OpenWRT Build System for CentOS 7

How does one create a "build system" (AKA Cross Compile / Cross-Compile / CrossCompile Build System) for OpenWRT?

Other Tutorials

• General Guide for building firmware: https://openwrt.org/docs/guide-user/additional-software/beginners-build-guide
  • Start of detailed Information: https://openwrt.org/docs/guide-developer/build-system/start
  • Pre-requisites for a : https://openwrt.org/docs/guide-developer/build-system/install-buildsystem
  • Some additional granular detail for working with specific versions: https://openwrt.org/docs/guide-developer/build-system/use-buildsystem
• Build environment tips: https://openwrt.org/docs/guide-developer/env
• SDK (for compiling specific packages): https://openwrt.org/docs/guide-developer/using_the_sdk
  • URL for SDK (look towards the bottom of the page for SDK): https://downloads.openwrt.org/releases/19.07.3/targets/mvebu/cortexa9/
• Third Party Site with nice information (Old, but this article itself notes even older OpenWRT documentation): http://www.acme-dot.com/building-openwrt-14-07-barrier-breaker-on-ubuntu-and-os-x/ (Don't follow the instructions as it GIT won't work anymore with non-SSL URLs)
• Entware Tips (The don't apply directly, but have some good tips if read from the context of OpenWRT)
  • Native Compilation: https://github.com/Entware/Entware-ng/wiki/Using-gcc-%28native-compilation%29
  • Compiling Packages from Source: https://github.com/Entware/Entware-ng/wiki/Compile-packages-from-sources
  • Creating Perl modules: https://github.com/Entware/Entware-ng/wiki/Self-installation-of-perl-modules

Environment

• Build System for Cross-Compiling: CentOS 7.8 (There seem to be some missing packages in CentOS 8, so CentOS 7.8 was chosen)
• OpenWRT 19.07.03 (Linux Kernel 4.14.180)
• Linksys WRT3200ACM Platform Information (see here for additional information)
  • Marvell Armada 385 88F6820 SoS (System on a Chip)
  • Manufacturer: Marvell, MVEBU (Marvell Embedded Business Unit)
  • CPU Information
    • ARM Family: Cortex-A (32-bit)
    • ARM Architecture: ARMv7-A
    • ARM Core: Cortex-A9
    • OpenWRT Target and Package Details
      • MVEBU (Marvell Embedded Business Unit)
        • Targets Available: cortexa53, cortexa72, cortexa9
        • Marvell Armada 37x/38x/XP (Armada 3700LP and Armada 7k/8k are 64 bit versions)
    • Equivalent DD-WRT / Entware Target and Package reference
      • ARMv7sf-k3.2: ARM version 7 with linux kernel 3.2
  • Sources:
    • https://en.wikipedia.org/wiki/List_of_ARM_microarchitectures
    • https://en.wikipedia.org/wiki/ARM_Cortex-A9
    • http://bin.entware.net/mipselsf-k3.4/

Steps (as of 9.2020 for the above Environment)

All of the below assumes one is storing files in the user's home directory.

OpenWRT recommends NOT compiling as a root users. To bypass this restriction;

• export FORCE_UNSAFE_CONFIGURE=1 (the export command adds the variable to the current user session, it will not persist if logging off, confirm with printenv or export -p)

Obtain Source Files from OpenWRT

• git clone -b openwrt-19.07 https://github.com/openwrt/openwrt.git (downloads the 19.07 branch of OpenWRT, which also includes the latest version, 19.07.03)
  • To see repository, in web browser: https://git.openwrt.org/openwrt/
  • git tag (essentially shows available versions to build)
  • git branch (shows local versions of OpenWRT downloaded)
  • git branch -r (shows versions available for download)
• Feeds
  • Documentation: https://openwrt.org/docs/guide-developer/feeds
  • ~/openwrt/feeds.conf.default
  • ./scripts/feeds update -a (A script from OpenWRT to makes sure all files are up to date)
  • ./scripts/feeds (by itself, no "sub-commands", shows available options)
  • ./scripts/feeds list -n (shows main categories of feeds)
    • Should show these Names: packages, luci, routing, and telephony
  • ./scripts/feeds list -r OneOfTheFeedNames (shows the available packages, names are equivalent to what would be found with opkg list)
  • Example: ./scripts/feeds install htop (an enhanced version of top)
    • Files will be downloaded here: ~/openwrt/package/feeds/packages/ and ~/openwrt/feeds/packages/admin
• Checkout (not needed if a single branch like 19.07 is downloaded)
  • git checkout openwrt-19.07 (if multiple branches have been downloaded, switch to using this one)
  • Oddly, this command is documented using the latest version of OpenWRT as of 9.2020, but it doesn't work as expected: git checkout v19.07.03 (from https://openwrt.org/docs/guide-developer/build-system/use-buildsystem)
• SDK
  • Download the SDK and uncompress it (which is equivalent to the git clone command above)
    • URL for SDK (look towards the bottom of the page for SDK): https://downloads.openwrt.org/releases/19.07.3/targets/mvebu/cortexa9/
    • Don't worry about the name ending in x86_64 as this indicates the platform that the cross compiling will take place on.
  • tar xf WhatEverFileName (be sure one is in the ~ directory)
  • ./scripts/feeds update -a
  • ./scripts/feeds install WhatEverPackageName
• Package Building
  • Sources
    • https://openwrt.org/docs/guide-developer/package-policies
    • https://openwrt.org/docs/guide-developer/packages
  • Using the full TAR package as an example
    • make menuconfig
    • disable all the Global settings to "build all"
    • Utilities (look in the Makefile file in the define Package/tarcustom (or WhatEverName) for where the package will be)
  • Copy the ~/openwrt/package/feeds/packages/tar/Makefile
  • Edit the PKG_SOURCE
• Headers
  • Potential Locations;
    • ~/openwrt/build_dir/target-arm_cortex-a9+vfpv3-d16_musl_eabi/linux-mvebu_cortexa9/linux-4.14.195/user_headers/include
  • Sources
    • https://forum.openwrt.org/t/how-to-obtain-headers-for-openwrts-kernel/50737/6
    • https://openwrt.org/docs/guide-developer/packages