B2G/Architecture

This document describes at a high level how Gaia and B2G work in the port of Gecko to Gonk. If you don't know what these things are yet, don't worry, they'll be introduced below.

Gaia/B2G/Gecko are under heavy development, so some items below discuss how the code will work, instead of how it currently works. This will be noted.

Terminology

Gaia : The user interface of b2g. Everything drawn to screen after b2g starts up is some part of Gaia. Gaia implements a lock screen, home screen, telephone dialer, text-messaging application, camera app, ... and many more. Gaia is written entirely in HTML, CSS, and JavaScript. Its only interface to the underlying operating system is through Open Web APIs, which are implemented by Gecko. Gaia works well when run on top of b2g; however, since it only uses standard web APIs, it works on other OSes and in other web browsers (albeit with degraded functionality). Third-party applications can be installed alongside Gaia.

Gecko : The "application runtime" of b2g. At a high level, Gecko implements the open standards for HTML, CSS, and JS and makes those interfaces run well on all the OSes that Gecko supports. This means that Gecko consists of, among other things, a networking stack, graphics stack, layout engine, virtual machine (for JS), and porting layers.

Gonk : The lower-level "operating system" of b2g. Gonk consists of a linux kernel and userspace hardware abstraction layer (HAL). The kernel and several userspace libraries are common open-source projects: linux, libusb, bluez, etc. Some other parts of the HAL are shared with the android project: GPS, camera, among others. You could say that Gonk is an extremely simple linux distribution. Gonk is a porting target of Gecko; there is a port of Gecko to Gonk, just like there is a port of Gecko to OS X, and a port of Gecko to Android. Since the b2g project has full control over Gonk, we can expose interfaces to Gecko that aren't possible to expose on other OSes. For example, Gecko has direct access to the full telephony stack and display framebuffer on Gonk, but doesn't have this access on any other OS.

Booting

After turning on a b2g phone, execution starts in the primary bootloader. From there, the process of loading the main OS kernel happens in the usual way: a succession of higher-level bootloaders bootstrap the next loader in the chain. At the end of the process, execution is handed off to the linux kernel.

There's not a lot to say about the boot process, but there are a few things worth knowing

• The bootloaders usually show the first "splash screen" seen during device boot, which usually displays a vendor logo.
• The bootloaders implement flashing an image onto the device. Different devices use different protocols. Most phones use the fastboot protocol, but the Galaxy S II uses the "odin" protocol.
• By the end of the bootstrapping process, the modem image is usually loaded and running on the modem firmware. How this happens is highly device-specific and possibly proprietary.

Kernel (Linux)

The linux kernel(s) in Gonk is reasonably close to upstream linux. There are a few modifications made by AOSP that are not in upstream yet. Vendors also modify the linux kernel and upstream those modifications on their own schedule. But in general, the linux kernel is close to stock.

The startup process for linux is well documented elsewhere on the internet, so it's not covered here. At the end of kernel startup, a userspace "init" process is launched, like in most other UNIX-like OSes. At this point in execution, only a ramdisk is mounted. The ramdisk is built during the b2g build process, and consists of critical utilities (like init), other startup scripts, and loadable kernel modules.

After launching the init process, the linux kernel services system calls from userspace and interrupts etc. from hardware devices. Many devices are exposed to userspace through sysfs (documented elsewhere on the internet). For example, here's some code that reads the battery state in Gecko (link to original code)

  FILE *capacityFile = fopen("/sys/class/power_supply/battery/capacity", "r");
  double capacity = dom::battery::kDefaultLevel * 100;
  if (capacityFile) {
    fscanf(capacityFile, "%lf", &capacity);
    fclose(capacityFile);
  }