."
>
> That does not mean I'm using systemd's network manager - which is
> something I know little about and have never used.

You should definitely try it out, it gets a lot of new features added
all the time, it uses the netlink interface, it reacts on udev events.

> The reason I mentioned systemd is precisely because with systemd, you
> get a hell of a lot happening parallel - and that's significiant in
> this case, because it's very clear that modules are being loaded in
> parallel with networking being brought up - and that is where the
> problems begin. In fact, modules themselves get loaded in paralllel
> with systemd.

So that's what I don't understand. You're saying that the ifupdown
service runs in parallel with systemd-modules-load.service, and
networking is a kernel module? Doesn't that mean it behaves as expected,
then? /shrug/
Have you tried adding an 'After=systemd-modules-load.service' dependency
to the ifupdown service? I don't think that DSA is that bad that it
registers its net devices outside of the process context in which the
insmod mv88e6xxx.ko is called. Quite the opposite, I think (but I
haven't actually taken a close look yet) that the component stuff
Saravana is proposing would do exactly that. So you "fix" one issue, you
introduce another.