netdev - Re: [RFC 3/4] wwan: add interface creation support

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Message-ID: <CAHNKnsQh7ikP4MCB0LhjpdqkMTjWq2ByWG4wToaXgzteYjUQaQ@mail.gmail.com>
Date:   Wed, 2 Jun 2021 15:45:41 +0300
From:   Sergey Ryazanov <ryazanov.s.a@...il.com>
To:     Johannes Berg <johannes@...solutions.net>
Cc:     Loic Poulain <loic.poulain@...aro.org>,
        linux-wireless@...r.kernel.org, netdev@...r.kernel.org,
        m.chetan.kumar@...el.com
Subject: Re: [RFC 3/4] wwan: add interface creation support

Hello Johannes,

On Wed, Jun 2, 2021 at 10:38 AM Johannes Berg <johannes@...solutions.net> wrote:
>> Wow, this is a perfect solution! I just could not help but praise this
>> proposal :)
>
> Heh.
>
>> When researching the latest WWAN device drivers and related
>> discussions, I began to assume that implementing the netdev management
>> API without the dummy (no-op) netdev is only possible using genetlink.
>> But this usage of a regular device specified by its name as a parent
>> for netdev creation is so natural and clear that I believe in RTNL
>> again.
>>
>> Let me place my 2 cents. Maybe the parent device attribute should be
>> made generic? E.g. call it IFLA_PARENT_DEV_NAME, with usage semantics
>> similar to the IFLA_LINK attribute for VLAN interfaces. The case when
>> a user needs to create a netdev on behalf of a regular device is not
>> WWAN specific, IMHO. So, other drivers could benefit from such
>> attribute too. To be honest, I can not recall any driver that could
>> immediately start using such attribute, but the situation with the
>> need for such attribute seems to be quite common.
>
> That's a good question/thought.
>
> I mean, in principle this is trivial, right? Just add a
> IFLA_PARENT_DEV_NAME like you say, and use it instead of
> IFLA_WWAN_DEV_NAME.
>
> It'd come out of tb[] instead of data[] and in this case would remove
> the need to add the additional data[] argument to rtnl_create_link() in
> my patch, since it's in tb[] then.

Yep, exactly.

> The only thing I'd be worried about is that different implementations
> use it for different meanings, but I guess that's not that big a deal?

The spectrum of sane use of the IFLA_PARENT_DEV_NAME attribute by
various subsystems and (or) drivers will be quite narrow. It should do
exactly what its name says - identify a parent device.

We can not handle the attribute in the common rtnetlink code since
rtnetlink does not know the HW configuration details. That is why
IFLA_PARENT_DEV_NAME should be handled by the RTNL ->newlink()
callback. But after all the processing, the device that is identified
by the IFLA_PARENT_DEV_NAME attribute should appear in the
netdev->dev.parent field with help of SET_NETDEV_DEV(). Eventually
RTNL will be able to fill IFLA_PARENT_DEV_NAME during the netdevs dump
on its own, taking data from netdev->dev.parent.

I assume that IFLA_PARENT_DEV_NAME could replace the IFLA_LINK
attribute usage in such drivers as MBIM and RMNET. But the best way to
evolve these drivers is to make them WWAN-subsystem-aware using the
WWAN interface configuration API from your proposal, IMHO.

-- 
Sergey