Re: [PATCH v1 1/2] driver core: fw_devlink: Add support for FWNODE_FLAG_BROKEN_PARENT

[Saravana Kannan]

On Tue, Aug 31, 2021 at 6:38 PM Vladimir Oltean <olteanv@xxxxxxxxx> wrote:
>
> On Wed, Sep 01, 2021 at 04:28:26AM +0300, Vladimir Oltean wrote:
> > On Wed, Sep 01, 2021 at 02:18:04AM +0300, Vladimir Oltean wrote:
> > > On Wed, Sep 01, 2021 at 01:02:09AM +0200, Andrew Lunn wrote:
> > > > Rev B is interesting because switch0 and switch1 got genphy, while
> > > > switch2 got the correct Marvell PHY driver. switch2 PHYs don't have
> > > > interrupt properties, so don't loop back to their parent device.
> > >
> > > This is interesting and not what I really expected to happen. It goes to
> > > show that we really need more time to understand all the subtleties of
> > > device dependencies before jumping on patching stuff.
> >
> > There is an even more interesting variation which I would like to point
> > out. It seems like a very odd loophole in the device links.
> >
> > Take the example of the mv88e6xxx DSA driver. On my board
> > (arch/arm64/boot/dts/marvell/armada-3720-turris-mox.dts), even after I
> > had to declare the switches as interrupt controller and add interrupts
> > to their internal PHYs, I still need considerable force to 'break' this
> > board in the way discussed in this thread. The correct PHY driver insists
> > to probe, and not genphy. Let me explain.
> >
> > The automatic device links between the switch (supplier, as interrupt-controller)
> > and PHYs (consumers) are added by fwnode_link_add, called from of_link_to_phandle.
> >
> > Important note: fwnode_link_add does not link devices, it links OF nodes.
> >
> > Even more important node, in the form of a comment:
> >
> >  * The driver core will use the fwnode link to create a device link between the
> >  * two device objects corresponding to @con and @sup when they are created. The
> >  * driver core will automatically delete the fwnode link between @con and @sup
> >  * after doing that.
> >
> > Okay?!
> >
> > What seems to be omitted is that the DSA switch driver's probing itself
> > can be deferred. For example:
> >
> > dsa_register_switch
> > -> dsa_switch_probe
> >    -> dsa_switch_parse_of
> >       -> dsa_switch_parse_ports_of
> >          -> dsa_port_parse_of
> >             -> of_find_net_device_by_node(of_parse_phandle(dn, "ethernet", 0));
> >             -> not found => return -EPROBE_DEFER
> >
> > When dsa_register_switch() returns -EPROBE_DEFER, it is effectively
> > an error path. So the reverse of initialization is performed.
> >
> > The mv88e6xxx driver calls mv88e6xxx_mdios_register() right _before_
> > dsa_register_switch. So when dsa_register_switch returns error code,
> > mv88e6xxx_mdios_unregister() will be called.
> >
> > When mv88e6xxx_mdios_unregister() is called, the MDIO buses with
> > internal PHYs are destroyed. So the PHY devices themselves are destroyed
> > too. And the device links between the DSA switch and the internal PHYs,
> > those created based on the firmware node links created by fwnode_link_add,
> > are dropped too.
> >
> > Now remember the comment that the device links created based on
> > fwnode_link_add are not restored.
> >
> > So probing of the DSA switch finally resumes, and this time
> > device_links_check_suppliers() is effectively bypassed, the PHYs no
> > longer request probe deferral due to their supplier not being ready,
> > because the device link no longer exists.
> >
> > Isn't this self-sabotaging?!

Yeah, this is a known "issue". I'm saying "issue" because at worst
it'd allow a few unnecessary deferred probes. And if you want to break
or get fw_devlink to ignore your child devices or your consumers,
there are simpler APIs to do it without having to intentionally defer
a probe.  Fixing this "issue" would just use up more memory and
increase boot time for no meaningful benefit.

> >
> > Now generally, DSA drivers defer probing because they probe in parallel
> > with the DSA master. This is typical if the switch is on a SPI bus, or
> > I2C, or on an MDIO bus provided by a _standalone_ MDIO controller.
> >
> > If the MDIO controller is not standalone, but is provided by Ethernet
> > controller that is the DSA master itself, then things change a lot,
> > because probing can never be parallel. The DSA master probes,
> > initializes its MDIO bus, and this triggers the probing of the MDIO
> > devices on that bus, one of which is the DSA switch. So DSA can no
> > longer defer the probe due to that reason.
> >
> > Secondly, in DSA we even have variation between drivers as to where they
> > register their internal MDIO buses. The mv88e6xxx driver does this in
> > mv88e6xxx_probe (the probe function on the MDIO bus). The rtl8366rb
> > driver calls realtek_smi_setup_mdio() from rtl8366rb_setup(), and this
> > is important. DSA provides drivers with a .setup() callback, which is
> > guaranteed to take place after nothing can defer the switch's probe
> > anymore.
> >
> > So putting two and two together, sure enough, if I move mv88e6xxx_mdios_register
> > from mv88e6xxx_probe to mv88e6xxx_setup, then I can reliably break this
> > setup, because the device links framework isn't sabotaging itself anymore.
> >
> > Conversely, I am pretty sure that if rtl8366rb was to call of_mdiobus_register()
> > from the probe method and not the setup method, the entire design issue
> > with interrupts on internal DSA switch ports would have went absolutely
> > unnoticed for a few more years.
> >
> > I have not tested this, but it also seems plausible that DSA can
> > trivially and reliably bypass any fw_devlink=on restrictions by simply
> > moving all of_mdiobus_register() driver calls from the .setup() method
> > to their respective probe methods (prior to calling dsa_register_switch),
> > then effectively fabricate an -EPROBE_DEFER during the first probe attempt.
> > I mean, who will know whether that probe deferral request was justified
> > or not?
>
> Pushing the thought even further, it is not even necessary to move the
> of_mdiobus_register() call to the probe function. Where it is (in .setup)
> is already good enough. It is sufficient to return -EOPNOTSUPP once
> (the first time) immediately _after_ the call to of_mdiobus_register
> (and have a proper error path, i.e. call mdiobus_unregister too).

Right, there are plenty of ways to intentionally break fw_devlink. I
hope that's not the point :) And I don't think -EOPNOTSUPP would work
because your device wouldn't be probed again.

>
> > Anyway, I'm not sure everyone agrees with this type of "solution" (even
> > though it's worth pointing it out as a fw_devlink limitation). In any
> > case, we need some sort of lightweight "fix" to the chicken-and-egg
> > problem, which will give me enough time to think of something better.

I think the generic DSA patch I gave would be the lightweight fix to
address this chicken-and-egg issue.

As for the long term fix, I'd really suggest looking into using the
component device model. I'd even be happy to help make any driver
core/component device improvements you might need.

I'm also interested in looking into improving the PHY probing so that
the genphy never probes a device that has a driver that could probe
it. Even outside of all this fw_devlink thing, they way PHY is handled
now, if any of the supplier really isn't ready yet (say a clock), then
the genphy gets used -- which isn't good.

-Saravana

> > I hope it is at least clearer now that there are subtleties and nuances,
> > and we cannot just assess how many boards are broken by looking at the
> > device trees. By design, all are, sure, but they might still work, and
> > that's better than nothing...