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Subject: Re: [RFC 3/4] wwan: add interface creation support
From:   Johannes Berg <johannes@sipsolutions.net>
To:     Sergey Ryazanov <ryazanov.s.a@gmail.com>
Cc:     Loic Poulain <loic.poulain@linaro.org>,
        linux-wireless@vger.kernel.org, netdev@vger.kernel.org,
        m.chetan.kumar@intel.com
Date:   Wed, 02 Jun 2021 14:56:04 +0200
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Hi Sergey,

> > 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.

Sure, I was more worried there could be multiple interpretations as to
what "a parent device" is, since userspace does nothing but pass a
string in. But we can say it should be a 'struct device' in the kernel.

> 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 didn't do that second part, but I guess that makes sense.

Want to send a follow-up patch to my other patch? I guess you should've
gotten it, but if not the new series is here:

https://lore.kernel.org/netdev/20210602082840.85828-1-johannes@sipsolutions.net/T/#t

> 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.

Right.

johannes