basic configuration and documentation

Author: drorgl

README.md

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+# Buildroot for TinyEMU
+
+This repo is a quick getting started guide on how to build kernel and rootfs using Buildroot for TinyEMU
+
+The current configuration is a minimal kernel with no networking, though it should be easy to change.
+
+If you're not ready to build your own, you can start get a ready made kernel, rootfs and bbl from [TinyEMU](https://bellard.org/tinyemu/), at the bottom there's [diskimage-linux-riscv-2018-09-23.tar.gz](https://bellard.org/tinyemu/diskimage-linux-riscv-2018-09-23.tar.gz).
+
+# BBL - Berkely boot loader
+To build BBL, you'll find the patches needed for [RISC-V Proxy Kernel](https://github.com/riscv-software-src/riscv-pk) in [diskimage-linux-riscv-2018-09-23.tar.gz](https://bellard.org/tinyemu/diskimage-linux-riscv-2018-09-23.tar.gz).
+
+# Getting Started
+
+## Building the docker image
+```
+docker build -t buildroot .
+```
+
+## Running the docker image
+```
+docker run -it buildroot
+```
+
+At this point you have a ready made machine to start building your rootfs and kernel:
+```
+make -j 5
+```
+
+Once its done, you'll find 2 files in output/images:
+```
+~/buildroot-2022.05# ls -l output/images
+total 10252
+-rw-r--r-- 1 root root  2172016 Jul 12 11:36 Image
+-rw-r--r-- 1 root root 20971520 Jul 12 11:36 rootfs.ext2
+```
+
+Now we want to copy them into our virtual riscv machine:
+```
+docker cp <container id>:/root/buildroot-2022.05/output/images/Image kernel32.bin
+docker cp <container id>:/root/buildroot-2022.05/output/images/rootfs.ext2 rootfs32.bin
+```
+
+Make sure you have bbl32.bin and TinyEMU configuration file:
+```
+/* VM configuration file */
+{
+    version: 1,
+    machine: "riscv32",
+    memory_size: 12,
+    bios: "bbl32.bin",
+    kernel: "kernel32.bin",
+    cmdline: "console=hvc0 debug ignore_loglevel earlycon=sbi root=/dev/vda rw",
+    drive0: { file: "rootfs32.bin" }
+}
+```
+
+And you're ready to go, either copy it to your SD card under /emu or run tinyemu from the command line
+```
+<tinyemu executable> riscv32.cfg
+```
+
+If you choose to run it on ESP32 through my [esp32-tinyemu](https://github.com/drorgl/esp32-tinyemu), you'll see something very similar to this:
+[![IMAGE ALT TEXT HERE](https://img.youtube.com/vi/f3a3xeTRj_A/0.jpg)](https://www.youtube.com/watch?v=f3a3xeTRj_A)
+
+# Quick Cheat Sheet for Buildroot
+configure buildroot:
+```
+make menuconfig
+```
+
+configure busybox:
+```
+make busybox-menuconfig
+```
+
+configure kernel:
+```
+make linux-menuconfig
+```
+
+rebuild whole system:
+```
+make clean all -j 5
+```
+
+persist your buildroot configuration:
+```
+make savedefconfig
+docker cp <container id>:/root/buildroot-2022.05/buildroot_rv32_config buildroot_config/buildroot_rv32_config
+```
+
+persist your kernel configuration:
+```
+make linux-update-defconfig
+docker cp <container id>:/root/buildroot-2022.05/linux_rv32_config buildroot_config/linux_rv32_config
+```
+
+persist your busybox configuration:
+```
+make busybox-update-config
+docker cp <container id>:/root/buildroot-2022.05/busybox_config buildroot_config/busybox_config
+```
+
+# Notes
+* This docker image was customized to use axel to download each dependency with multiple connections, you might not need it on your network.
+* The configured overlay skips init and goes straight to bash, you'll need to remove it to work with getty (BR2_ROOTFS_OVERLAY and BR2_TARGET_GENERIC_GETTY_OPTIONS)

buildroot_config/buildroot_rv32_config

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+BR2_riscv=y
+BR2_RISCV_32=y
+BR2_WGET="axel -n 10  "
+BR2_TOOLCHAIN_EXTERNAL=y
+BR2_TARGET_GENERIC_GETTY_PORT="hvc0"
+BR2_TARGET_GENERIC_GETTY_OPTIONS="-n -L -l /bin/sh"
+BR2_ROOTFS_OVERLAY="rootfs_overlay"
+BR2_ROOTFS_POST_IMAGE_SCRIPT="board/qemu/post-image.sh"
+BR2_ROOTFS_POST_SCRIPT_ARGS="$(BR2_DEFCONFIG)"
+BR2_LINUX_KERNEL=y
+BR2_LINUX_KERNEL_CUSTOM_VERSION=y
+BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="5.18.7"
+BR2_LINUX_KERNEL_USE_CUSTOM_CONFIG=y
+BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE="linux_rv32_config"
+BR2_PACKAGE_BUSYBOX_CONFIG="busybox_config"
+# BR2_PACKAGE_IFUPDOWN_SCRIPTS is not set
+# BR2_PACKAGE_URANDOM_SCRIPTS is not set
+BR2_PACKAGE_NANO=y
+BR2_TARGET_ROOTFS_EXT2=y
+BR2_TARGET_ROOTFS_EXT2_SIZE="20M"
+# BR2_TARGET_ROOTFS_TAR is not set
+BR2_TARGET_OPENSBI=y
+BR2_TARGET_OPENSBI_PLAT="generic"
+# BR2_TARGET_OPENSBI_INSTALL_DYNAMIC_IMG is not set
+# BR2_TARGET_OPENSBI_INSTALL_JUMP_IMG is not set
+BR2_PACKAGE_HOST_MKPASSWD=y