# github: torvalds
git clone -b v2.6.39 https://github.com/torvalds/linux.git

# git.kernel.org
git clone -b v2.6.39.4 git:https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git

# tgz: kernel.org
curl -O https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/snapshot/linux-2.6.39.4.tar.gz
tar xzf linux-2.6.39.4.tar.gz

make help

sudo apt install universal-ctags
sudo apt install cscope
sudo apt install global

make TAGS
make cscope
make gtags

├── [3.1M]  GPATH
├── [196M]  GRTAGS
├── [ 63M]  GTAGS
├── [ 82M]  TAGS
├── [542K]  cscope.files
├── [232M]  cscope.out
├── [ 46M]  cscope.out.in
├── [335M]  cscope.out.po