netdev - 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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Date:   Thu, 2 Sep 2021 15:59:23 +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,
        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,
        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 03:35:32PM +0300, Vladimir Oltean wrote:
> On Thu, Sep 02, 2021 at 01:19:27PM +0100, Russell King (Oracle) wrote:
> > On Thu, Sep 02, 2021 at 01:50:50AM +0300, Vladimir Oltean wrote:
> > > The central point of that discussion is that DSA seems "broken" for
> > > expecting the PHY driver to probe immediately on PHYs belonging to the
> > > internal MDIO buses of switches. A few suggestions were made about what
> > > to do, but some were not satisfactory and some did not solve the problem.
> > 
> > I think you need to describe the mechanism here. Why wouldn't a PHY
> > belonging to an internal MDIO bus of a switch not probe immediately?
> > What resources may not be available?
> 
> As you point out below, the interrupt-controller is what is not available.
> There is a mechanism called fw_devlink which infers links from one OF
> node to another based on phandles. When you have an interrupt-parent,
> that OF node becomes a supplier to you. Those OF node links are then
> transferred to device links once the devices having those OF nodes are
> created.
> 
> > If we have a DSA driver that tries to probe the PHYs before e.g. the
> > interrupt controller inside the DSA switch has been configured, aren't
> > we just making completely unnecessary problems for ourselves?
> 
> This is not what happens, if that were the case, of course I would fix
> _that_ and not in this way.
> 
> > Wouldn't it be saner to ensure that the interrupt controller has been
> > setup and become available prior to attempting to setup anything that
> > relies upon that interrupt controller?
> 
> The interrupt controller _has_ been set up. The trouble is that the
> interrupt controller has the same OF node as the switch itself, and the
> same OF node. Therefore, fw_devlink waits for the _entire_ switch to

...and the same struct device, not "OF node" repeated twice, silly me.

> finish probing, it doesn't have insight into the fact that the
> dependency is just on the interrupt controller.
> 
> > From what I see of Marvell switches, the internal PHYs only ever rely
> > on internal resources of the switch they are embedded in.
> > 
> > External PHYs to the switch are a different matter - these can rely on
> > external clocks, and in that scenario, it would make sense for a
> > deferred probe to cause the entire switch to defer, since we don't
> > have all the resources for the switch to be functional (and, because we
> > want the PHYs to be present at switch probe time, not when we try to
> > bring up the interface, I don't see there's much other choice.)
> > 
> > Trying to move that to interface-up time /will/ break userspace - for
> > example, Debian's interfaces(8) bridge support will become unreliable,
> > and probably a whole host of other userspace. It will cause regressions
> > and instability to userspace. So that's a big no.
> 
> Why a big no? 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.