Download the kernel source of android

git clone https://android.googlesource.com/kernel/msm
look out git branch -a how many branches we can download.
Find the latest branch

git checkout -b remotes/origin/android-msm-wahoo-4.4-pie

If you have error of this:

execute command repo, then has information below:

root@retool-x:/media/sf_source_code/kernel_pie/bin# ./repo
./repo: line 1: syntax error near unexpected token `newline'
./repo: line 1: `'

You must install command repo through apt install repo instead of curl**>bin/repo.