Introduction
============

QUALCOMM MSM Interface (QMI) defines the interface between MSM and
attached Terminal Equipment (TE). RmNet interface is a new logical
device in QMI framework for data services. RmNet in accordance with
QMI architecture defines channels for control and data transfers and
for example it uses Data I/O channel for IP data transfer and control
I/O channel for QMI messaging (functionality similar to AT commands).
RmNet may be used in place of legacy USB modem interface.

Tethered networking is one of the function from MSM which can also be
supported using QMI protocol. There are other standard protocols exists
such as CDC-ECM and Windows proprietary RNDIS. On the host-side system,
the gadget rmnet device looks like a ethernet adapter.

Hardware description
====================

QMI is a messaging protocol to expose various functionalities of MSM
and one of the functionality to be tethered networking which is being
exposed over QMI using RmNet protocol. This usb gadget has one bulk-in,
one bulk-out and one interrupt-in endpoint.

Design:
=======
RmNet function driver design follows two approaches:

Approach 1:
-----------
Single function driver is used to communicate with
Modem(both for data and control). Most of the initial
MSM targets are following this approach.

The main disadvantage with this approach is there are
multiple RmNet drivers for any change in DATA and Control
Layer. There is no re-use in the code.

Approach 2:
-----------
RmNet driver is divided into 3 components

1. USB:
This component has the functionality to deal with composite layer.
Allocates Interfaces, Endpoints, listens to connect/disconnect
interrupts and gives connect/disconnect notifications to DATA and
CONTROL modules.

2. Data:
This component talks to modem to transfer IP data. Usually DATA
and CONTROL go over same channel. However, to achieve higher
data rates new transport channel for DATA may be used.

3. Control:
This component talks to modem to transfer rmnet control data.

Software description
====================
The RmNet suports following data and control transports:
as follows:
	1. SMD Interface
	2. SDIO Interface
	3. BAM Interface
	4. SMD Control Interface

SMD interface uses the Shared memory for the RmNet driver to communicate
with the MSM modem processor.
SDIO interface acts as a link for communication of RmNet driver with the
MDM modem processor.

USB INTERACTION:
----------------

The RmNet function driver binds with the USB using the struct usb_function.
The function is added using the usb_function_add().
Control Transfers: The RmNet handles two Class-specific control
transfers: SEND_ENCAPSULATED_COMMAND and GET_ENCAPSULATED_RESPONSE.
The asynchronous protocol QMI which consists of the QMI requests/responses
is used for handling the transfers between the RmNet and the Host where the
host sends a new QMI request before receiving the response for the current
QMI request.

Control & Data flow:
1. Host issues a SEND_ENCAPSULATED command to send a QMI request.
SMD: If the SMD control channel has enough room to accomodate a QMI request,
it is written into the SMD buffer. Otherwise, append/add that request to
qmi_request queue. A tasklet is scheduled to drain all QMI requests in
qmi_request queue.
SDIO: Add each request in the qmi_request queue and is processed until
the queue is empty.

2. Append/Add QMI response from modem to qmi_response queue.
A notification on an interrupt end point is used to communicate the QMI
response to host.

3. Host issues a GET_ENCAPSULATED command to retrieve the QMI response.
The response from qmi_response queue will be sent to the host.

4. After the connection is fully established data can be sent to
bulk-out endpoint and data can be received from bulk-in endpoint.

5. Host can send QMI requests even after the connection is established.

RmNet gadget driver is completely unaware of QMI/IP protocol. It just
acts as a bridge between the modem and the PC.

All the request/response queues in the driver can be accessed either
from tasklet/workqueue or from interrupt context (either usb or smd/sdio
interrupt handler). Hence a spinlock is used to protect all data/control req
lists.


SMD Interface:
--------------

1. Each QMI request/response can at most be 2048 bytes. Eight 2KB buffers
are allocated using kmalloc for storing maximum of 8 requests/responses.

2. Four 2KB buffers are allocated using kmalloc for data transfers on
each bulk endpoint.

Data structures:
struct qmi_buf		-	Buffer to handle QMI requests & responses
struct rmnet_smd_info	-	Control & Data SMD channel private data
struct rmnet_dev	-	Endpoint and driver specific data

Workqueues:
rmnet_connect_work	-	Called on device connection.
				Opens SMD channels; enables endpoints
rmnet_disconnect_work	-	Called on device disconnection.
				Closes SMD channels.

Tasklets:
rmnet_control_rx_tlet
rmnet_control_tx_tlet	-	Control transfer data reception and transmission
				handler

rmnet_data_rx_tlet
rmnet_data_tx_tlet	-	Data transfer data reception and transmission handler


SMD control interface
----------------------
This function driver implements exchnage of control informtion with
modem over SMD. Uses smd_read/write commands to read or write rmnet
ctrl packets. Exposes a call back function to usb component to write
control packet and at the same time call a call back usb component
callback to send data to usb host.

Data structures and Interfaces are very similar to control interfaces
explained in "SMD Interface"

BAM MUX interface
------------------
BAM Mux interface is very similar to SDIO MUX interface. However there
are differences in the way BAM and SDIO operate but all of the details
are masked by MUX Driver.

Refer to the SDIO interfaces for more information on data structures

SDIO Interface:
---------------

1. Each QMI request/response buffer is allocated depending on the size
of data to be transmitted for the request/response.

2. A 2KB network buffer is allocated for data transfer on bulk-out
endpoint. The SDIO allocates the required buffer for data transfers
on an bulk-in endpoint.

Data structures:
struct rmnet_sdio_qmi_buf      -       Buffer to handle QMI requests/responses.
struct rmnet_dev               -       Endpoint and driver specific data

Workqueues:
rmnet_connect_work             -       Device connection handler. Opens SDIO
                                       channels; enables and allocate buffer for
                                       endpoints
rmnet_disconnect_work          -       Device disconnection handler. Closes
                                       SDIO channels; Frees allocated buffers.
rmnet_control_rx_work          -       Control data reception handler.
rmnet_data_rx_work             -       Network data reception handler.


Two SMD/SDIO channels (control and data) are used as communication channels
between Modem and Apps processor. The driver opens the SMD/SDIO channels
on USB device connection. Data is either read from/written to the channels
as one complete packet.

SMD/SDIO provides a notification whenever the Modem processor completes
read/write of a packet.  Based on these SMD/SDIO notifications all the
pending read/write requests will be handled. Tasklets(SMD)/Workqueues(SDIO)
are used to get the requests done.

There is another variant of rmnet driver called rmnet_smd_sdio which is used
on some boards.  This driver allows the transport (SMD/SDIO) to be chosen
at runtime. This is required because either MDM processor or MODEM processor
is only active at a time for data transfers. As SMD and SDIO interfaces
are different, different endpoint completion handlers are used. This driver
leverage the existing rmnet over smd and rmnet over sdio drivers. The control
messages (QMI) always routed over SDIO. After the control messages exchange,
user space will come to know about the available data transport (SMD/SDIO).
User space notify the same to driver and the corresponding transport is
activated. It is assumed that transport will not change while a USB cable
is connected.

Rmnet over SMD and rmnet over SDIO doesn't expose any of its interfaces to
either kernelspace or userspace. But rmnet over smd/sdio expose a sysfs
interface for userspace to notify the available transport to driver.

The sysfs file can be found at
/sys/class/usb_composite/rmnet_smd_sdio/transport

The below command activates the SMD transport
echo 0 > /sys/class/usb_composite/rmnet_smd_sdio/transport

The below command activates the SDIO transport
echo 1 > /sys/class/usb_composite/rmnet_smd_sdio/transport

-EINVAL is returned if a write is attempted to transport when a USB cable
is not connected.