.. SPDX-License-Identifier: GPL-2.0-only

.. Copyright (C) 2020-21 Intel Corporation

.. _iosm_driver_doc:

===========================================
IOSM Driver for Intel M.2 PCIe based Modems
===========================================
The IOSM (IPC over Shared Memory) driver is a WWAN PCIe host driver developed
for linux or chrome platform for data exchange over PCIe interface between
Host platform & Intel M.2 Modem. The driver exposes interface conforming to the
MBIM protocol [1]. Any front end application ( eg: Modem Manager) could easily
manage the MBIM interface to enable data communication towards WWAN.

Basic usage
===========
MBIM functions are inactive when unmanaged. The IOSM driver only provides a
userspace interface MBIM "WWAN PORT" representing MBIM control channel and does
not play any role in managing the functionality. It is the job of a userspace
application to detect port enumeration and enable MBIM functionality.

Examples of few such userspace application are:
- mbimcli (included with the libmbim [2] library), and
- Modem Manager [3]

Management Applications to carry out below required actions for establishing
MBIM IP session:
- open the MBIM control channel
- configure network connection settings
- connect to network
- configure IP network interface

Management application development
==================================
The driver and userspace interfaces are described below. The MBIM protocol is
described in [1] Mobile Broadband Interface Model v1.0 Errata-1.

MBIM control channel userspace ABI
----------------------------------

/dev/wwan0mbim0 character device
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The driver exposes an MBIM interface to the MBIM function by implementing
MBIM WWAN Port. The userspace end of the control channel pipe is a
/dev/wwan0mbim0 character device. Application shall use this interface for
MBIM protocol communication.

Fragmentation
~~~~~~~~~~~~~
The userspace application is responsible for all control message fragmentation
and defragmentation as per MBIM specification.

/dev/wwan0mbim0 write()
~~~~~~~~~~~~~~~~~~~~~~~
The MBIM control messages from the management application must not exceed the
negotiated control message size.

/dev/wwan0mbim0 read()
~~~~~~~~~~~~~~~~~~~~~~
The management application must accept control messages of up the negotiated
control message size.

MBIM data channel userspace ABI
-------------------------------

wwan0-X network device
~~~~~~~~~~~~~~~~~~~~~~
The IOSM driver exposes IP link interface "wwan0-X" of type "wwan" for IP
traffic. Iproute network utility is used for creating "wwan0-X" network
interface and for associating it with MBIM IP session. The Driver supports
upto 8 IP sessions for simultaneous IP communication.

The userspace management application is responsible for creating new IP link
prior to establishing MBIM IP session where the SessionId is greater than 0.

For example, creating new IP link for a MBIM IP session with SessionId 1:

  ip link add dev wwan0-1 parentdev-name wwan0 type wwan linkid 1

The driver will automatically map the "wwan0-1" network device to MBIM IP
session 1.

References
==========
[1] "MBIM (Mobile Broadband Interface Model) Errata-1"
      - https://www.usb.org/document-library/

[2] libmbim - "a glib-based library for talking to WWAN modems and
      devices which speak the Mobile Interface Broadband Model (MBIM)
      protocol"
      - http://www.freedesktop.org/wiki/Software/libmbim/

[3] Modem Manager - "a DBus-activated daemon which controls mobile
      broadband (2G/3G/4G) devices and connections"
      - http://www.freedesktop.org/wiki/Software/ModemManager/