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

Hello Johannes,

On Wed, Jun 2, 2021 at 10:38 AM Johannes Berg [off-list ref] wrote:

quoted

Wow, this is a perfect solution! I just could not help but praise this
proposal :)

Heh.

quoted

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