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

[Vladimir Oltean <olteanv@gmail.com>]

On Tue, Aug 31, 2021 at 07:19:40PM -0700, Saravana Kannan wrote:
> On Tue, Aug 31, 2021 at 6:38 PM Vladimir Oltean <olteanv@gmail.com> 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.

But I mean, if the goal of fw_devlink is to infer a probing order based
on phandles, and it is faced with a long chain of devices, then any
-EPROBE_DEFER of a device on top of the chain will break the probing
order for all devices beneath it. It is self-defeating, it is already
memory used for nothing.

> > >
> > > 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.

Yes, -EPROBE_DEFER is what I meant.

> >
> > > 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.

I think this is the real problem which needs to be addressed. The
trouble is, I don't know if phy_attach_direct can find out the reason
for which d->driver is NULL, i.e. that there was a driver which matched
and attempted the probe, but returned -EPROBE_DEFER.

> -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...