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

On Wed, Jun 2, 2021 at 3:56 PM Johannes Berg [off-list ref] wrote:

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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.

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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 (local)

Yes, I saw the second version of your RFC and even attempted to
provide a full picture of why this attribute should be generic.

I will send a follow-up series tonight with parent device exporting
support and with some usage examples.

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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.

-- 
Sergey