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.
Projects[edit | edit source]
Build frameworks and environments[edit | edit source]
- 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
Components[edit | edit source]
- The Linux Kernel
- "init" implementations: OpenRC, runit
- GUIs: LXQt, LXDE, Xfce, FLTK
- coreboot - lightweight BIOS
C standard library variants[edit | edit source]
- General purpose GNU C Library glibc, 7.9 MB
- uClibc, 560 KB
- musl, 527 KB
- dietlibc, 185 KB
- klibc - primarily for booting Linux systems
- Bionic - originally developed by Google for the Android embedded system operating system
Distributions[edit | edit source]
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 systems[edit | edit source]
- I/O and buses:
- - peripheral USB device implementation
- - Industrial I/O
- Pulse-width modulation -
- - Generic Voltage and Current Regulator support
- SLOB - Simple List Of Blocks memory allocator
- Devicetree - describes hardware components for the kernel
Multimedia[edit | edit source]
- ASoC - ALSA System on Chip
- TFT (LCD) - Thin-film-transistor liquid-crystal display
File systems and storage[edit | edit source]
- UBIFS - Unsorted Block Image FS
- SquashFS - a compressed read-only FS with low overhead
- Memory Technology Device block device -
- More: YAFFS, JFFS2
MMU less[edit | edit source]
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-time[edit | edit source]
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:
Further readings about Embedded Linux: