Session Pre-requisites

sudo apt install libncurses5-dev flex bison # for menuconfig tool.
sudo apt install subversion libssl-dev
sudo apt-get install qemu-system
sudo apt-get install device-tree-compiler

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Session Goals:

1. Download/Choose Kernel.
2. Understand kernel versioning system [stable-mainline-long-term-support]
3. Understand kernel source code tree.
4. Understand Device tree.
5. Understand menuconfig/kconfig/kbuild.
6. Compiling Kernel for raspi3/4
7. Identify Kernel size.
8. Compiling device tree.
9. Booting kernel on Qemu.

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1. Actions to Build Kernel

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• Choose Kernel Version.
• Downloading Source code.
• Understand Kernel config.
• Identify Kernel Version.
• Compiling kernel for specific target.
• Build artifcats [Targets]:
  • VMLinux: elf version.
  • zImage: compressed version (u-boot understand it through bootz command)
  • uImage: for uBoot.

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1. Download/Choose Kernel.

a. from vendor

source: https://www.raspberrypi.com/documentation/computers/linux_kernel.html\

git clone --depth=1 -b rpi-4.19.y https://github.com/raspberrypi/linux.git # raspberrypi linux version.
[Kernel.org](https://www.kernel.org/) # Main branches [Kernel Archives]

b. from kernel main branch.

1. [Kernel.org](https://www.kernel.org/) # Main branches [Kernel Archives]
2. wget <version link from kernel.org>
3. tar xf <compressed filename>

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2. Understand Kernel source code structure.

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3. Understanding Kernel config mechanism.

1. menuconfig ---> read KConfig ---> output .config

make ARCH=<target> menuconfig

# choose configs
make ARCH=<target> <filename>

3. Kbuild ---> read .config ---> add parameters inside code as Macros.

4. build :)

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4. Identify Kernel Version.

make ARCH=<target> kernelrelease 

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5. Compiling kernel for specific target.

0. Most Important Targets:

a. VMLinux b. zImage c. uImage. d. distclean

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2. Most Used make targets.

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a. distclean

make ARCH=<target> distclean

b. vmLinux: elf

make -j 4 ARCH=<target> CROSS_COMPILE=`prefix`

# output
> main directory for Linux.

c. zImage: compressed.

make -j4 ARCH=<target> CROSS_COMPILE=`prefix` Image

# output
<linux_directory>/arch/<target>/boot

d. uImage: zImage + uBoot header.

make -j4 ARCH=<target> CROSS_COMPILE=`prefix` LOADADDR=0x80008000 uImage

# output
<linux_directory>/arch/<target>/boot

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6. Compiling Device tree.

a. Find device tree for target. Device tree for Raspi3

b. compile device tree.

dtc <name>.dts -o <name>.dtb # convert it into binary.

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7. Booting kernel on Qemu

qemu-system-aarch64 \
    -M raspi3b \
    -cpu cortex-a72 \
    -append "rw earlyprintk loglevel=8 console=ttyAMA0,115200" \
    -kernel <Image> \
    -dtb <dtb>.dtb \
    -m 1G -smp 4 \
    -serial stdio \
    -usb -device usb-mouse -device usb-kbd \
        -device usb-net,netdev=net0 \
        -netdev user,id=net0,hostfwd=tcp::5555-:22