We normally avoid sending ZLPs by padding NTBs with a zero byte
if the NTB is shorter than dwNtbOutMaxSize, resulting in a short
USB packet instead of a ZLP.  But in the case where the NTB length
is exactly dwNtbOutMaxSize and this is an exact multiplum of
wMaxPacketSize, then we must send a ZLP.

This fixes an issue seen on a Sierra Wireless MC7710 device
where the transmission would fail whenever we ended up padding
the NTBs to max size.
Signed-off-by: NBjørn Mork <bjorn@mork.no>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

MBIM devices can support up to 256 generic streams called
Device Service Streams (DSS). The MBIM spec says

   The format of the Device Service Stream payload depends
   on the device service (as identified by the corresponding
   UUID) that is used when opening the data stream.

Example use cases are serial AT command interfaces and NMEA
data streams. We cannot make any assumptions about these
device services.

Adding support for Device Service Stream by extending
the MBIM session to VLAN mapping scheme, allocating
VLAN IDs 256 to 511 for DSS, using the DSS SessionID
as the lower 8bit of the VLAN ID.

Using a netdev for DSS keeps the device framing intact and
allows userspace to do whatever it want with the streams.
For example, exporting an AT command interface using DSS
session #0 to a PTY for use with a terminal application like
minicom:

  vconfig add wwan0 256
  ip link set dev wwan0 up
  ip link set dev wwan0.256 up
  socat INTERFACE:wwan0.256,type=2 PTY:,echo=0,link=/tmp/modem

Device configuration must be done using MBIM control commands
over the /dev/cdc-wdmx device. The userspace management
application should coordinate host VLAN configuration and the
device MBIM configuration using the device capabilities to
find out if it needs to set up PTY mappings etc.
Signed-off-by: NBjørn Mork <bjorn@mork.no>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

MBIM devices can support up to 256 independent IP Streams.
The main network device will only handle SessionID 0. Mapping
SessionIDs 1 to 255 to VLANs using the SessionID as VLAN ID
allow userspace to use these streams with traditional tools
like vconfig.
Signed-off-by: NBjørn Mork <bjorn@mork.no>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

The CDC Mobile Broadband Interface Model (MBIM) specification
extends CDC NCM by
 - removing the redundant ethernet header from the point-to-point
   USB channel
 - adding support for multiple IP (v4 and/or v6) sessions multiplexed
   on the same USB channel
 - adding a MBIM control channel encapsulated in CDC
 - adding Device Service Streams (DSS), which are non IP generic data
   streams multiplexed on the same USB channel as the IP sessions

MBIM devices are managed using the dedicated control channel, and no
data will flow on the data channel until a control session has been
established.  This driver has no knowledge of MBIM control messages.
It just exports the control channel to a /dev/cdc-wdmX character
device for userspace management applications. Such an application is
therefore required to use this driver.

This patch implements basic MBIM support, reusing the NCM and WDM driver
APIs, currently limited to IP sessions with SessionID 0. DSS and
multiplexed IP sessions are not yet supported.
Signed-off-by: NGreg Suarez <gsuarez@smithmicro.com>
Signed-off-by: NBjørn Mork <bjorn@mork.no>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>