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?! > > 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). > 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 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...
