Table of Contents
- Making a SquashFS image of the root filesystem
- Re-compile kernel with squashfs as inbuilt module
- Copy the dtb and kernel image to SD card
- Set bootcmd env in the board U-boot console
- Reboot the board
Now that we have our filesystem with necessary libraries ready and kernel modules installed to the root filesystem, we are going to put everything we have compiled/generated so far - bootloader, kernel and the root filesystem - into the SD card and boot from it.
Making a SquashFS image of the root filesystem
The SquashFS is a read only filesystem with high compression and read speed. We are going to create a SquashFS image of our root filesystem and flash that image to the secondary partition of SD card (we created the second partition with 512MB size).
Install the squashfs-tools package on your Ubuntu development system with the following command
sudo apt install squashfs-tools
Create squashfs image of root directory
mksquashfs ~/rpi3/nfs ~/rpi3/sdCardFiles/rootfs.sqfs -noappend
Flash the squashfs image to SD card
Unmount the SD card partitions using the following commands first before flashing
# Replace <USERNAME> with your Ubuntu system username. sudo umount /media/<USERNAME>/rootfs
Using the dd command, flash the squashfs image to the second partition of the memory card.
# The second patition name might vary. Check `dmesg` output after inserting the SD card into the system. sudo dd if=~/rpi3/sdCardFiles/rootfs.sqfs of=/dev/mmcblk0p2 bs=1M
Re-compile kernel with squashfs as inbuilt module
cd ~/rpi3/linux make menuconfig
Make squashfs as inbuilt kernel module by going into File systems —> Miscellaneous filesystems —> SquashFS 4.0 - Squashed file system support.
Save the config file and recompile the kernel by running following commands.
export ARCH=arm64 export CROSS_COMPILE=aarch64-rpi3-linux-uclibc- export PATH=$PATH:~/x-tools/aarch64-rpi3-linux-uclibc/bin/ make -j`nproc`
Copy the dtb and kernel image to SD card
cp ~/rpi3/linux/arch/arm64/boot/Image /media/<USERNAME>/boot/kernel.img cp ~/rpi3/linux/arch/arm64/boot/dts/broadcom/bcm2710-rpi-3-b.dtb /media/<USERNAME>/boot/
Set bootcmd env in the board U-boot console
Insert SD card into the Raspberry Pi. Power up the board. Stop at U-boot console and run the following commands to boot the Raspberry Pi 3B completely from the SD card.
setenv bootargs earlyprintk root=/dev/mmcblk0p2 rootfstype=squashfs rootwait noinitrd setenv bootcmd 'mmc dev 0; fatload mmc 0:1 0x2000000 kernel.img; fatload mmc 0:1 0x200000 bcm2710-rpi-3-b.dtb; booti 0x2000000 - 0x200000;' saveenv
The explanation of the first command as follows -
- The bootargs env value is what will be passed as the kernel command line arguments while booting.
- The root=/dev/mmcblk0p2 tells the kernel to look for the root filesystem from second partition of the kernel.
- The rootfstype=squashfs tells the filesystem type. It is squash filesystem.
- The rootwait tells the kernel to wait until the SD card is ready for the root filesystem.
- The noinitrd tells there is no initramfs created.
The explanation of the second command as follows -
- The bootcmd env value is a bunch of U-boot commands which will be executed every time the board boots.
- The mmc dev 0 selects the first mmc device. Here the SD card. You can check the mmc details with the command mmc info in the U-boot console.
- The fatload mmc 0:1 0x2000000 kernel.img loads the kernel.img image from the first partition of SD card to the RAM address 0x2000000
- The fatload mmc 0:1 0x200000 bcm2710-rpi-3-b.dtb loads the device tree file from the first partition of SD card to the RAM address 0x200000
- The booti 0x2000000 - 0x200000 boots the image (kernel.img) at address 0x2000000 with the dtb file (bcm2710-rpi-3-b.dtb) at address 0x200000 with no initramfs(-).
The final command saveenv saves the environment variables on SD card.
Reboot the board
Enter the following command in the U-boot console to reboot the Raspberry Pi 3B from the SD card.
Congratulations on successfully booting the board from SD card.