GBridge (Greybus Bridge)

Greybus is a protocol layer for UniPro bus.
It was supposed to be used by modular phones to control modules.

GBridge is an application that can be used to support other bus / network,
such as TCP/IP or bluetooth. 

I) SVC
The SVC take a central piece in Greybus.
It is in charge of module hot plug / hot unplug detection,
create / destroy connections, etc.
In modular phones, the SVC is a micro-controller but here,
we don't have a micro-controller to detect modules,
so we have to simulate it.

SVC protocol has been made for UniPro.
Most of the SVC operations are useless or not adapted for non UniPro modules.
The current implementation of SVC protocol only implements the mandatory
operations though some of them are not correctly implemented
(such as DME operations).

II) Netlink
GBridge is using netlink to communicate with Greybus.
Currently, netlink is only used to transmit Greybus operations,
but it's planned to use it to control Greybus when SVC protocol is not adapted.

II) Controller
The controller is actually the link between the SVC, netlink and all the
modules.
The controller is actually handling some operations made by SVC in phones,
such as modules detection and connection.

a) Bluetooth controller
The Bluetooth controller scans continuously to detect new bluetooth module.
When a Bluetooth module with the "GREYBUS" string in its name show up,
the controller will generate an hotplug event and create a connection.
Currently, the controller open a RFCOMM socket that is not available for BLE.
It's planned to use L2CAP instead of RFCOMM to support both Bluetooth and BLE.