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Embedded Linux on ARM

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An unique module combining various previous modules you have learnt by combing Linux administration, Hardware knowledge, Linux as OS, C/Computer programming areas. This is a complete module on Embedded OS, as of now no books are written on this with such practical aspects. Here is a consolidated material to get real hands-on perspective about building custom Embedded Linux distribution in ARM.

Published in: Technology
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Embedded Linux on ARM

  1. 1. Team Emertxe Embedded Operating System Linux
  2. 2. Contents
  3. 3. Embedded Operating System - Linux Contents ● Embedded Systems Introduction ● Linux as Embedded Operating System ● Embedded Development and its Environment ● Overview of the Target – Peripherals and Interfacing ● Booting Sequences ● Embedded Linux Kernel ● File Systems
  4. 4. Linux as Embedded OS
  5. 5. Embedded Linux ● Open Source & Free Software Fundamentals ● Why to choose Linux ● Architecture ● Choices to Make
  6. 6. Open Source How it all started? ● With GNU (GNU is not UNIX) ● Richard Stallman made the initial announcement in 1983, Free Software Foundation (FSF) got formed during 1984 ● Volunteer driven GNU started developing multiple projects, but making it as an operating system was always a challenge ● During 1991 a Finnish Engineer Linus Torvalds developed core OS functionality, called it as “Linux Kernel” ● Linux Kernel got licensed under GPL, which laid strong platform for the success of Open Source ● Rest is history!
  7. 7. ● Multiple Linux distributions started emerging around the Kernel ● Some applications became platform independent ● Community driven software development started picking up ● Initially seen as a “geek-phenomenon”, eventually turned out to be an engineering marvel ● Centered around Internet ● Building a business around open source started becoming viable ● Redhat set the initial trend in the OS business Open Source How it evolved?
  8. 8. OS Databases Server/Cloud Enterprise Consumer Education CMS eCommerce Open Source Where it stands now?
  9. 9. ● 4 Freedoms – Copy, – Study, – Change, – Use ● The above freedom are for both commercial and non- commercial use ● Free Software Licenses – GNU GPL and GNU FDL Free Software What does it mean?
  10. 10. ● Quality and Reliability of Code ● Availability of Code ● Hardware Support ● Communication Protocols and Software Stds ● Available Tools ● Community Support ● Licensing ● Vendor Independence ● Cost Embedded Linux Why Choose!
  11. 11. ● Modularity and Structure ● Readability of Code ● Extensibility ● Configurable ● Predictability ● Error Recovery ● Longevity Embedded Linux Why Choose! – Quality and Reliability of Code
  12. 12. ● ARM – Suits well for Embedded – Include THUMB – reduce code bandwidth – High density code than PPC, x86. ● Power PC – Intended for PC – Have become popular in embedded ● Strong ARM – Faster CPU – Higher Performance – PDAs, Setup box etc., ● MIPS And many more !! Embedded Linux Ported Architectures
  13. 13. ● Which kernel to use? ● Which development environment: ● Which compiler, debugger, dev boards? ● Which drivers and libraries? ● Support and training? Embedded Linux Choices to Make
  14. 14. Target Development & Environment
  15. 15. Embedded Development & Environment ● Typical System Components ● Hardware Tools and Interfacing ● Software Environments Tools ● Toolchains
  16. 16. Typical System Components Hardware Boot Loaders Operating System Libraries Applications Tools
  17. 17. Embedded Development & Environment Hardware Tools and Interfacing Possible Connections Serial Network USB JTAG Emulators OTA
  18. 18. Embedded Development & Environment Software Environment Tools Serial Network minicom gtkterm TFTP NFS
  19. 19. Toolchain Introduction
  20. 20. Toolchain Introduction ● Set of applications used to perform a complex task or to create a product, which is typically another computer program or a system of programs. ● Tools are linked (or chained) together by specific stages ● Output or resulting environment state of each tool becomes the input or starting environment for the next one ● By default the host contains some development tools which ate called as native toolchain
  21. 21. Toolchain Introduction ● When you use these tool the code generation would be for native architecture (say x86) ● In case of developing embedded systems these toolchains does not serve the purpose ● The targets some times might have the following restrictions too! – Less memory footprints – Slow compared to host – You may not wish to have in a developed system finally ● So cross compiling toolchain are used
  22. 22. Toolchain Introduction - Flow Native Tools Cross Compiling Tools
  23. 23. Toolchain Components
  24. 24. Toolchain Components Compiler Binary Utilities C / C++ Libraries Kernel Headers Debuggers
  25. 25. Toolchain Components – GCC ● Abbreviated as GNU Compiler Collection, one of the famous free software compiler ● Can compile many programming languages and generate code for many different types of architecture
  26. 26. Toolchain Components – Binary Utilities ● Set of programming tools for creating and managing binary programs, object files, libraries, profile data, and assembly source code ● Includes an assembler, linker and several other software tools ● Often used with a compiler and libraries to design programs for Linux ● GNU Binary Utilities are called binutils
  27. 27. Toolchain Components – binutils ● as - the assembler, that generates binary code from assembler source code ● ld - the linker ● ar, ranlib - to generate .a archives, used for libraries ● objdump, readelf, size, nm, strings - to inspect binaries. Very useful analysis tools ! ● strip - to strip useless parts of binaries in order to reduce their size
  28. 28. Toolchain Components - Libraries ● The C library is an essential component which provides interface between the applications and the kernel ● Provides macros, type definitions, and functions for tasks like string handling, mathematical computations, input/output processing, memory allocation and several other operating system services
  29. 29. Toolchain Components - Libraries ● Several C libraries are available: glibc, uClibc, dietlibc, etc ● The selection of the library has to be made while generation the toolchain as gcc is compiled against the selected library ● Some common used libraries – glibc – uclibc
  30. 30. Toolchain Components – Libraries - glibc ● C library from the GNU project ● Good performance optimization, standards compliance and portability ● Well maintained and used on all GNU / Linux host systems ● Require large memory footprint making it not suitable for small embedded systems ● Used in systems that run many different kernels and different hardware architectures ● License: LGPL ● Website: http://www.gnu.org/software/libc/
  31. 31. Toolchain Components – Libraries - uclibc ● Smaller than the GNU C Library, but nearly all applications supported by glibc also work perfectly with uClibc ● Recommended for lower footprint embedded systems ● Excellent configuration options at the cost of ABI differences for different configurations ● Features can be enabled or disabled according to space requirements ● Works on most embedded architectures with embedded Linux ● Focus on size rather than performance with less compile time
  32. 32. Toolchain Components – Libraries - uclibc
  33. 33. Toolchain Components – Libraries - uclibc ● Created for support uclinux, a Linux for MMU less system ● Actively maintained, large developer and user base ● Used on a large number of production embedded products, including consumer electronic devices
  34. 34. Toolchain Components – Kernel Headers ● The compiled applications and the libraries sometimes need to interact with the kernel ● So the Linux kernel need to expose the available interfaces for the libraries and applications to use, like – System calls and its numbers – MACRO definitions – Data structures, etc.
  35. 35. Toolchain Components – Kernel Headers ● Therefore, compiling the C library requires kernel headers, and many applications also require them. ● You may find these in <linux/...> and <asm/...> and a few other directories corresponding to the ones visible in include/ in the kernel sources ● Compatibility with running kernel has to be considered
  36. 36. Toolchain Building
  37. 37. Toolchain Building ● Toolchain building is one of the time consuming process ● A very clear picture of the system architecture is sometimes required ● We might have to identify three systems in this case, like – Build System – which is used to create the toolchain – Host – which will be used to execute the created toolchain – Target – which will execute the binaries created by the toolchain ● So by considering the above points some possible build options are provided in next slide
  38. 38. Toolchain Building ● Possible Build System ● Build Considerations ● Toolchain Build Possibilities – Prebuilt – Home Built – Automated Tools ● Crosstool NG ● Buildroot
  39. 39. Toolchain Building – Build Systems Native Build Cross Build Canadian BuildCross Native Build
  40. 40. Toolchain Building - Considerations ● Before building the toolchain following decisions have to be made – Which library to be used? – What version of the components to selected? – Certain important configurations like ● Architecture features like floating point ● ABI selections ● Networking features etc., ● So you might have to put good amount of time in investigations
  41. 41. Toolchain Building – Pre-built ● Get a pre-built open toolchain either from a web or from your hardware vendor ● This is the easiest solution, as the toolchain is already built, and has supposedly been tested by the vendor ● The drawback is that you don't have flexibility on your toolchain features (which C library? hard-float or soft-float? which ABI?)
  42. 42. Toolchain Building – Home Built ● Building toolchain from scratch is one of the tiring and time consuming task which might go upto several weeks. ● Several information to be know. Lots of components to be considered to build. ● Proper decision on the components and its configuration have to be made ● Need kernel headers and C library sources ● There are version dependency issues, patches required to make something work etc. ● The order of build is to be known, but have complete freedom of choice
  43. 43. Toolchain Building – Home Built ● Can get some idea from the crosstool matrix by Kegel (Note this is outdated)
  44. 44. Toolchain Building – Automated Tools ● Lots of open community has built several scripts or more elaborate systems to ease the process of building a toolchain ● More easy procedure since no need of breaking your heads on dependency resolutions ● Provides easy configuration mechanism ● Recipes and patches needed to build a toolchain made of particular versions of the various components are shared and easily available ● Some common automated tools are discussed in following slides
  45. 45. Toolchain Building – Automated Tools – Crosstool-NG ● Updated version of Crosstool, with a kernel-like menuconfig like configuration system ● Supports uClibc, glibc, eglibc, hard and soft float, many architectures ● Support different target OS: Linux, bare metal ● Debug facilities like native and cross gdb, gdb server ● Actively maintained ● Targeted at building toolchains ● http://crosstool-ng.org/
  46. 46. Toolchain Building – Automated Tools – Buildroot ● Buildroot is a set of Makefiles and patches that makes it easy to generate a complete embedded Linux system ● Generates root file system images ready to be used ● Complete build system based on the Linux Kernel configuration system and supports a wide range of target architectures ● It automates the building process of your embedded system and eases the cross-compilation process ● Supports multiple filesystem types for the root filesystem image
  47. 47. Toolchain Building – Automated Tools – Buildroot ● Can generate an (e)glibc or uClibc cross-compilation toolchain, or re-use your existing glibc, eglibc or uClibc cross-compilation toolchain ● Supports several hundreds of packages for userspace applications and libraries ● http://buildroot.uclibc.org
  48. 48. Buildroot Introduction
  49. 49. Buildroot Introduction ● Buildroot is a set of Makefiles and patches that makes it easy to generate a complete embedded Linux system ● Generates root file system images ready to be used ● Complete build system based on the Linux Kernel configuration system and supports a wide range of target architectures ● It automates the building process of your embedded system and eases the cross-compilation process ● Supports multiple filesystem types for the root filesystem image
  50. 50. Buildroot Introduction ● Can generate an (e)glibc or uClibc cross-compilation toolchain, or re-use your existing glibc, eglibc or uClibc cross-compilation toolchain ● Supports several hundreds of packages for userspace applications and libraries ● http://buildroot.uclibc.org
  51. 51. Buildroot Configuration
  52. 52. Buildroot Configuration ● Download buildroot package from http://buildroot.uclibc.org ● Untar the package and change directory to buildroot $ tar xvf buildroot-<year>-<month>.tar.bz2 $ cd buildroot-<year>-<month> ● Buildroot supports Linux kernel like configuration options like menuconfig, xconfig etc., ● To configure type $ make menuconfig ● You should get a curses based configurator
  53. 53. Buildroot Configuration ● Select the target architecture you want to work with ● You may select the toolchain components like – kernel headers – binutils – uclibc – gcc etc., ● You can ignore selecting these components buy selecting the target architecture, but the default selected components would be used while building
  54. 54. Buildroot Building
  55. 55. Buildroot Building ● To start the build process just type $ make ● Make sure you don't use make -jN option. Instead you can use BR2_JLEVEL option to run compilation of each individual package with make -jN ● BR2_JLEVEL can be set while configuration at Build options Number of jobs to run simultaneously→
  56. 56. Buildroot Building ● The make command will generally perform: – Download source files (as required) – Configure, build and install the cross compilation toolchain, or simply import an external toolchain – Configure, build and install selected target packages – Build kernel, bootloader images if selected – Create the root filesystem in selected format ● Buildroot output is stored in a single directory named output/ which will be found in the root directory of buildroot
  57. 57. Buildroot Building - Output ● The left side shows contents the output directory of the buildroot folder ● This directory contains several subdirectories ● The following slides discuss its contents images build staging buildroot-<YYYY>.<MM> target host output
  58. 58. Buildroot Building - Output ● All the built images like kernel, bootloader, filesystem are stored here ● These are the files you need to put on the target system images build staging buildroot-<YYYY>.<MM> target host output
  59. 59. Buildroot Building - Output ● All the components are built here (this includes tools needed by Buildroot on the host and packages compiled for the target) ● Contains one sub directory for each of these components images build staging buildroot-<YYYY>.<MM> target host output
  60. 60. Buildroot Building - Output ● Contains a hierarchy similar to a root filesystem hierarchy ● Contains the headers and libraries of the cross-compilation toolchain and all the userspace packages selected for the target ● This directory is not intended to be the root filesystem for the target: it contains a lot of development files, unstripped binaries and libraries that make it far too big for an embedded system ● These development files are used to compile libraries and applications for the target that depend on other libraries images build staging buildroot-<YYYY>.<MM> target host output
  61. 61. Buildroot Building - Output ● Contains almost the complete root filesystem for the target: everything needed is present except the device files in /dev/ and It doesn’t have the correct permissions ● Therefore, this directory should not be used on your target ● Instead, you should use one of the images built in the images/ directory ● If you need an extracted image of the root filesystem for booting over NFS, then use the tarball image generated in images/ and extract it as root ● Contains only the files and libraries needed to run the selected target applications: the development files (headers, etc.) are not present, the binaries are stripped images build staging buildroot-<YYYY>.<MM> target host output
  62. 62. Buildroot Building - Output ● Contains the installation of tools compiled for the host that are needed for the proper execution of Buildroot, including the cross- compilation toolchain images build staging buildroot-<YYYY>.<MM> target host output
  63. 63. Buildroot More Infos!! ● http://buildroot.uclibc.org/downloads/manual/manual.html
  64. 64. Target Overview
  65. 65. EmxDev Boards – EmxARM9A03 Target Overview ● Know your Target Controller ● Target Architecture ● Target Board
  66. 66. EmxDev Boards – EmxARM9A03 Target Overview - Target Controller ● ARM920T – 200MIPS at 180MHz, MMU – 16KB Inst & Data Cache – ICE ● Memories – 128 ROM, 16 KB SRAM – EBI – SDRAM, SM,CF, NOR, NAND ● Ethernet MAX 10/100 ● USB 2.0 FS Host ● USB 2.0 FS Device ● MCI ● SSC ● USART – Smart Card – RS485, RS232 – IrDA – Modem ● I2C, SPI ● Debug Unit
  67. 67. EmxDev Boards – EmxARM9A03 Target Overview - Target Architecture
  68. 68. EmxDev Boards – EmxARM9A03 Target Overview - Base Board
  69. 69. EmxDev Boards – EmxARM9A03 Target Overview - Application Board
  70. 70. Booting Sequences
  71. 71. Booting Sequence Linux as general System Start-up BIOS / Boot Monitor Stage 1 Boot Loader Master Boot Record Stage 2 Boot Loader LILO, GRUB, etc Kernel Linux Init User Application
  72. 72. EmxDev Boards – EmxARM9A03 Booting Sequence ● Controller's Booting Sequence ● Boot Loader Stages
  73. 73. EmxDev Boards – EmxARM9A03 Booting Sequence - Controller's Boot Sequence Device Setup SPI Data Flash Parallel Boot OR Download from Data Flash Download from EEPROM Download from 8 bit Device DBGU Serial Download USB Download DFU Protocol RUN RUN RUN RUN RUN TWI EEPROM Boot
  74. 74. ● Stage 1 Boot Loader ● Stage 2 Boot Loader EmxDev Boards – EmxARM9A03 Booting Sequence – Boot Loader Stages
  75. 75. ● Pointer to Stage 2 Boot Loader EmxDev Boards – EmxARM9A03 Booting Sequence – Stage 1 Boot Loader ROM (ROM Code) SRAM Memory (serial / parallel) (S1BL) Stage 2 Boot Loader Kernel Data Memory (parallel)
  76. 76. ● Pointer to Kernel Image ● We use U-Boot as S2BL EmxDev Boards – EmxARM9A03 Booting Sequence – Stage 2 Boot Loader Memory (parallel) Stage 2 Boot Loader Kernel Data
  77. 77. U-Boot Introduction
  78. 78. U-Boot Introduction - General ● The "Universal Bootloader" ("Das U-Boot") is a monitor program ● Free Software: full source code under GPL ● Can get at: //www.denx.de/wiki/U-Boot ● Production quality: used as default boot loader by several board vendors ● Portable and easy to port and to debug ● Many supported architectures: PPC, ARM, MIPS, x86, m68k, NIOS, Microblaze
  79. 79. U-Boot Introduction - General ● More than 216 boards supported by public source tree ● Simple user interface: CLI or Hush shell ● Environment variable storing option on different media like EEPROM, Flash etc ● Advanced command supports
  80. 80. U-Boot Introduction – Design Principles ● Easy to port to new architectures, new processors, and new boards ● Easy to debug: serial console output as soon as possible ● Features and commands configurable ● As small as possible ● As reliable as possible
  81. 81. U-Boot Source Tree
  82. 82. U-Boot Source Code Browsing ● Untar the U-Boot code – tar xvf u-boot-<version>.tar.bz2 ● Enter the U-Boot directory – cd u-boot-<version> ● The following slide discuss the contents of the U-Boot directory
  83. 83. U-Boot Source Tree ● The left side of the slide shows the source content of the U-Boot ● The directory structure might vary depending on the picked version. ● The considered version is u-boot-2015-01 ● Lets us discuss some important directories and files driver api arch board common configs post scripts dts examples fs include lib test tools Makefile disk net doc u-boot-<version>
  84. 84. U-Boot Source Tree ● Machine/arch independent API for external apps driver api arch board common configs post scripts dts examples fs include lib test tools Makefile disk net doc u-boot-<version>
  85. 85. U-Boot Source Tree ● All architecture dependent functions ● CPU specific information – <core>/cpu.c – <core>/interrupt.c – <core>/start.S driver api arch board common configs post scripts dts examples fs include lib test tools Makefile disk net doc u-boot-<version>
  86. 86. U-Boot Source Tree ● Platform, board level files. Eg, atmel, icecube, oxc etc., ● Contains all board specific initialization – <boardname>/flash.c – <boardname>/<boardname>_emac.c – <boardname>/<boardname>.c – <boardname>/soc.h – <boardname>/platform.S driver api arch board common configs post scripts dts examples fs include lib test tools Makefile disk net doc u-boot-<version>
  87. 87. U-Boot Source Tree ● All architecture independent functions ● All the commands driver api arch board common configs post scripts dts examples fs include lib test tools Makefile disk net doc u-boot-<version>
  88. 88. U-Boot Source Tree ● Default configuration files for boards driver api arch board common configs post scripts dts examples fs include lib test tools Makefile disk net doc u-boot-<version>
  89. 89. U-Boot Source Tree ● Partition and device information for disks driver api arch board common configs post scripts dts examples fs include lib test tools Makefile disk net doc u-boot-<version>
  90. 90. U-Boot Source Tree ● You can find all the README files here driver api arch board common configs post scripts dts examples fs include lib test tools Makefile disk net doc u-boot-<version>
  91. 91. U-Boot Source Tree ● Various device drivers files driver api arch board common configs post scripts dts examples fs include lib test tools Makefile disk net doc u-boot-<version>
  92. 92. U-Boot Source Tree ● Contains Makefile for building internal U-Boot fdt driver api arch board common configs post scripts dts examples fs include lib test tools Makefile disk net doc u-boot-<version>
  93. 93. U-Boot Source Tree ● Example code for standalone application driver api arch board common configs post scripts dts examples fs include lib test tools Makefile disk net doc u-boot-<version>
  94. 94. U-Boot Source Tree ● File system directories and codes driver api arch board common configs post scripts dts examples fs include lib test tools Makefile disk net doc u-boot-<version>
  95. 95. U-Boot Source Tree ● Various header files – configs/<boardname>.h – <core>.h driver api arch board common configs post scripts dts examples fs include lib test tools Makefile disk net doc u-boot-<version>
  96. 96. U-Boot Source Tree ● Processor specific libraries – board.c – <arch>linux.c – div0.c driver api arch board common configs post scripts dts examples fs include lib test tools Makefile disk net doc u-boot-<version>
  97. 97. U-Boot Source Tree ● Networking related files. driver api arch board common configs post scripts dts examples fs include lib test tools Makefile disk net doc u-boot-<version>
  98. 98. U-Boot Source Tree ● Power On Self Test driver api arch board common configs post scripts dts examples fs include lib test tools Makefile disk net doc u-boot-<version>
  99. 99. U-Boot Source Tree ● Contains the sources for various helper programs driver api arch board common configs post scripts dts examples fs include lib test tools Makefile disk net doc u-boot-<version>
  100. 100. U-Boot Source Tree driver api arch board common configs post scripts dts examples fs include lib test tools Makefile disk net doc u-boot-<version>
  101. 101. U-Boot Source Tree ● Various tools directories and files driver api arch board common configs post scripts dts examples fs include lib test tools Makefile disk net doc u-boot-<version>
  102. 102. U-Boot Source Tree ● Top level make file for Uboot build and configuration driver api arch board common configs post scripts dts examples fs include lib test tools Makefile disk net doc u-boot-<version>
  103. 103. U-Boot Building
  104. 104. U-Boot Building ● The include/configs/ directory contains one configuration file for each supported board – It defines the CPU type, the peripherals and their configuration, the memory mapping, the Uboot features that should be compiled in, etc. – It is a simple .h file that sets preprocessor constants. See the README file for the documentation of these constants. ● Assuming that your board is already supported by Uboot, there should be a config corresponding to your board, for example include/configs/at91rm9200ek.h
  105. 105. U-Boot Building ● We need to configure U-Boot for the required board which is generally done as make <board_name>_config ● The board_name can be found in include/configs/ directory ● The newer version supports kernel like configuration options like make menuconfig ● Compile Uboot, by specifying the cross compiler prefix. make CROSS_COMPILE=<cross_compile_path>
  106. 106. U-Boot Building ● cross_compile_path could be the command itself if already exported in PATH variable, else you can specify the installation path of command ● For example for arm platform it would look like make CROSS_COMPILE=arm-linux- ● The result would be u-boot.bin which has to be stored in flash memory (in most of the cases) ● The invocation of the stored image depends on the target architecture. The memory used to store would play the role here
  107. 107. U-Boot Introduction
  108. 108. U-Boot Responsibility Execute from flash (If configured). Do POST Relocate to RAM Setup console for user interaction Setup device driver for kernel (& RFS) image Choose the kernel (& RFS) image Download the kernel (& RFS) image Choose the kernel (& RFS) imageChoose the kernel (& RFS) image Setup kernel command line arguments Jump to kernel start address Codeflow
  109. 109. U-Boot Important Commands
  110. 110. U-Boot Utilities ● Environment Variables ● Commands – Information – Environment – Network – Boot – Data Transfer – Memory
  111. 111. U-Boot Environment Variables ● bootcmd : Contains the command that U-Boot will automatically execute at boot time after a configurable delay, if the process is not interrupted ● bootargs : contains the arguments passed to the Linux kernel ● serverip : Server (Host) ip address for network related commands ● ipaddr : Local ip address of the target ● ethaddr : MAC address. Will be set once
  112. 112. U-Boot Important Environment Variables ● netmask : The network mask to communicate with the server ● bootdelay : Time in seconds to delay the boot process so that the u-boot can be interrupted before executing bootcmd ● autostart : If set the loaded image in memory will be executed automatically
  113. 113. U-Boot Important Commands – Information ● help : Help command. Can be used to list all supported built commands ● flinfo : Display flash informations (NOR and SPI Flash) ● nand info : Display NAND flash informations
  114. 114. U-Boot Important Commands – Environment ● printenv : Print all set environment variables ● setenv : Set the environment variable ● saveenv : Save environment variable to configured memory
  115. 115. U-Boot Important Commands – Network ● ping : Checks for network connectivity
  116. 116. U-Boot Important Commands – Boot ● boot : Runs the default boot command, stored in bootcmd variable ● bootm : Boot memory. Starts a kernel image loaded at the specified address in RAM Example: bootm <address>
  117. 117. U-Boot Important Commands – Data Transfer ● loadb ● loads ● loady : Load a file from the serial line to RAM ● tftpboot : Loads a file from the network to RAM Example: tftpboot <address>
  118. 118. U-Boot Important Commands – Memory ● erase : Erase the content of NOR flash ● protect : Protect the content of NOR flash ● cp : Write to NOR flash ● nand : Erase, read, write to NAND flash
  119. 119. Embedded Linux Kernel General Information
  120. 120. Embedded Linux Kernel General Information ● Where to get? ● Kernel Subsystem ● Source Code Browsing
  121. 121. Embedded Linux Kernel General Information – Where to get? Note: Snapshot of www.kernel.org. Expect changes on updates
  122. 122. Embedded Linux Kernel General Information – Kernel Subsystem ● Process Scheduler: – To provide control, fair access of CPU to process, while interacting with HW on time ● Memory Manager: – To access system memory securely and efficiently by multiple processes. Supports Virtual Memory in case of huge memory requirement ● Virtual File System: – Abstracts the details of the variety of hardware devices by presenting a common file interface to all devices
  123. 123. Embedded Linux Kernel General Information – Kernel Subsystem ● Network Interface: – provides access to several networking standards and a variety of network hardware ● Inter Process Communications: – supports several mechanisms for process-to- process communication on a single Linux system
  124. 124. Embedded Linux Kernel General Information – Source Code Browsing ● Untar the Linux kernel code – tar xvf linux-<version>.<compression_format> ● Enter the Linux kernel directory – cd linux-<version> ● The following slide discuss the contents of the Linux directory
  125. 125. Embedded Linux Kernel General Information – Source Code Browsing ● The left side of the slide shows the source content of the Linux kernel ● The directory structure might vary depending on the picked version. ● Lets us discuss some important directories and files init arch block crypto drivers firmware scripts security ipc kernel lib mm net sound tools usr Makefile fs samples include linux-<version>
  126. 126. Embedded Linux Kernel General Information – Source Code Browsing ● Architecture specific kernel code ● Has sub directories per supported architecture ● Example: – arm – powerpc – X86 ● We can also find low level memory management, interrupt handling, early inits, assembly code and much moreinit arch block crypto drivers firmware scripts security ipc kernel lib mm net sound tools usr Makefile fs samples include linux-<version>
  127. 127. Embedded Linux Kernel General Information – Source Code Browsing ● Contains core block layer files init block crypto drivers firmware scripts security ipc kernel lib mm net sound tools usr Makefile fs samples include linux-<version> arch
  128. 128. Embedded Linux Kernel General Information – Source Code Browsing ● Cryptographic API for use by kernel itself init block crypto drivers firmware scripts security ipc kernel lib mm net sound tools usr Makefile fs samples include linux-<version> arch
  129. 129. Embedded Linux Kernel General Information – Source Code Browsing ● Contains system's device drivers ● Sub directories contain classes of device drivers like video drivers, network card drives, low level SCSI drivers etc., init block crypto drivers firmware scripts security ipc kernel lib mm net sound tools usr Makefile fs samples include linux-<version> arch
  130. 130. Embedded Linux Kernel General Information – Source Code Browsing ● Contains the device firmwares which will be uploaded to devices with help of drivers init block crypto drivers firmware scripts security ipc kernel lib mm net sound tools usr Makefile fs samples include linux-<version> arch
  131. 131. Embedded Linux Kernel General Information – Source Code Browsing ● File system related code ● Contains both generic file system code (VFS) and different files system code ● Sub directories of supported file system ● Examples: – ext2 – ext3 – fat init block crypto drivers firmware scripts security ipc kernel lib mm net sound tools usr Makefile fs samples include linux-<version> arch
  132. 132. Embedded Linux Kernel General Information – Source Code Browsing ● Most of the header files used in the .c file of the kernel source ● It has further sub directories including asm-generic ● Architecture specific header file would be found in arch/<arch>/include/ init block crypto drivers firmware scripts security ipc kernel lib mm net sound tools usr Makefile fs samples include linux-<version> arch Note: File level organization will vary based on different versions of kernel sources especially architecture and machine related header files
  133. 133. Embedded Linux Kernel General Information – Source Code Browsing ● Initialization code for kernel ● Best directory to start with to know on how kernel works ● Has main.c of kernel init block crypto drivers firmware scripts security ipc kernel lib mm net sound tools usr Makefile fs samples include linux-<version> arch
  134. 134. Embedded Linux Kernel General Information – Source Code Browsing ● Contains kernel's inter process communication code like shared memory, semaphores and other forms init block crypto drivers firmware scripts security ipc kernel lib mm net sound tools usr Makefile fs samples include linux-<version> arch
  135. 135. Embedded Linux Kernel General Information – Source Code Browsing ● Generic kernel level code which can't fit anywhere else ● Contain upper level codes for signal handling, scheduling etc., ● The architecture specific kernel code will be in arch/<arch_name>/kernel init block crypto drivers firmware scripts security ipc kernel lib mm net sound tools usr Makefile fs samples include linux-<version> arch
  136. 136. Embedded Linux Kernel General Information – Source Code Browsing ● Contains kernel's library code ● Common string operations, code for debugging and command line parsing code can be found here ● The architecture specific library code will be in arch/<arch_name>/lib init block crypto drivers firmware scripts security ipc kernel lib mm net sound tools usr Makefile fs samples include linux-<version> arch
  137. 137. Embedded Linux Kernel General Information – Source Code Browsing ● Contains memory management code ● The architecture specific memory management code would be found in arch/<arch_name>/mm ● Example: – arch/x86/mm/init.c init block crypto drivers firmware scripts security ipc kernel lib mm net sound tools usr Makefile fs samples include linux-<version> arch
  138. 138. Embedded Linux Kernel General Information – Source Code Browsing ● The kernels networking code init block crypto drivers firmware scripts security ipc kernel lib mm net sound tools usr Makefile fs samples include linux-<version> arch
  139. 139. Embedded Linux Kernel General Information – Source Code Browsing ● Some sample programs init block crypto drivers firmware scripts security ipc kernel lib mm net sound tools usr Makefile fs samples include linux-<version> arch
  140. 140. Embedded Linux Kernel General Information – Source Code Browsing ● Contains scripts that are used while kernel configuration init block crypto drivers firmware scripts security ipc kernel lib mm net sound tools usr Makefile fs samples include linux-<version> arch
  141. 141. Embedded Linux Kernel General Information – Source Code Browsing ● Contains code for different security models init block crypto drivers firmware scripts security ipc kernel lib mm net sound tools usr Makefile fs samples include linux-<version> arch
  142. 142. Embedded Linux Kernel General Information – Source Code Browsing ● Contains all the sound card drivers init block crypto drivers firmware scripts security ipc kernel lib mm net sound tools usr Makefile fs samples include linux-<version> arch
  143. 143. Embedded Linux Kernel General Information – Source Code Browsing ● Certain configuration and testing tools init block crypto drivers firmware scripts security ipc kernel lib mm net sound tools usr Makefile fs samples include linux-<version> arch
  144. 144. Embedded Linux Kernel General Information – Source Code Browsing ● Contains code that builds a cpio-format archive containing a root file system image, which will be used for early userspace init block crypto drivers firmware scripts security ipc kernel lib mm net sound tools usr Makefile fs samples include linux-<version> arch
  145. 145. Embedded Linux Kernel General Information – Source Code Browsing ● This is top level Makefile for the whole source tree ● Contains useful rules and variables like default gcc compilation flags init block crypto drivers firmware scripts security ipc kernel lib mm net sound tools usr Makefile fs samples include linux-<version> arch
  146. 146. Embedded Linux Kernel Configuration
  147. 147. Embedded Linux Kernel Configuration ● The kernel configuration is based on multiple Makefiles ● As discussed already the top level Makefile would be used for this purpose ● The configuration you should know the target. You can find of the target as mentioned below $ cd linux-<version> $ make help ● Now you may look for “Configuration targets:” section of the output and decide one
  148. 148. Embedded Linux Kernel Configuration ● Once you decide on the target you may try the following command $ make target ● The modified configurations would be saved on a file called as .config which can be found on the top level of the linux-<version> directory. ● All the target options use the same .config file, so you may use any interchangeably.
  149. 149. Embedded Linux Kernel Configuration ● Some most commonly used target are – make config – make menuconfig – make xconfig ● Configuring Architecture specific targets ● Configuring for specific architecture from scratch
  150. 150. Embedded Linux Kernel Configuration – make config user@hostname:linux-<version>$ make config scripts/kconfig/conf --oldaskconfig Kconfig * * Linux/<ARCH> <version> Kernel Configuration * Patch physical to virtual translations at runtime (ARCH_PATCH_PHYS_VIRT) [Y/n/?] ● The above image show snap shot typical output of make config command ● Updates current config utilizing a line-oriented program ● No user friendly approach. Could be used if you have limited host installations ● The problem with this approach is that, It force you to follow an sequence of questions while configuration. ● Have to use “Ctrl C” to exit
  151. 151. Embedded Linux Kernel Configuration – make menuconfig ● The above image shows the snapshot of typical output of make menuconfig command
  152. 152. Embedded Linux Kernel Configuration – make menuconfig ● Most commonly used method and simple method ● Can be used if graphics is unavailable ● Requires libncurses-dev installation ● Easy to navigate between options, using arrow keys ● Use <Help> to know more on menuconfig
  153. 153. Embedded Linux Kernel Configuration – make xconfig ● The above image shows the snapshot of typical output of make xconfig command
  154. 154. Embedded Linux Kernel Configuration – make xconfig ● Most commonly used graphical method of configuration ● Easy to use, better search option ● Use Help menu to know more on xconfig ● Requires libqt-dev packages installation
  155. 155. Embedded Linux Kernel Configuration – Architecture Specific ● Most preferably used in Embedded Linux configuration ● You can find then at arch/<arch>/configs/ ● These files are best possible minimal .config file you can have for your board ● Just type the following on the command to know available target $ make help – Now you may look for “Architecture specific targets:” section of the output to look for default configuration for your target architecture ● Now the following command $ make <controller_name>_defconfig
  156. 156. Embedded Linux Kernel Configuration – Architecture Specific ● The previous command would rewrite the existing .config file. ● Now you can use any of the general configuration method to discussed above to configure further if required ● If you feel the you are done and need to preserve your configuration then you can save it by $ make savedefconfig ● The above line will create a file call defconfig on root of kernel source directory ● Now you can mv it to the config directory by the following command $ mv defconfig /arch/<arch>/configs/my_defconfig
  157. 157. Embedded Linux Kernel Configuration – From Scratch ● Its possible to configure a kernel from scratch without using any default configuration ● It would obvious if your a board vendor where you might have to do for your board ● Point to be kept in mind in this case – Make sure you alteast select a proper architecture for your board – Most of the architecture dependent basic things would be set by default, so just leave it as it is, unless you know what you change – Might have to change certain thing like select a correct device driver for your board
  158. 158. Embedded Linux Kernel Building
  159. 159. ● Assuming the required configuration are done, The next step would be to compile the kernel. ● Type the following command on the prompt to start the compilation make ● Can use the below command if you have multicore CPU make -j ● The above command will speed up your compilation process ● You may even specify the no of jobs you want to run simultaneously based on your CPU configuration Embedded Linux Kernel Building - Compilation
  160. 160. ● Once the compilation is done you will get the kernel image in the following location arch/<arch>/boot ● make install this is rarely used in embedded dev as the kernel image is single file, But still can be done by modifying its behavior arch/<arch>/boot/install.sh ● You can install all the configured modules by the following command make INSTALL_MOD_PATH=<dir>/ module_install ● The above line direct the module installation on the path provided by the INSTALL_MOD_PATH variable and this is important to avoid installation in host root path Embedded Linux Kernel Building - Compilation
  161. 161. ● Most of the embedded system uses U-Boot as its second stage boot loader ● U-Boot require the kernel image to be converted into a format which it can load. This converted format is called as uImage ● The discussion done here is on how create the uImage from vmlinux ● vmlinux is the output of the kernel compilation which you would find on the root directory of the kernel directory ● vmlinux consists of multiple information like ELF header, COFF and binary Embedded Linux Kernel Building – Kernel Image
  162. 162. ● So it required to extract the binary file from the vmlinux first, Which is done by the following command arm-linux-objcopy -O binary vimlinux linux.bin ● After extraction the U-Boot header can be added using mkimage command, This is done by the following command mkimage -A arm -O linux -T kernel -C none -a 20008000 -e 20008000 -n “Embedded Linux” -d linux.bin uImage.arm ● After all the above steps the kernel image is ready for deployment on target Embedded Linux Kernel Building - Kernel Image
  163. 163. Embedded Linux Kernel Deploy
  164. 164. Embedded Linux Kernel Deploy ● Assuming the host is already configured with TFTP server and Target is running U-Boot with TFTP client ● Copy uImage.arm in /var/lib/tftpboot/ ● Copy the kernel image to the target board as mentioned below U-boot> tftp <TEXTBASE_ADDRESS> uImage.arm ● TEXTBASE_ADDRESS is defined configuring u-boot ● Once the image is transferred you can boot the image as U-boot> bootm ● Your kernel should be loaded and executed now :)
  165. 165. File Systems
  166. 166. File Systems ● Introduction ● Building FS from scratch
  167. 167. Introduction
  168. 168. File Systems Introduction ● File system is an approach on how the data can be organized in order to have a meaningful read or write in a system ● File systems provides a very easy way of identifying data like where it begins and ends ● The group of such data can be called as “Files” ● The method used to manage these groups of data can be called as “File systems”
  169. 169. File Systems Introduction ● There are several kinds file system having different structure and logic, properties of speed, flexibility, security, size and more ● The most commonly used media to store the data are – Storage Devices ● Magnetic Tapes ● Hard Drive ● Optical Discs ● Flash Memories ● RAM (Temporary) – Network like NFS – Virtual like procfs
  170. 170. File Systems Introduction ● An OS can support more than one file system ● In this topic we are going to concentrate on Unix and Unix like file systems ● File systems can be discussed in the following context – Contents – Types – Partitions
  171. 171. File System Contents Linux Directory Structure
  172. 172. File System Contents Linux Directory Structure ● When it is the matter of data, different operating system uses different approaches to organize it ● In Unix and Unix like systems, applications and users see a single global hierarchy of files and directories, which can be composed of several file systems. ● The access of the data are done using a process called as mounting ● The location on where the data should be located called as mount point is to be informed to the OS. ● The following slides discuss about the Linux Directory Structure
  173. 173. File System Contents Linux Directory Structure ● The left side of the slide shows the most important files in Linux system ● The organization of a Linux root file system in terms of directories is well defined by the Filesystem Hierarchy Standard ● Most Linux systems conform to this specification – Applications expect this organization – It makes it easier for developers and users as the file system organization is similar in all systems tmp sys usr var / bin dev home media proc sbin boot etc lib mnt root
  174. 174. File System Contents Linux Directory Structure ● Place for most commonly used terminal commands ● Common Linux commands you need to use in single-user modes are located under this directory. ● Commands used by all the users of the system are located here. ● Examples: – ls – ping – grep – cp bin tmp sys usr var dev home media proc sbin boot etc lib mnt root
  175. 175. File System Contents Linux Directory Structure ● Contains files needed to start up the system, including the Linux kernel, a RAM disk image and bootloader configuration files ● Kernel image (only when the kernel is loaded from a file system, not common on non-x86 architectures) boot tmp sys usr var bin dev home media proc sbin etc lib mnt root
  176. 176. File System Contents Linux Directory Structure ● Device files ● These include terminal devices, usb, or any device attached to the system. ● Examples: – /dev/tty1 – /dev/usbmon0 dev tmp sys usr var bin home media proc sbin boot etc lib mnt root
  177. 177. File System Contents Linux Directory Structure ● Contains configuration files required by all programs. ● This also contains startup and shutdown shell scripts used to start/stop individual programs. ● Examples: – /etc/resolv.conf – /etc/logrotate.conf etc tmp sys usr var bin dev home media proc sbin boot etc lib mnt root
  178. 178. File System Contents Linux Directory Structure ● Home directories for all users to store their personal files. ● Example: – /home/arun – /home/adil home tmp sys usr var bin dev media proc sbin boot etc lib mnt root
  179. 179. File System Contents Linux Directory Structure ● Contains library files that supports the binaries located under /bin and /sbin ● Library file names are either ld* or lib*.so.* ● Examples: – ld-2.11.1.so – libncurses.so.5.7 lib tmp sys usr var bin dev home media proc sbin boot etc mnt root
  180. 180. File System Contents Linux Directory Structure ● Temporary mount directory for removable devices. ● Examples: – /media/cdrom – /media/floppy – /media/cdrecorder media tmp sys usr var bin dev home proc sbin boot etc lib mnt root
  181. 181. File System Contents Linux Directory Structure ● Temporary mount directory where sysadmins can mount file systems. mnt tmp sys usr var bin dev home media proc sbin boot etc lib root
  182. 182. File System Contents Linux Directory Structure ● Contains information about system process. ● This is a pseudo file system contains information about running process. For example: /proc/{pid} directory contains information about the process with that particular pid. ● This is a virtual file system with text information about system resources. Examples: – /proc/uptime proc tmp sys usr var bin dev home media sbin boot etc lib mnt root
  183. 183. File System Contents Linux Directory Structure ● Root user's home directory root tmp sys usr var bin dev home media proc sbin boot etc lib mnt
  184. 184. File System Contents Linux Directory Structure ● Just like /bin, /sbin also contains binary executables ● But, the Linux commands located under this directory are used typically by system administrator, for system maintenance purpose. ● Examples: – iptables – reboot – fdisk – ifconfig – swapon sbin tmp sys usr var bin dev home media proc boot etc lib mnt root
  185. 185. File System Contents Linux Directory Structure ● Mount point of the sysfs virtual file system sys tmp usr var bin dev home media proc sbin boot etc lib mnt root
  186. 186. File System Contents Linux Directory Structure ● Directory that contains temporary files created by system and users. ● Files under this directory are deleted when system is rebooted. tmp sys usr var bin dev home media proc sbin boot etc lib mnt root
  187. 187. File System Contents Linux Directory Structure ● Contains binaries, libraries, documentation, and source-code for second level programs. ● /usr/bin contains binary files for user programs. If you can’t find a user binary under /bin, look under /usr/bin. For Examples: at, awk, cc, less, scp ● /usr/sbin contains binary files for system administrators. If you can’t find a system binary under /sbin, look under /usr/sbin. For Examples: atd, cron, sshd, useradd, userdel ● /usr/lib contains libraries for /usr/bin and /usr/sbin ● /usr/local contains users programs that you install from source. For example, when you install apache from source, it goes under /usr/local/apache2usr tmp sys var bin dev home media proc sbin boot etc lib mnt root
  188. 188. File System Contents Linux Directory Structure ● var stands for variable files. ● Content of the files that are expected to grow can be found under this directory. ● This includes — – /var/log - system log files – /var/lib - packages and database files – /var/mail - emails – /var/spool - print queues – /var/lock - lock files – /var/tmp - temp files needed across reboots var tmp sys usr bin dev home media proc sbin boot etc lib mnt root
  189. 189. File System Types
  190. 190. File System Types Introduction ● Variety of choices available based on – Speed – Size – Reliability – Access restrictions ● Some types are specifically targeted based on the storage media ● The following slide discuss some of the most commonly used types
  191. 191. File System Types Introduction ● RAM File Systems – initrd – initramfs ● Disk File Systems – ext2 – ext3 – ext4 – Flash File Systems ● Intro to MTD ● JFFS2
  192. 192. File System Formats Variants ● Distributed File Systems – NFS ● Special Purpose File Systems – squashfs
  193. 193. File System Types RAM File Systems
  194. 194. File System Types RAM File Systems - initrd ● Abbreviated as initial RAM disk ● RAM based block device with fixed size formatted and mounted like disk at /dev/ram ● The size problem – Sometimes might be a waste of space if its not fully utilized – You can extend the limit later on run time as it has to be reformatted ● Requires a file system driver compiled statically into kernel to interpret data at run time ● ext2 format mostly used for creation
  195. 195. File System Types RAM File Systems - initrd ● Typical boot process using initrd is as followed – Bootloader loads the kernel and initrd into ram – Kernel converts the initrd into a normal RAM disk and frees the memory used by initrd – initrd is mounted as read write as root – /linuxrc is executed (could be a executable, shell script) – linuxrc mounts the real root file system – linuxrc places the root file system at the root directory using the pivot_root system call – The normal boot is done on the root file system (run /sbin/init) – Initrd file system is removed
  196. 196. File System Types RAM File Systems - initramfs ● Abbreviated as initial RAM file system ● Successor of initrd, with an advantage of flexible size which can grow and shrink and does not need a driver ● Gets integrated with kernel image ● cpio archive of initial file system that gets loaded into memory during Linux startup process ● Kernel mounts the file system and starts init ● Faster booting as the it is loaded at boot time, hence applications can start faster
  197. 197. File System Types RAM File Systems - initramfs ● Could be used as intermediate stage before loading real file system ● The path of file system to be included has to be set while kernel configuration ● The kernel build process would take care of integrating it with the kernel image ● Just search INITRAMFS on the configuration menu to know where to set the path ● Just build the kernel once the path is set
  198. 198. File System Types Disk File Systems
  199. 199. File System Types Disk (Block) File System ● Has ability to store data randomly at addresses in a short amount of time ● Multiple users (or processes) access to various data on the disk without regard to the sequential location of the data ● Disk file systems are usually block-oriented. ● Files in a block-oriented file system are sequences of blocks, often featuring fully random-access read, write, and modify operations ● No erasing required before write
  200. 200. File System Types Disk (Block) File System ● Examples – Hard drives, floppys, RAM Disks – Compact Flash, SD Cards ● Though this device are based of flash devices, the integrated controllers take care of the low level handling of data like block devices
  201. 201. File System Types Disk (Block) File System - ext2 ● Abbreviated as Second Extended File System ● Matured and stable GNU/Linux file system ● Does not support journaling support or barriers ● Not recommend as for / and /home partitions since it take long time for file system check ● Most compatible choice for /boot partition ● Recommend for very small partitions (say < 5 MB), because other file system types need more space for metadata ● Used to be the default file system in several Linux distribution
  202. 202. File System Types Disk (Block) File System - ext2 ● ext2 is still preferred choice of flash based storage due the absence of journal
  203. 203. File System Types Disk (Block) File System – ext3 ● Abbreviated as Third Extended File System ● An extension of ext2 with journaling support and write barriers ● Backward compatible with ext2 and extremely stable ● ext2 can be converted to ext3 directly without backup
  204. 204. File System Types Disk (Block) File System – ext4 ● Abbreviated as Fourth Extended File System ● Compatible with both ext2 and ext3 and used as default file system in many distributions ● Supports huge individual file data and overall file system size ● ext4 allows unlimited number of sub directories compared to ext3 ● Has many new features like – Journal checksum – Fast fsck etc. ● Can turn off the Journal feature if required
  205. 205. File System Types Disk (Block) File System - Flash File System ● Flash file system is designed for storing files on flash memory–based storage devices ● They are optimized for the nature and characteristics of flash memory for the following reasons – Erasing blocks – Random access – Wear leveling ● Examples – NAND and NOR Flashes ● In Linux the flash memories use a device file called MTD for interaction
  206. 206. File System Types Flash File System - MTD ● MTD stands form Memory Technology Device ● Created to have abstraction layer between the hardware specific device drivers and higher-level applications ● MTD helps to use the same API with different flash types and technologies ● MTD does not deal with block devices like MMC, eMMC, SD, Compact Flash etc., Since these are no raw flashes but they have Flash Translation layer. The FTL make these devices look like block devices
  207. 207. File System Types Flash File System - MTD
  208. 208. File System Types Flash File System - MTD ● MTD devices can be looked at /proc/mtd ● Two types of drivers created – mtdchar drivers ● Can be seen in /dev as mtd<n> or mtd<n>ro with major number 90 ● Even minor number for rw type and odd minor number for ro type – mtdblock drivers ● Can be seen in /dev as mtdblock<n> allowing rw in block level ● Does not handle bad block check and any wear-leveling
  209. 209. File System Types Flash File System - MTD ● MTD device are usually partitioned ● These partitions can be used fro different purposes like boot code area, kernel image, data partitions etc ● The MTD partitions can be described by – Specifying in board device tree – In kernel code (Hard code) – Through kernel command line (Modification) ● Each partition becomes a separate MTD device
  210. 210. File System Types Flash File System – MTD - Partitions ● Using Device Tree
  211. 211. File System Types Flash File System – JFFS2 ● Abbreviated as Journalling Flash File System 2 ● Standard filesystem for MTD flash ● Compression support for zlib, rubin, rtime and lzo ● Better performance, power down reliability, wear leveling and ECC
  212. 212. File System Types Distributed File Systems
  213. 213. File System Types Distributed File System ● A network file system is a file system that acts as a client for a remote file access protocol, providing access to files on a server. ● Data accessed by all users can be kept on a central host, with clients mounting this directory at boot time ● Data consuming large amounts of disk space may be kept on a single host
  214. 214. File System Types Special Purpose File Systems
  215. 215. File System Types Special Purpose File System ● Sometimes the way we handle data might depend on the requirements ● There some file system types which can offer different features like – Compressions – Backups – Access etc.
  216. 216. File System Types Special Purpose File System - squashfs ● Read-only compressed file system for block devices ● Can be used for read only data like binaries, kernel etc ● Very good compression rate and read access performance ● Mostly used in live CDs and live USB distros ● Can support compressions like – gzip – LZMA – LZO etc
  217. 217. File System Partitions
  218. 218. File System Partitions ● Logically dividing the available storage space into sections that can be accessed independently to one another for – File management – Multiple users – Data Security – Efficiency and reliability etc., ● Entire storage can be created as a single partition if required, But multiple partition can help in number of way like dual or multiboot system, swap etc.
  219. 219. File System Partitions Linux System ● Two major partitions – Data partition – Linux data partition ● A root partition and one pr more data partition – Swap partition - Expansion of physical memory, extra memory on storage (like virtual RAM) ● Rarely used in embedded system because of the nature of the storage devices used
  220. 220. File System Partitions Mount Points ● All partitions are attached to the system via a Mount Point ● A mount point defines the place of a particular data set in a file system ● Usually all the partitions are connected through the root (/) partition ● The / will have empty directories which will be the starting point of partitions ● Some core partitions can be mounted on startup by describing it in /etc/fstab
  221. 221. File System Partitions Schemes – Desktop – Example 1 ● One possible Desktop scheme is shown here ● Simplest and standard one to have ● Partition for OS which get mounted as / (root) ● Data partition mounted as /home ● Augment to RAM, referred and mounted as swap / (root) /home swap HardDrive
  222. 222. File System Partitions Schemes - Desktop – Example 2 ● Similar to previous one with a separate boot partition which will contain the boot images / (root) /home swap /boot HardDrive
  223. 223. File System Partitions Schemes - Embedded – Example 1 ● The partition scheme in embedded systems depends on the system architecture ● This scheme shows two stages of boot loaders each in different memories ● Rest all the sections goes in other memory Bootloader 1 Kernel Image RFS Image Bootloader 2 Memory1Memory2Memory3 Data
  224. 224. File System Partitions Schemes - Embedded – Example 2 ● This scheme shows stage 1 of boot loader in different memory ● Rest all the sections including stage 2 boot loader goes in other memory Bootloader 1 Kernel Image RFS Image Bootloader 2 Memory1Memory2 Data
  225. 225. File System Partitions Schemes - Embedded – Example 3 ● This scheme shows all the contents share in a single memory Bootloader 1 Kernel Image RFS Image Bootloader 2 Memory1 Data
  226. 226. File Systems Building from scratch ● File systems can be created manually but – Would be time consuming – Lots of dependencies – Lots of components have to be integrated like binaries, libraries, scripts, configuration files etc. – Customization for embedded would be challenging – Many more.. ● So BusyBox is an alternative solution
  227. 227. BusyBox Introduction
  228. 228. BusyBox Introduction ● Often called as the Swiss Army Knife of Embedded Linux ● BusyBox combines tiny versions of many common UNIX utilities into a single small executable ● It provides replacements for most of the utilities you usually find in GNU fileutils, shellutils, etc ● Written with size-optimization and limited resources in mind
  229. 229. BusyBox Introduction ● The utilities in BusyBox generally have fewer options than their full-featured GNU cousins; with expected functionality and behave very much like their GNU counterparts ● Provides a fairly complete environment for any small or embedded system ● Extremely modular so you can easily include or exclude commands (or features) at compile time ● This makes it easy to customize your embedded systems ● Sizes less than < 500 KB (statically compiled with uClibc) or less than 1 MB (statically compiled with glibc)
  230. 230. BusyBox Building
  231. 231. BusyBox Building - Configuration ● Download the latest stable sources from http://busybox.net ● You may try the following targets for configuration make defconfig ● Configures all options for regular users. make allnoconfig ● Unselects all options. Good to configure only what you need. ● Linux kernel like configuration, make menuconfig or make xconfig also available
  232. 232. BusyBox Building - Compilation ● BusyBox, by specifying the cross compiler prefix. make CROSS_COMPILE=<cross_compile_path> ● cross_compile_path could be the command itself if already exported in PATH variable, else you can specify the installation path of command ● For example for arm platform it would look like make CROSS_COMPILE=arm-linux- ● The cross compiler prefix can be set in configuration interface if required BusyBox Setting Build Option Cross Compiler prefix→ →
  233. 233. BusyBox Installation ● To install BusyBox just type make install ● The default installation path would the current directory, will should see _install directory ● The installation path can be customized in configuration if required as mentioned below BusyBox Setting Installation Option BusyBox installation→ → prefixs ● The installation directory will contain Linux like directory structure with symbolic links to busybox executable
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  235. 235. Thank You

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