Embedded Linux Workshop IndiaWilson Wingston Sharonwingston.sharon@gmail.com

What are embedded systems?• Embedded systems are self contained intelligent electronic

control systems.

• They contain a microcontroller / microprocessor and peripherals interfaced to perform a particular series of tasks.

• Embedded systems today run almost every electronic device imaginable from TV’s to washing machine.

• Embedded systems also can be used for self-contained intelligent automated systems.

Simple Embedded Example• An AVR microcontroller. Atmega328.

• Contains all hardware necessary to perform computational tasks onboard provided power is given to it.

• Communicates with the outside world via gpio pins.

• Can be used to read data from a sensor connected to it and analyze that. (e.g.: IR

• Can be interfaces to motors and set to control the movement of the motors depending on the sensor input.

• Then, the microcontroller can be put on-board a bot and be asked to perform line following!

Programming simple MC’s• Microcontrollers are defined by their architecture and bus bit

width.• The atmega is an 8 bit AVR series microcontroller.

• To program the atmega we need• A Compiler to compile C code for the atmega328• A Flash/EPROM burning device

• The cross compiler convers our code from C to the hex op code that can be placed in the atmega’s internal memory.

• The atmega executes each instruction sequentially!

The problem!• When we program in native C for a microcontroller, we need to be

intimately aware of the underlying hardware.

• To control the atmega’s behavior we need to engage in register level programming.

• This makes the code non-portable as the program is now written to run on only one controller!

• Even different atmega that has different pinouts and register names will require a complete rewrite of most of the code before the program can work on it.

• As more advanced microcontrollers / microprocessors emerged into the market, register level programming needed an alternative to avoid over specializing developers.

Hardware available• Hardware manufactures keep increasing complexity and

system performance.

• The higher processing power comes with the price of too much registers with individual internal controlling methodology

• Hardware manufacturers needed to abstract their hardware to be able to support easier development.

• Software developers needed a generalized framework where they cab build their applications without worrying about their hardware.

Hardware v/s software• Objective : Getting your code to sense / control external devices.

• The more complex your hardware is, the more requirements it will have in respect to code to write control mechanism.

• If a stand-alone application is required to be developed• Multiple (internal / external) devices have to be managed in the background• I/O of different devices must be managed and processed as per demand.• Interrupts / clocks / power must be managed to keep the microcontroller running.

• This calls for increased debugging / non portability and results in increased development time / bugs in the system.

• If hardware is as complicated and powerful as a computer (SBC) then we need code comparable to that of an Operating System (DOS) to be able to run it!!

Line between HW / SW• Very few processors can be programmed by flash burning with

ICSP. (e.g: ARM5)

• Modern communication standards are replacing “legacy” RS-232 with USB, I2C ,Ethernet etc.

• The software control of these protocols in the Atmega register level way is too complex.

• Harware manufacturers release “Drivers” or libraries for controlling their hardware to software developers which allows for more efficient usage of the underlying hardware.

Embedded Linux

Hardware

• Processor• RAM• GPIO• Clocks• UART• I2C

Kernel

• Kernel developers work on hardware control of the devices.

Embedded Linux• In 1990s, an underground software movement consisting of the

worlds leading developers and programmers wrote a completely free Operating System!

• As more people used it with the FOSS philosophy, improvements, fixes and support for multiple processors creeped in!

• This resulted in Linux (the very same kernel ) to run on many processors and provide a similar level of functionality.

• A Global collaborative effort for improvements and upgrades make linux so popular with hardware developers

• Most of the time, linux gets fixes and support for new hardware as soon as they are available!

Application Developers• Embedded developers prefer a non black box OS distribution.

• Although Software application are completely abstracted away from the hardware, it is still requirement that slight changes / improvements in the OS code could make the application a lot more efficiently on the developers embedded target.

• The HAL (Hardware Abstraction Layer) lets you focus on image recognition and not memory management!

• The Open Source Linux Kernel Project provids a HAL that is ported to wide range of processors and has driver support for almost every hardware device in the market.

Linux -> Embedded Linux• Linux for x86 and amd64 (desktop architectures) require

almost 100 - 500mb.

• Embedded Devices have more strict requirements in terms of memory and processing power.

• Embedded Linux kernels can go as low of 11Mb when placed in RAM.

• A non distribtion based linux – with only kernel and a minimal filesystem for a “dos” – like usage is usually run.

• Any custom linux libraries for hardware / software can be installed to help with application development.

Starting the Hardware• When the hardware is switched on, the OS is present in some

onboard memory peripheral.

• First there is code called a bootloader that initializes all the required hardware on the board.

• Bootloaders are small programs (4 – 16K) written for and compiled for specific hardware to be executed immediately after start.

• The bootloader starts the board and loads the kernel from where-ever it is into RAM.

• Once the Kernel starts executing from RAM, it takes over and starts a linux session!

Types of Bootloaders• Intel Motherboard : PHOENIX BIOS : • This bootloader is present on most intel based laptops. • It starts the laptop hardware and loads “NTLDR” the windows

bootmanager.• This code is hardwired into the mother board.

• Embedded Hardware• Bootloader is usually places in a NAND Flash memory.• Bootloaders are very small.• They load, uncompress the linux kernel and relenquish control..

Kernel• Designed as a Finnish UG (B.tech eq) student’s hobby project.

• First was made as a UNIC port for a motorola 64Kb machine that made Linux designed for Portability.

• The groundwork and FOSS nature allowed the kernel to be ported to (and thus support) almost every hardware platform on/off the market.

• The base for extending the kernel through “Device Drivers” have hardware manufactures / driver developers to release support for any hardware available.

• Kernel is just a runtime HAL! It just has instructions for running the hardware – something has to give it instructions -> RootFS..

Linux system Design

Filesystem (UserLand)• Filesystem : Collection of directories

• These directories follow a tree heirarchy and contain• Executable files or programs that the kernel loads into memory• Libraries for application to link to at run-time• User Application that can be simply installed onto it• Setting files that control the Linux OS’s behaviour.

• Hardware devices are also linked as special file nodes in the filesystem to connect them to the Kernel’s HAL.

• USB drives / HDDs / SD cards are mounted onto the filesystem and can be browsed as usual.

Cross Compilers and Toolchains• Different Hardware – Same Source code?

• Cross Compilers are called the translators to machine language for different architectures.

• Hardware manufactures and developers develop a toolchain for their architectures.

• The toolchain contain all the utilities required to compile, debug code and link for the processor.

• There is a GNU toolchain for AVR and ARM architectures.• The same source code when used with different cross compilers allow

for targeting different platforms.

• The changes in code required for a particular hardware is managed with localised “patches”.

Applications• Headless units : Devices without the need for a graphical display• Routers• Set Top Box

• GUI based Applications• Touch Smartphones• GPS car navigation multimedia systems.

• Application developers have:• System level functionality if required• Shared libraries for efficient management of resources• Linux kernel provided complete HAL• Same code workable of various devices

• Android is a Linux Kernel and FS example!• Android will run on any phone that linux can work in.• Phone developers have a unified Free OS to work with.• Cheaper and more wide variety of applications!

Drivers• Run time modules attached to the linux kernel to manage hardware

peripherals

• USB Wifi• Camera• GPS

• Unified driver API that makes it easy to write Driver Code that integrated to the main Kernel.

• Hardware that is accepted to the main repository (upstream) means that everybody has access to the driver for that hardware!

• Linux drivers need not be released as source – which means hardware manufactures can release their driver in binary format. (becomes proprietary)

Libraries• C library• Provides an interface to the kenel functions via calls from userland.

• Stripped down minimal C libraries are there for use in embedded devices.• GlibC (Full Featured)• uCibC (Minimal Variant)

• POSIX support• Allows for communication job sceduling, multiprocessing and IPC in

a unified framework.• ALSA• Advanced Linux Sound Architecture for Hardware DSP support

Custom Applications• Compiled with appropriate cross-compiler as UNIX / POSIX

Compliant applications

• BusyBox• Provides an embedded shell functionality in embedded devices• cd ls mkdir echo cat and all standard linux commands all work• I/O can be managed over a serial line• Can be thought of as a terminal equivalent• Commands allow for direct control of the kernel• Helps navigate the filesystem

• Qt GUI applications can also be built if LCD is present.

Run Time Linux• Serial Console

• Apps that can be autostarted

• Daemons or “services” that provide background application functionality

• Kernel Threads for Real-Time interrupt management

• RTOS supprt in RT-Linux Project.

Memory Considerations• Linux works primarily on processors with a hardware MMU.

(memory management unit)

• MMU enforces copy and access violation protection in RAM between kernel, hardware and user application to make sure system can be kept stable at all times.

• Virtual Memory allows for run-time linking and delinking of un responsive kernel modules / application to keep the system functioning even in the event of a crash.

Try for yourself• devmem2• Memory inspector

• ps• Running processors

• cat files in /proc• Gives you current system information

Open source Licenses• Basic funda

• Us at your own Risk• No guarantee• We’ll help if we CAN. We don’t need to.

• GPLv2• GNU Public Licence• Source must accompany binary• Linkng to non GPL software not possible.

• LGPL• Link to non GPL software possible• To provide for non open source driver development• LGPL source must be provided

• Modified Free-BSD• No source delivery required• For proprietary kernels • Broken and non FOSS supported Forks