#!/usr/bin/env bash
# Copyright (C) 2024-Ω Miquel Sabaté Solà <mikisabate@gmail.com>
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.

#####
# Build the Linux Kernel and bundle it into a tarball that can then be used on
# another machine. After you call this script it will:
#   1. Either call `defconfig` or re-use an existing .config as per user request.
#   2. Call `menuconfig` so the user can double check the configuration.
#   3. Build the Kernel.
#   4. Copy into a tarball the resulting image, modules, dtbs, and headers. An
#      install.sh script is also bundled in.
#
# After that just send the resulting tarball to the machine where you want to
# install this kernel. In there you just need to untar the given bundle and then
# run the `install.sh` script from within the resulting directory. This will:
#   1. Copy recursively all the files into their proper paths.
#   2. Make an initramfs for the new image.
#   3. Run your $EDITOR to update the /boot/extlinux/extlinux.conf file.
#
# *NOTE*: this script expects the `ARCH` environment variable to be set, and if
# that doesn't match the current architecture, then it will expect to have
# `CROSS_COMPILE` set as well.
# *NOTE*: because of the funny machines I'm testing this in, only Debian was
# tested. Hence, we assume tools such as `update-initramfs` exist instead of
# using `dracut`.
#####

set -e

##
# Initial checks.

# Most of the times, we are safer if we just set the `ARCH` env. variable.
if [ -z "$ARCH" ]; then
    echo "kbuild (error): \$ARCH has to be defined."
    exit 1
fi

# Is cross compilation the thing we want?
if [ "$ARCH" != "$(uname -m)" ]; then
    # Confusingly enough, riscv64 is supposed to be passed as 'riscv' to the
    # kernel. Hence, don't take that as being different.
    if [ "$(uname -m)" != "riscv64" ] || [ "$ARCH" != "riscv" ]; then
        # We are indeed cross compiling. If `CROSS_COMPILE` is not set at this
        # point we should raise an error.
        if [ -z "$CROSS_COMPILE" ]; then
            echo "kbuild (error): \$CROSS_COMPILE has to be defined."
            exit 1
        fi
    fi
fi

# To avoid embarrassing moments, let's make sure that we are in the Linux repo.
if [ ! -f "Kbuild" ]; then
    echo "kbuild (error): we are not in the Linux Kernel."
    exit 1
fi

##
# .config

if [ ! -f ".config" ]; then
    echo "kbuild (info): no '.config' file found"
    make defconfig
fi

config_options=("From a default configuration (defconfig)" "From an existing .config file")
echo "kconfig (info): how do you want to configure the build?"
PS3="> "
select _opt in "${config_options[@]}" "Quit"; do
    case "$REPLY" in
    1)
        make defconfig
        echo "kbuild (info): running 'menuconfig' to double check."
        make menuconfig
        break
        ;;
    2)
        echo "kbuild (info): running 'menuconfig' to double check."
        make menuconfig
        break
        ;;
    3)
        echo "kbuild (info): bye!"
        exit 0
        ;;
    *)
        echo "kbuild (info): not an option. Try again..."
        continue
        ;;
    esac
done

##
# Build the kernel.

echo "kbuild (info): building the Linux kernel..."
make -j "$(nproc)"

##
# Installing into a local bundle.

name=$(cat include/config/kernel.release)

if [ -z "$INSTALL_PATH" ]; then
    INSTALL_PATH="buildroot-$name"
fi
mkdir -p "$INSTALL_PATH/usr/lib/modules/$name"
pushd "$INSTALL_PATH" >/dev/null
ln -s usr/lib lib
popd >/dev/null

make modules_install INSTALL_MOD_PATH="$INSTALL_PATH"
make dtbs_install INSTALL_DTBS_PATH="$INSTALL_PATH/boot/dtbs/$name"
make headers_install INSTALL_HDR_PATH="$INSTALL_PATH/usr"

unlink "$INSTALL_PATH/lib"

cp "arch/$ARCH/boot/Image.gz" "$INSTALL_PATH/boot/vmlinuz-$name"
cp .config "$INSTALL_PATH/boot/config-$name"

##
# Setup install.sh script.

cat <<SCRIPT > "$INSTALL_PATH/install.sh"
#!/usr/bin/env bash

set -ex

if [ "\$EUID" -ne 0 ];then
   echo "kbuild (error): run this as root."
   exit 1
fi

if [ ! -f /sbin/update-initramfs ] && [ ! -f /usr/bin/dracut ]; then
   echo "kbuild (error): you need either 'dracut' or 'update-initramfs'."
   exit 1
fi

cp -rv usr /
cp -rv boot /

export PATH=/sbin:\$PATH
if command -v dracut &> /dev/null
then
    dracut --kver=$name -f
else
    update-initramfs -k $name -u
fi

if [ -z "\$EDITOR" ]; then
   EDITOR=vi
fi
\$EDITOR /boot/extlinux/extlinux.conf
SCRIPT

##
# Create tarball.

tar czf "$INSTALL_PATH.tar.gz" "$INSTALL_PATH"
rm -r "$INSTALL_PATH"

echo "kbuild (info): your build is now ready at '$INSTALL_PATH.tar.gz'."