To set up build environment for Android kernel development

setup_android_build.sh

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# Download Android NDK to be able to compile runtime cmd-line tools on ARM
# - https://developer.android.com/ndk/downloads/index.html
wget https://dl.google.com/android/repository/android-ndk-r15b-linux-x86_64.zip

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# Download up-to-date adb/fastboot platform tools to interface with Android (adb/fastboot)
# - https://developer.android.com/studio/releases/platform-tools.html
wget https://dl.google.com/android/repository/platform-tools-latest-linux.zip

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# Download prebuilt gcc toolchain for kernel compilation
# Pick the correct architecture: ARMv7/arm, ARMv8/aarch64
# - https://android.googlesource.com/platform/prebuilts
# - for Nexus 6, use arm-eabi-4.8 instead instead of arm-linux-androideabi-
git clone https://android.googlesource.com/platform/prebuilts/gcc/linux-x86/arm/arm-eabi-4.8
git clone https://android.googlesource.com/platform/prebuilts/gcc/linux-x86/aarch64/aarch64-linux-android-4.9

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# Set environment settings for cross-compilation
# - ARMv7/arm
export PATH=/home/user/tools/arm-eabi-4.8/bin:$PATH
export ARCH=arm
export SUBARCH=arm
export CROSS_COMPILE=arm-eabi-
# - ARMv8/aarch64 
export PATH=/home/user/tools/aarch64-linux-android-4.9/bin:$PATH
export ARCH=arm64
export CROSS_COMPILE=aarch64-linux-android-

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# Download kernel (Nexus 6)
# - https://android.googlesource.com/kernel/msm/
git clone https://android.googlesource.com/kernel/msm.git
git checkout android-msm-shamu-3.10-marshmallow-mr2

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# Compile a custom kernel image that can load kernel modules
make shamu_defconfig
make menuconfig
# - enable loadable kernel modules
make -j4
# - image is found at msm/arch/arm/boot/zImage-dtb

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# Extract RAM disk and build boot image
# - Kernel is compiled to a zImage which needs to be combined with a ramdisk to create a boot image.
# - We do that by extracting the raw boot partition dump from the device, and then extract the ramdisk.
sudo apt install abootimg
# - log into device via adb and view partition layout
adb shell
cat /proc/partitions
# - look for the boot partition and extract it (boot -> /dev/block/mmcblk0p14)
ls /dev/block/platform/msm_sdcc.1/by-name/
dd if=/dev/block/platform/msm_sdcc.1/by-name/boot of=cur-boot.img
# - extract the ramdisk from raw boot image dump
# - we need 2 files: bootimg.cfg, initrd.img
abootimg -x cur-boot.img
# - build the new boot image by combining our zImage kernel image with the above 2 files
abootimg --create myboot.img -f bootimg.cfg -k zImage -r initrd.img

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# Boot new image (without flashing it, it is not permanent)
adb reboot bootloader
fastboot boot myboot.img


#======================================================================================================
# Custom kernel module
make CFLAGS_MODULE=-fno-pic ARCH=arm CONFIG_HELLOWORLD=m M=drivers/helloworld
adb -s $DEVICE_ID push drivers/helloworld/helloworld.ko /data/local/tmp
adb -s $DEVICE_ID shell su -c "cat /proc/kmsg" &
adb -s $DEVICE_ID shell su -c "insmod /data/local/tmp/helloworld.ko"


#======================================================================================================
# Debug commands for Nexus 6 (shamu)

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# Leave only CPU1 and CPU2 online at maximum frequency
cd /data/local/tmp
stop thermal-engine
stop mpdecision
echo 1 > /sys/devices/system/cpu/cpu1/online
echo 1 > /sys/devices/system/cpu/cpu2/online
echo 0 > /sys/devices/system/cpu/cpu3/online
echo userspace > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
echo userspace > /sys/devices/system/cpu/cpu1/cpufreq/scaling_governor
echo userspace > /sys/devices/system/cpu/cpu2/cpufreq/scaling_governor
echo 2649600 > /sys/devices/system/cpu/cpu0/cpufreq/scaling_setspeed
echo 2649600 > /sys/devices/system/cpu/cpu1/cpufreq/scaling_setspeed
echo 2649600 > /sys/devices/system/cpu/cpu2/cpufreq/scaling_setspeed

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# Diagnostic debug logs on frequency, voltage and temperature
cat /d/clk/krait0_clk/measure
cat /d/regulator/krait0/voltage
cat /d/clk/l2_clk/measure
cat /sys/devices/virtual/thermal/thermal_zone0/temp