Embedded Linux

Core components of an embedded Linux system are boot loader, custom Linux kernel and root file system. Source code is compiled by GCC cross compiler. Common boot loader is Das U-Boot – the Universal Boot Loader. Root FS usually is based on BusyBox – software suite that provides many Unix utilities in a single executable file. Local debugging often is limited, so gdbserver can be used.

Basic embedded Linux system can be compiled manually from modules above. There are tools for complete system generation from old and simple Buildroot to huge Yocto Project, which creates customized embedded Linux distributions. PC distributions Debian, Ubuntu ant others have build for ARM processors.

Raspberry Pi, BeagleBoard and Nvidia Jetson are examples of popular single-board computers supporting embedded Linux.

Unlike regular desktop or server distributions of Linux these versions of Linux usually have much fewer libraries and application, firmware is smaller and typically boot from flash memory or SD card.

ProjectsEdit

Build frameworks and environmentsEdit

Wear OS – Android for smartwatches and other wearables.

AOSP – Android Open Source Project

Buildroot – old simple patch and make based build environment

Yocto Project and OpenEmbedded – a BitBake based build framework and environment

BitBake – make-like build tool

OpenWrt – open wireless router

Embeddable Linux Kernel Subset – for ancient 16-bit x86 machines and emulators

ComponentsEdit

The Linux Kernel

"init" implementations: OpenRC, runit

GUIs: LXQt, LXDE, Xfce, FLTK

coreboot – lightweight BIOS

C standard library variantsEdit

General purpose GNU C Library glibc, 7.9 MB

uClibc, 560 KB

musl, 527 KB

dietlibc, 185 KB

Newlib

klibc – primarily for booting Linux systems

Bionic – originally developed by Google for the Android embedded system operating system

DistributionsEdit

Distributions for ARM processors:

Arch Linux ARM – port of Arch Linux for ARM processors

Armbian – Debian and Ubuntu based

RedSleeve – port of Red Hat Enterprise Linux

emteria.OS – proprietary Android based

Lightweight multi-platform distributions:

Common multi-platform distributions:

Features of Linux kernel for embedded systemsEdit

I/O and buses:

GPIO

drivers/gpio_src, tools/gpio_src, General Purpose Input/Output_doc

SPI

drivers/spi_src, tools/spi_src, Serial Peripheral Interface_doc

I²C

⚲ UAPI: Instantiate I2C devices from user-space_doc

drivers/i2c_src, https://i2c.wiki.kernel.org

I2C/SMBus Subsystem_doc

drivers/net/can_src – Controller Area Network bus

drivers/mfd_src – Multi-Function Devices

drivers/usb/gadget_src – peripheral USB device implementation

drivers/iio_src – Industrial I/O

drivers/pwm_src – Pulse-width modulation

drivers/regulator_src – Generic Voltage and Current Regulator support

drivers/fpga_src

drivers/slimbus_src

SLOB – Simple List Of Blocks memory allocator

Devicetree – describes hardware components for the kernel

MultimediaEdit

Audio

ASoC – ALSA System on Chip

snd_soc_card_id

ASoC – ALSA SoC Layer_doc

https://www.alsa-project.org/wiki/ASoC

TFT (LCD) – Thin-film-transistor liquid-crystal display

fbtft_display_id

File systems and storageEdit

UBIFS – Unsorted Block Image FS

SquashFS – a compressed read-only FS with low overhead

drivers/mtd_src – Memory Technology Device block device

More: YAFFS, JFFS2

MMU lessEdit

Linux was originally designed on a processor with a memory management unit (MMU). Most embedded systems do not have a MMU, as we discussed earlier (Embedded Systems/Memory) and are called microcontrollers. Microcontrollers without a MMU are cheaper consumes less power.

Benefits of using a processor with a MMU:

can isolate running "untrusted" machine code from running "critical" code, so the "untrusted" code is guaranteed (in the absence of hardware failures) not to interfere with the "critical" code

makes it easier for the OS to present the illusion of virtual memory

can run "normal" Linux (could also run "μClinux", but what's the point?)

μClinux – "MicroController Linux" is a version of the Linux kernel that supports Altera NIOS, ARM, Freescale M68K (including DragonBall, ColdFire, PowerQUICC and others), Hitachi H8, MIPS, and Xilinx MicroBlaze processors.

Disabling parameter CONFIG_MMU enables nommu mode.

Real-timeEdit

People use a variety of methods to combine real-time tasks with Linux:

Run the real-time tasks on a dedicated microcontroller; communicate with a (non-real-time) PC that handles non-real-time tasks. This is pretty much your only choice if you need real-time response times below 1 microsecond.

Run the real-time tasks in a "underlying" dedicated real-time operating system; run Linux as a "nested operating system" inside one low-priority task on top of the real-time operating system. Some of these systems claim real-time response times below 500 microseconds.

use a Linux kernel designed to emphasize real-time tasks, and run the real-time tasks with a high priority (perhaps even as a kernel thread).

Parameter CONFIG_PREEMPT_RT enables real-time preemption.

SCHED_DEADLINE – scheduling policy for RT.

Examples of RT distributions:

Red Hat Enterprise Linux for Real Time