add post: buildroot setup

content/blog/buildroot-setup.md

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++++
+title = "Set-up a cross-compile environment for buildroot Linux system running on Orange PI Zero 3"
+date = 2023-12-19 00:27:00
+draft = false
+
+[taxonomies]
+categories = ["embedded-linux"]
+tags = ["embedded-linux", "linux", "buildroot"]
+
+[extra]
+lang = "en"
+toc = true
+comment = false
+copy = true
+math = false
+mermaid = false
+outdate_alert = false
+outdate_alert_days = 120
+display_tags = true
+truncate_summary = false
++++
+
+Assuming host system Ubuntu 22.04. Install the pre-requisite packages needed for this lab:
+
+```
+sudo apt install build-essential git autoconf bison flex texinfo help2man gawk libtool-bin libncurses5-dev unzip
+```
+
+### Setting up the TFTP server
+
+Let’s install a TFTP server on your development workstation:
+
+```
+sudo apt install tftpd-hpa
+```
+
+Then edit the file `/etc/default/tftpd-hpa` to change this setting:
+
+```
+TFTP_DIRECTORY="/srv/tftp"
+```
+
+Now restart the service:
+
+```
+systemctl restart tftpd-hpa
+```
+
+## Grab and compile buildroot
+
+```
+git clone https://git.buildroot.net/buildroot
+cd buildroot
+```
+
+Load the default config for Orange PI Zero 3. And build it. Note that we shouldn't change the kernel version as it's been assigned to a custom version compatible with Orange PI.
+
+```
+make orangepi_zero3_defconfig
+make
+```
+
+Once the build process is finished you will have an image called "sdcard.img" in the output/images/ directory. Flash this image to your SD card and boot.
+
+On login prompt enter `root` and no password. TA-DA - and you're in your very own custom linux system!
+
+For more details see *Using a build system, example with Buildroot* chapter in the slides [bootlin embedded linux qemu slides](https://bootlin.com/doc/training/embedded-linux-qemu/embedded-linux-qemu-labs.pdf).
+
+## Compiling a kernel module
+
+Now we'll setup the cross-compile variables.
+
+Include the following in your PATH:
+```
+export PATH="$PATH:~/buildroot/output/host/bin"
+```
+And set the following variables:
+```
+export ARCH=arm64
+export CROSS_COMPILE=aarch64-linux-
+```
+
+Make a new directory somewhere in your home folder called `hello`. Inside it create a C file `hello.c`:
+
+```C
+#include <linux/module.h>    /* Needed by all modules */
+#include <linux/kernel.h>    /* Needed for KERN_INFO */
+
+int init_module(void){
+    printk(KERN_INFO "Hello World!\n");
+
+    return 0;
+}
+
+void cleanup_module(void){
+    printk(KERN_INFO "Goodbye World.\n");
+}
+
+MODULE_LICENSE("GPL");
+
+```
+
+Create `Makefile`:
+
+```
+CC = $(CROSS_COMPILE)gcc
+
+obj-m := hello.o
+KDIR := ~/buildroot/output/build/linux-custom
+
+all:
+        $(MAKE) -C $(KDIR) M=$(PWD)
+
+```
+
+Noting to adjust the `KDIR` according to where you have buildroot. Assuming Homefolder here.
+
+Now run `make`. You'll get a bunch of files as result. We are interested in `hello.ko` file.
+
+### Transfer the compiled module
+
+We need to transfer this file to our board. We'll use the tftp service we setup earlier.
+
+First copy the `hello.ko` file to `/srv/tftp` directory. Then in the board's prompt:
+
+```
+cd /root
+tftp -gr hello.ko <server-ip>
+```
+
+Once successfully copied, clear the system log, and load the module:
+
+```
+sudo dmesg -c
+insmod hello.ko
+```
+
+Now check for the message, running `dmesg` again. To remove this module, simply `rmmod hello.ko`. Check the output again and you'll see Goodbye world.
+
+This tests that we have a successful cross-compiler environment working to compile and study device driver development on Orange PI Zero 3 running Linux!