net: cdc_mbim: optionally use VLAN ID 4094 for IP session 0

net: cdc_mbim: optionally use VLAN ID 4094 for IP session 0

The cdc_mbim driver maps 802.1q VLANs to MBIM IP and DSS
sessions. MBIM IP session 0 is handled as an exception and
is mapped to untagged frames.

This patch adds optional support for remapping MBIM IP
session 0 to 802.1q VLAN ID 4094 instead. The default
behaviour is not changed. The new behaviour is triggered
by adding a link for this previously unsupported VLAN.

The untagged mapping was chosen initially to support the
assumed most common use case: Most current MBIM devices only
support a single IP session (i.e. session 0 only), and using
untagged frames lets the users completely ignore the
additonal complexity of the multiplexing layer.

But when the multiplexing features of MBIM are used, then
this netdev gets a double meaning: It becomes the master
interface for all the VLAN subdevs the additional sessions
are mapped to, while still serving as the untagged IP
interface for session 0.

This can be problematic, especially when using Device Service
Streams (DSS), as have become apparent recently with the
availability of devices with real DSS support. Some use cases
need to e.g set a MTU which is higher than allowed for IP
Session 0. The dual role also leads to the situation where
the IP Session 0 interface cannot be taken down without
breaking unrelated IP or DSS sessions - a devastating side
effect which applications managing a simple IP session cannot
be expected to be aware of. A typical DHCP client will assume
that it should bring the interface down after releasing the
IP lease.

These problems can be avoided by tagging IP session 0 packets
too, making this session similar to all other multiplexed
sessions. This redefines the main netdev as an upper master
interface only.

Cc: Greg Suarez <gsuarez@smithmicro.com>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>