Create a bootable image (boot.img) (legacy)

For legacy devices where boot.img contains ramdisk, you'll have to extract ramdisk from a prebuilt image itself and then append to ours.

If you don't have existing boot.img, you can download the prebuilt boot.img matching your kernel from KernelSU releases or Android.com

It is STRONGLY recommended to download boot.img which is known to work on your devices, or else it'll be unbootable. You can test them by using fastboot boot <boot.img> to see if it boots on your device.

Refer to official tutorial for more information: https://source.android.com/docs/setup/build/building-kernels#id-bootimage-no-init-boot

Clone the mkbootimg source

git clone https://android.googlesource.com/platform/system/tools/mkbootimg/ --depth 1
cd mkbootimg

Unpack the working boot.img

Place the working boot.img in the mkbootimg directory so we can work easier

./unpack_bootimg.py --boot_img boot.img --out boot

The unpacked boot image will be in boot/ directory, also upon unpacking you should see something like this:

boot magic: ANDROID!
kernel_size: 44493028
ramdisk size: 2713820
os version: 15.0.0
os patch level: 2025-06
boot image header version: 4
command line args: 
boot.img signature size: 0

Remember the details because you'll need them later (except the kernel & ramdisk size)

Create our own boot image

Place the Image to the current directory, and now execute this command to create a new boot image

./mkbootimg.py --kernel Image --ramdisk boot/ramdisk --os_version <os version> --os_patch_level <os patch level> --header_version 4 --cmdline <command line args> --out my_boot.img

Fill the missing fields with the information above, and if command line args is empty then you can ignore the argument. The new boot image will be named my_boot.img in the current working directory.

Congrats on your custom kernel :D, now you're ready to flash them on your devices!