Embedded Systems/Linux

Linux on embedded systems Embedded Linux systems can range from consumer electronics like smartphones and smart TVs to industrial machinery, medical devices, automotive systems, communication equipment and more. Linux brings the power, flexibility, and stability to these resource-constrained devices. Core components of an Linux on embedded systems are boot loader, custom Linux kernel and root file system. Source code is compiled by Cross_compiler#GCC_and_cross_compilation 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 architecture 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. Projects Build frameworks and environments Wear OS – Android for smartwatches and other wearables. Android_(operating_system)#AOSP 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 The Linux Kernel "init" implementations: OpenRC, runit GUIs: LXQt, LXDE, Xfce, FLTK coreboot – lightweight BIOS C standard library variants 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 (software) Bionic – originally developed by Google for the Android embedded system operating system Further reading: Comparison of C/POSIX standard library implementations for Linux Choosing System C library Distributions Distributions for ARM architecture ARM processors: Arch Linux ARM – port of Arch Linux for ARM architecture ARM processors Armbian – Debian and Ubuntu_(operating_system) Ubuntu based RedSleeve – port of Red Hat Enterprise Linux emteria.OS – proprietary Android (operating system) Android based Lightweight cloud oriented systems: Alpine Linux – uses musl, BusyBox and OpenRC. Fedora_Linux#CoreOS Fedora CoreOS RHCOS – Red Hat Enterprise Linux CoreOS for OpenShift Container Platform https://MicroShift.io/ – lightweight Kubernetes distribution designed for edge and IoT computing environments Lightweight multi-platform distributions: Gentoo Linux Tiny Core Linux Kali Linux Lubuntu Slackware Void Linux Xubuntu Common multi-platform distributions: Debian Fedora (operating system) Fedora openSUSE Ubuntu_(operating_system) Ubuntu Features of Linux kernel for embedded systems I/O and buses: GPIO drivers/gpio, tools/gpio, General Purpose Input/Output driver-api/gpio SPI drivers/spi, tools/spi, Serial Peripheral Interface spi I2C ⚲ UAPI: Instantiate I2C devices from user-space i2c/instantiating-devices.html?#method-4-instantiate-from-user-space drivers/i2c, https://i2c.wiki.kernel.org I2C/SMBus Subsystem i2c drivers/net/can – CAN bus Controller Area Network bus drivers/mfd – Multi-Function Devices drivers/usb/gadget – peripheral USB device implementation drivers/iio – Industrial I/O drivers/pwm – Pulse-width modulation drivers/regulator – Generic Voltage and Current Regulator support drivers/fpga drivers/slimbus SLOB – Simple List Of Blocks memory allocator Devicetree – describes hardware components for the kernel Multimedia Audio ASoC – ALSA System on Chip snd_soc_card ASoC – ALSA SoC Layer sound/soc https://www.alsa-project.org/wiki/ASoC TFT (LCD) – Thin-film-transistor liquid-crystal display fbtft_display File systems and storage UBIFS – Unsorted Block Image FS SquashFS – a compressed read-only FS with low Overhead (computing) overhead drivers/mtd – Memory Technology Device block device More: YAFFS, JFFS2 MMU less 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 People use a variety of methods to combine Real-time computing 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). Examples of RT distributions: Xenomai RTAI RTLinux MontaVista#Linux MontaVista Linux Red Hat Enterprise Linux for Real Time Real-time Ubuntu Several RT features provide real time capabilities: RT preemption RT scheduling CPU partitioning and isolation Dynticks (Tickless, nohz) ... Further readings about Embedded Linux: Embedded Linux Conference (ELC) @ EOSS 2023 https://elinux.org Category:Embedded_Linux Linux kernel interfaces ARM support arch/arm, ARM Architecture arch/arm https://lwn.net/Kernel/Index/#Architectures-ARM arch/arm64, ARM64 Architecture arch/arm64 architecture-specific initialization Learning the vi Editor Linux