linux-kernel - Re: fwnode_for_each_child_node() and OF backend discrepancy

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Date:   Tue, 28 Jun 2022 15:47:59 +0200
From:   Michael Walle <michael@...le.cc>
To:     Andy Shevchenko <andy.shevchenko@...il.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Horatiu Vultur <horatiu.vultur@...rochip.com>
Subject: Re: fwnode_for_each_child_node() and OF backend discrepancy

[adding Horatiu Vultur, because we now digress to the bug
in the switch, rather than that odd OF behavior]

Am 2022-06-28 15:29, schrieb Andy Shevchenko:
> On Tue, Jun 28, 2022 at 3:23 PM Michael Walle <michael@...le.cc> wrote:
>> 
>> >> I was trying to fix the lan966x driver [1] which doesn't work if there
>> >> are disabled nodes in between.
>> >
>> > Can you elaborate what's wrong now in the behaviour of the driver? In
>> > the code it uses twice the _available variant.
>> 
>> Imagine the following device tree snippet:
>>   port0 {
>>     reg = <0>;
>>     status = "okay";
>>   }
>>   port1 {
>>     reg = <1>;
>>     status = "disabled";
>>   }
>>   port@2 {
>>     reg = <2>;
>>     status = "okay";
>>   }
>> 
>> The driver will set num_phys_ports to 2. When port@2 is probed, it
>> will have the (correct!) physical port number 2. That will then
>> trigger various EINVAL checks with "port_num >= num_phys_ports" or
>> WARN()s.
> 
> It means the above mentioned condition is wrong: it should be
> 
> "port_idx >= num_phys_ports" (if the port_idx doesn't exists, that's
> the bug in the first place)

I can't follow you here. Please note, that you need the actual
physical port number. It's not a made up number, but corresponds
to a physical port on that ethernet switch. So you can't just skip
the disabled ones. port@2 must have port number 2.

>> So the easiest fix would be to actual count all the child nodes
>> (regardless if they are available or not), assuming there are as
>> many nodes as physical ports.
>> 
>> But num_phys_ports being a property of the hardware
> 
> So, name is wrong, that's how I read it, it should be
> num_of_acrive_phys_ports (or alike).

See above, it is not just an iterator but corresponds to
a hardware property.

>> I don't
>> think it's good to deduce it by counting the child nodes anyway,
> 
> Right.
> 
>> but it should rather be a (hardcoded) property of the driver.
> 
> Also good to update.

Horatiu, can we determine the actual number of ports (or maybe
determine if its a LAN9668 or a LAN9662) from the hardware itself
in an easy way? That way we wouldn't need a new compatible string,
but could use the generic "lan966x" one.

-michael

[1] 
https://elixir.bootlin.com/linux/v5.19-rc4/source/drivers/net/ethernet/microchip/lan966x/lan966x_main.c