Embedded Linux Kernel - Build your custom kernel

Team Emertxe
Embedded Linux Kernel

Contents
● General Information
● Configuration
● Build
● Deploy

General Information

General Information
● Where to get?
● Kernel Subsystem
● Source Code Browsing

General Information – Where to get?
Note: Snapshot of www.kernel.org. Expect changes on updates

General Information – Kernel Subsystem
●
Process Scheduler:
– To provide control, fair access
of CPU to process, while
interacting with HW on time
●
Memory Manager:
– To access system memory
securely and efficiently by
multiple processes. Supports
Virtual Memory in case of
huge memory requirement
●
Virtual File System:
– Abstracts the details of the
variety of hardware devices
by presenting a common file
interface to all devices

General Information – Kernel Subsystem
● Network Interface:
– provides access to several
networking standards and a
variety of network hardware
● Inter Process
Communications:
– supports several
mechanisms for process-to-
process communication on a
single Linux system

General Information – Source Code Browsing
● Untar the Linux kernel code
– tar xvf linux-<version>.<compression_format>
● Enter the Linux kernel directory
– cd linux-<version>
● The following slide discuss the contents of the Linux
directory

● The left side of the slide shows
the source content of the Linux
kernel
● The directory structure might
vary depending on the picked
version.
● Lets us discuss some important
directories and files
init
arch
block
crypto
drivers
firmware
scripts
security
ipc
kernel
lib
mm
net
sound
tools
usr
Makefile
fs samples
include
linux-<version>

● Architecture specific kernel
code
● Has sub directories per
supported architecture
● Example:
– arm
– powerpc
– X86
● We can also find low level
memory management, interrupt
handling, early inits, assembly
code and much moreinit
arch
block
crypto
drivers
firmware
scripts
security
ipc
kernel
lib
mm
net
sound
tools
usr
Makefile
fs samples
include
linux-<version>

● Contains core block layer files
init
block
crypto
drivers
firmware
scripts
security
ipc
kernel
lib
mm
net
sound
tools
usr
Makefile
fs samples
include
linux-<version>
arch

● Cryptographic API for use by
kernel itself
init
block
crypto
drivers
firmware
scripts
security
ipc
kernel
lib
mm
net
sound
tools
usr
Makefile
fs samples
include
linux-<version>
arch

● Contains system's device drivers
● Sub directories contain classes
of device drivers like video
drivers, network card drives,
low level SCSI drivers etc.,
init
block
crypto
drivers
firmware
scripts
security
ipc
kernel
lib
mm
net
sound
tools
usr
Makefile
fs samples
include
linux-<version>
arch

● Contains the device firmwares
which will be uploaded to
devices with help of drivers
init
block
crypto
drivers
firmware
scripts
security
ipc
kernel
lib
mm
net
sound
tools
usr
Makefile
fs samples
include
linux-<version>
arch

● File system related code
● Contains both generic file
system code (VFS) and different
files system code
● Sub directories of supported file
system
● Examples:
– ext2
– ext3
– fat
init
block
crypto
drivers
firmware
scripts
security
ipc
kernel
lib
mm
net
sound
tools
usr
Makefile
fs samples
include
linux-<version>
arch

● Most of the header files used in
the .c file of the kernel source
● It has further sub directories
including asm-generic
● Architecture specific header
file would be found in
arch/<arch>/include/
init
block
crypto
drivers
firmware
scripts
security
ipc
kernel
lib
mm
net
sound
tools
usr
Makefile
fs samples
include
linux-<version>
arch
Note: File level organization will vary based
on different versions of kernel sources
especially architecture and machine related
header files

● Initialization code for kernel
● Best directory to start with to
know on how kernel works
● Has main.c of kernel
init
block
crypto
drivers
firmware
scripts
security
ipc
kernel
lib
mm
net
sound
tools
usr
Makefile
fs samples
include
linux-<version>
arch

● Contains kernel's inter process
communication code like shared
memory, semaphores and other
forms
init
block
crypto
drivers
firmware
scripts
security
ipc
kernel
lib
mm
net
sound
tools
usr
Makefile
fs samples
include
linux-<version>
arch

● Generic kernel level code which
can't fit anywhere else
● Contain upper level codes for
signal handling, scheduling
etc.,
● The architecture specific kernel
code will be in
arch/<arch_name>/kernel
init
block
crypto
drivers
firmware
scripts
security
ipc
kernel
lib
mm
net
sound
tools
usr
Makefile
fs samples
include
linux-<version>
arch

● Contains kernel's library code
● Common string operations,
code for debugging and
command line parsing code can
be found here
● The architecture specific library
code will be in
arch/<arch_name>/lib
init
block
crypto
drivers
firmware
scripts
security
ipc
kernel
lib
mm
net
sound
tools
usr
Makefile
fs samples
include
linux-<version>
arch

● Contains memory management
code
● The architecture specific
memory management code
would be found in
arch/<arch_name>/mm
● Example:
– arch/x86/mm/init.c
init
block
crypto
drivers
firmware
scripts
security
ipc
kernel
lib
mm
net
sound
tools
usr
Makefile
fs samples
include
linux-<version>
arch

● The kernels networking code
init
block
crypto
drivers
firmware
scripts
security
ipc
kernel
lib
mm
net
sound
tools
usr
Makefile
fs samples
include
linux-<version>
arch

● Some sample programs
init
block
crypto
drivers
firmware
scripts
security
ipc
kernel
lib
mm
net
sound
tools
usr
Makefile
fs samples
include
linux-<version>
arch

● Contains scripts that are used
while kernel configuration
init
block
crypto
drivers
firmware
scripts
security
ipc
kernel
lib
mm
net
sound
tools
usr
Makefile
fs samples
include
linux-<version>
arch

● Contains code for different
security models
init
block
crypto
drivers
firmware
scripts
security
ipc
kernel
lib
mm
net
sound
tools
usr
Makefile
fs samples
include
linux-<version>
arch

● Contains all the sound card
drivers
init
block
crypto
drivers
firmware
scripts
security
ipc
kernel
lib
mm
net
sound
tools
usr
Makefile
fs samples
include
linux-<version>
arch

● Certain configuration and
testing tools
init
block
crypto
drivers
firmware
scripts
security
ipc
kernel
lib
mm
net
sound
tools
usr
Makefile
fs samples
include
linux-<version>
arch

● Contains code that builds a
cpio-format archive containing
a root file system image, which
will be used for early userspace
init
block
crypto
drivers
firmware
scripts
security
ipc
kernel
lib
mm
net
sound
tools
usr
Makefile
fs samples
include
linux-<version>
arch

● This is top level Makefile for
the whole source tree
● Contains useful rules and
variables like default gcc
compilation flags
init
block
crypto
drivers
firmware
scripts
security
ipc
kernel
lib
mm
net
sound
tools
usr
Makefile
fs samples
include
linux-<version>
arch

Configuration

Configuration
● The kernel configuration is based on multiple Makefiles
● As discussed already the top level Makefile would be
used for this purpose
● The configuration you should know the target. You can
find of the target as mentioned below
$ cd linux-<version>
$ make help
● Now you may look for “Configuration targets:” section of
the output and decide one

Configuration
● Once you decide on the target you may try the following
command
$ make target
● The modified configurations would be saved on a file
called as .config which can be found on the top level of
the linux-<version> directory.
● All the target options use the same .config file, so you
may use any interchangeably.

Configuration
● Some most commonly used target are
– make config
– make menuconfig
– make xconfig
● Configuring Architecture specific targets
● Configuring for specific architecture from scratch

Configuration – make config
user@hostname:linux-<version>$ make config
scripts/kconfig/conf --oldaskconfig Kconfig
*
* Linux/<ARCH> <version> Kernel Configuration
*
Patch physical to virtual translations at runtime (ARCH_PATCH_PHYS_VIRT) [Y/n/?]
●
The above image show snap shot typical output of make config
command
●
Updates current config utilizing a line-oriented program
●
No user friendly approach. Could be used if you have limited host
installations
●
The problem with this approach is that, It force you to follow an
sequence of questions while configuration.
●
Have to use “Ctrl C” to exit

Configuration – make menuconfig
●
The above image shows the snapshot of typical output of make menuconfig
command

Configuration – make menuconfig
● Most commonly used method and simple method
● Can be used if graphics is unavailable
● Requires libncurses-dev installation
● Easy to navigate between options, using arrow keys
● Use <Help> to know more on menuconfig

Configuration – make xconfig
●
The above image shows the snapshot of typical output of make xconfig
command

Configuration – make xconfig
● Most commonly used graphical method of configuration
● Easy to use, better search option
● Use Help menu to know more on xconfig
● Requires libqt-dev packages installation

Configuration – Architecture Specific
● Most preferably used in Embedded Linux configuration
● You can find then at arch/<arch>/configs/
● These files are best possible minimal .config file you can have
for your board
● Just type the following on the command to know available
target
$ make help
– Now you may look for “Architecture specific targets:” section of the
output to look for default configuration for your target architecture
● Now the following command
$ make <controller_name>_defconfig

Configuration – Architecture Specific
● The previous command would rewrite the existing .config file.
● Now you can use any of the general configuration method to
discussed above to configure further if required
● If you feel the you are done and need to preserve your
configuration then you can save it by
$ make savedefconfig
● The above line will create a file call defconfig on root of
kernel source directory
● Now you can mv it to the config directory by the following
command
$ mv defconfig /arch/<arch>/configs/my_defconfig

Configuration – From Scratch
●
Its possible to configure a kernel from scratch without using
any default configuration
●
It would obvious if your a board vendor where you might
have to do for your board
● Point to be kept in mind in this case
– Make sure you alteast select a proper architecture for your board
– Most of the architecture dependent basic things would be set by
default, so just leave it as it is, unless you know what you
change
– Might have to change certain thing like select a correct device
driver for your board

Building

●
Assuming the required configuration are done, The next
step would be to compile the kernel.
●
Type the following command on the prompt to start the
compilation
make
●
Can use the below command if you have multicore CPU
make -j
●
The above command will speed up your compilation process
● You may even specify the no of jobs you want to run
simultaneously based on your CPU configuration
Building - Compilation

●
Once the compilation is done you will get the kernel image
in the following location arch/<arch>/boot
● make install this is rarely used in embedded dev as the
kernel image is single file, But still can be done by
modifying its behavior arch/<arch>/boot/install.sh
●
You can install all the configured modules by the following
command
make INSTALL_MOD_PATH=<dir>/ module_install
●
The above line direct the module installation on the path
provided by the INSTALL_MOD_PATH variable and this is
important to avoid installation in host root path
Building - Compilation

● Most of the embedded system uses U-Boot as its second
stage boot loader
● U-Boot require the kernel image to be converted into a
format which it can load. This converted format is called
as uImage
● The discussion done here is on how create the uImage
from vmlinux
● vmlinux is the output of the kernel compilation which you
would find on the root directory of the kernel directory
● vmlinux consists of multiple information like ELF header,
COFF and binary
Building – Kernel Image

● So it required to extract the binary file from the vmlinux
first, Which is done by the following command
arm-linux-objcopy -O binary vimlinux linux.bin
● After extraction the U-Boot header can be added using
mkimage command, This is done by the following
command
mkimage -A arm -O linux -T kernel -C none -a
20008000 -e 20008000 -n “Embedded Linux” -d
linux.bin uImage.arm
● After all the above steps the kernel image is ready for
deployment on target
Building - Kernel Image

Deploy
● Assuming the host is already configured with TFTP server
and Target is running U-Boot with TFTP client
● Copy uImage.arm in /var/lib/tftpboot/
● Copy the kernel image to the target board as mentioned
below
U-boot> tftp <TEXTBASE_ADDRESS> uImage.arm
● TEXTBASE_ADDRESS is defined configuring u-boot
● Once the image is transferred you can boot the image as
U-boot> bootm
● Your kernel should be loaded and executed now :)

Embedded Linux Kernel - Build your custom kernel

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