RootCanal: Add a readme

# RootCanal

RootCanal is a virtual Bluetooth Controller.

Its goals include, but are not limited to: Bluetooth Testing and Emulation.

## Usage

RootCanal is usable:

- With the Cuttlefish Virtual Device.

- As a Host standalone binary.

- As a Bluetooth HAL.

- As a library.

### Cuttlefish Virtual Device

Cuttlefish enables RootCanal by default, refer to the Cuttlefish documentation

for more informations

### Host standalone binary

```bash

m root-canal # Build RootCanal

out/host/linux-x86/bin/root-canal # Run RootCanal

```

Note: You can also find a prebuilt version inside [cvd-host_package.tar.gz from Android CI][cvd-host_package]

[cvd-host_package]: https://ci.android.com/builds/latest/branches/aosp-master/targets/aosp_cf_x86_64_phone-userdebug/view/cvd-host_package.tar.gz

RootCanal when run as a host tool, exposes 4 ports by default:

- 6401: Test channel port

- 6402: HCI port

- 6403: BR_EDR Phy port

- 6404: LE Phy port

### Bluetooth HAL

A HAL using RootCanal is available as `android.hardware.bluetooth@1.1-service.sim`

## Channels

### HCI Channel

The HCI channel uses the standard Bluetooth UART transport protocol (also known as H4) over TCP.

You can refer to Vol 4, Part A, 2 of the Bluetooth Core Specification for more information.

Each connection on the HCI channel creates a new virtual controller.

### Test Channel

The test channel uses a simple custom protocol to send test commands to RootCanal.

You can connect to it using [scripts/test_channel.py](scripts/test_channel.py).

### Phy Channels

The physical channels uses a custom protocol described in [packets/link_layer_packets.pdl](packets/link_layer_packets.pdl)

with a custom framing.

**Warning:** The protocol can change in backward incompatible ways, be careful when depending on it.