Embedded linux system architecture
25 May 2020
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Linux Uboot DirverU-boot(bootloader), Linux Kernel and Rootsf are three key components of a embedded linux system.
Caution: The configurations and operations are mainly based on nxp i.mx series
1. U-boot
U-boot is the abbreviation and shorten of ‘Universal Boot Loader’, which support multiple architectures like ARM, MIPS, RISC-V and x86.
1.1 basic of uboot and flashing
The official Uboot source code can be found at DENX-U-Boot, also most of the SOCs manufactures will give their customized U-boot configuration, for example we can find most of the configurations of NXP’s Socs here.
Before make an Uboot for one board some configurations should be done first,
Using the following command makefile will create .config under the fold.
make ARCH=arm clean
make ARCH=arm XXXX_defconfig
//make ARCH=arm mx6ull_14x14_evk_defconfig , for freescale i.mx6u board SD card start
//make ARCH=arm mx6ull_14x14_evk_ emmc _defconfig
Following command can make the corresponding U-boot file according to the .config
make -j8 ARCH=arm CROSS_COMPILE=arm-none-eabi- `
We can see the U-boot as a big bare-metal program. So the way used to flash Uboot to SD card is same with bare-metal program. To flash the program some header information should be added to U-boot.bin file, which
Here we use the dd command to flash.
According to the official image vector table of nxp IVT, we need to flash the U-boot to the address offset 1kBytes.
sudo dd if=u-boot.imx of=/dev/sdb bs=512 seek=2 conv=fsync
<p>Table. IVT of nxp i.mx6ull</p></div>
1.2 porting uboot to hardware
To adding support of our customized board, most of the time, we only need the modify the U-boot from soc manufacture. There are three important path we need modify(take imx6u as example, U-boot version>2018):
/configs/<board_name>
/include/configs/<board_name>
/arch/arm/cpu/armv7/mash-imx/mx6/Kconfig //adding config TARGET and source "board/<vendor>/<board_name>
/board/<vendor>/<board_name>
The above configuration is the basic structure for a customized board. For the detailed configuration, I will write in another blog about adding support of LCD, ethernet, and other devices.
2. Linux kernel
2.1 Compile the official kernel
The structure of Linux kernel is very similar with U-boot project. Also, we can use nxp offical linux kernel as our template to customize our kernel (git switch x.xx.x–imx:using the branch with –imx).
make ARCH=arm clean
make ARCH=arm XXX_defconfig (imx_v7_defconfig for nxp offical imx6)
make menuconfig(optional if dont need config just esc)
make ARCH=arm -j10 CROSS_COMPILE= arm-linux-gnueabihf-
Note: Sometimes make cannot go through due to gcc version. In that case using older arm-linux-nueabihf- from source with export PATH=/{your gcc address}/bin:$PATH
2.2 Customize our linux kernel
To customize a linux kernel, two important file are needed:
- xxx_defconfig (in arch/arm/configs/)
- xxx.dts (in arch/arm/boot/dts )
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3. Rootfs
rootfs can be understand as the container files of linux environment, configuration, and programs in the mpu.
3.1 Roofts make
Rootfs can be made from various sources, busybox, buildroot, yocto, openembedded.
3.2 mount rootfs from nfs server
setenv bootargs ‘console=ttymxc0,115200 nfsvers=4 root=/dev/nfs nfsroot=192.168.0.188:/home/jie/workdir/rootfs ip=192.168.0.16:192.168.0.188:192.168.0.1:255.255.255.0::eth1:off’