| Peter | @ | NthCode | . | com |
Executive Summary
If you're having trouble estimating the time and complexity of porting Google's Android mobile platform to your embedded device, our experience porting Android to a Nokia N810 Internet Tablet may provide you with useful information.
Android Overview
Android is a software platform for mobile phones created by Google. It is based on the Linux kernel, a number of software libraries written in 'C,' a Java-like virtual machine for running application code, and a full suite of mobile phone applications.
Android costs nothing and the source code is freely available. Its license terms (see the Apache license link at the end) are commercial-friendly, saying, in essence, "Do what you like with it, just dont sue us if something goes wrong." The one exception to this rule is the Linux kernel, which is licensed under the GNU Public License. Because of this, manufacturers must release their device's Linux kernel source code after product shipment.
Taiwan's HTC shipped the first Android phone, the G1, in October 2008. Other mobile manufacturers, including Motorola, LG, and Samsung are all reportedly creating Android-based phones. Additionally, from NthCode's home base in China, we see many startups and smaller manufacturers leveraging Android in an attempt to gain a foothold in the global smartphone market.
The Android platform provides all the services one would expect in a mobile phone, including phone calling, networking, messaging, and multimedia. Our analysis of the source code so far is that these services are complete and built on top of an excellent architecture, but do not implement every last feature found in modern mobile phones. This is the 1.0 release, after all.
For application developers, Android builds on top of the popular open-source Eclipse integrated development environment. Developers can use Eclipse to write Java applications, which are then compiled to Java bytecode before being translated for running on Android's Dalvik Virtual Machine (VM).
Google claims their Dalvik VM runs more efficiently in terms of speed and memory tradeoffs than Java -- slyly suggesting that Google wrote their own VM for performance reasons. However, while we haven't seen supporting data, we suppose Google really created Dalvik to escape from needing to cede control or pay a license fee to Sun, the creator and steward of the Java platform.
By translating from Java to Dalvik bytecode, Google is able to leverage the mature tools and large existing community of Java developers. The downside is that developers will need to take the additional step of running the Google translation tool on their compiled Java code before their application will run on an Android phone (Google automates this step in their Android Eclipse plug-in, however).
Google also provides an Android emulator. The emulator is a virtualized ARM microprocessor that runs the same system code -- and almost exactly the same Linux kernel -- that runs on a device. Google provides an Eclipse add-on so that Android programs can be compiled and debugged in Eclipse while running in their emulator. The Android emulator is as complete an emulator as we have ever seen -- just make sure your developers have enough RAM in their PCs to be able to run Eclipse and the emulator together.
So what's in it for Google?
Google has given mobile phone manufacturers everything they want -- a free, open, mobile operating system -- and, in return, only asked that the devices they create can run Android applications. We can only speculate that Google expects to eventually earn more from consumers using Google services on Android devices than it costs Google to develop and maintain Android.
Overview of the Nokia N810
We work with embedded Linux systems at NthCode, so we thought it would be a fun challenge (and would help with a project we're working on) if we could port Android to an existing Linux device.
Luckily, we already had two Nokia N810 Internet Tablets in the office. The N810 is a handheld mobile device that comes with a high-resolution 4.1-inch touch screen and a slide-out QWERTY keyboard. The N810 has a 400 MHz Texas Instruments OMAP 2420 CPU, 128MB RAM and WiFi networking. The one thing the N810 can't do is make phone calls -- this is why Nokia markets it as an Internet Tablet that can surf the web, rather than a phone. At the time of this writing, the N810 costs about USD $425.
An active community of enthusiasts enjoys hacking the N810's Linux-based software. While some of those enthusiasts had already ported a pre-release version of the Android SDK to the N810, we discovered that no one had yet ported the Android 1.0 release to it. So we decided to see how long it would take us, and what we could learn in the process.
Development Steps
As mentioned earlier, the Android system software runs on top of a Linux kernel. This Linux kernel provides services to applications -- such as file access, process scheduling, and inter-process communication.
Google made a number of modifications to the standard Linux kernel for Android. Likewise, Nokia modified the standard Linux kernel to support their hardware, such as the keypad, touch screen, and file system.
We quickly discovered that Nokia's N810 changes are made against an earlier 2.6.21 Linux kernel. And, unlike earlier Android prereleases, the Android 1.0 modifications are made against a later 2.6.25 Linux kernel. We investigated and discovered that between these two versions, a year had elapsed and the Linux community had made thousands of changes to kernel source code.
So, to make both Nokia's and Google's modifications work together on the N810 we either needed to port the N810 changes to work on the newer 2.6.25 Linux kernel, or port the Android changes to the N810's earlier 2.6.21 Linux kernel.
What Did Google Change in the Kernel?
We checked the differences between the Android kernel and the standard Linux kernel and found that Google had changed 75 files and added an additional 88. We have prepared an annotated list of changed files at the end of this document, and a brief summary here.
Goldfish -- 44 Files
The Android emulator runs a virtual CPU that Google calls Goldfish. Goldfish executes ARM926T instructions and has hooks for input and output -- such as reading key presses from or displaying video output in the emulator.
These interfaces are implemented in files specific to the Goldfish emulator and will not be compiled into a kernel that runs on real devices. So we safely ignored these files in our work.
YAFFS2 -- 35 Files
Unlike PCs, which store files on disks, mobile phones store files in solid-state flash memory chips. The HTC G1 uses NAND flash, a type of flash memory that is becoming more popular due to its combination of high density and low cost.
YAFFS2 is an acronym for "Yet Another Flash File System, 2nd edition" and provides a high-performance interface between the Linux kernel and NAND flash devices. YAFFS2 was already freely available for Linux. However, it is not part of the standard 2.6.25 Linux kernel, and so Google added it to Android.
Bluetooth -- 10 files
Google made changes to 10 files in the Bluetooth communications stack. These changes fix apparent bugs related to Bluetooth headsets, as well as add Bluetooth debugging and access control functions.
Scheduler -- 5 files
The Android kernel also contains slight changes to the CPU process scheduler and time-keeping algorithms. We don't know the history of these changes, and the impact was not evident based on a cursory examination.