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Understanding Boot

There are several phases to BeaglePlay boot. The simplest place to take control of the system is using Distro Boot. It is simplest because it is very generic, not at all specific to BeaglePlay or AM62, and was included in the earliest Debian images shipping pre-installed in the on-board flash.

Over time, BeaglePlay images will include SystemReady support to provide for the most generic boot support allowing execution of

Distro Boot

For some background on distro boot, see the u-boot documentation on distro boot.

In Typical /boot/firmware/extlinux/extlinux.conf file, you can see line 1 provides a label and subsequent indented lines provide parameters for that boot option.

Listing 4 Typical /boot/firmware/extlinux/extlinux.conf file
1label Linux eMMC
2    kernel /Image
3    append root=/dev/mmcblk0p2 ro rootfstype=ext4 rootwait net.ifnames=0 quiet
4    fdtdir /
5    #fdtoverlays /overlays/<file>.dtbo
6    initrd /initrd.img

It is important to note that this file is not on the root file system of BeaglePlay. It is sitting on a separate FAT32 partition that is mounted at /boot/firmware. You can see the mounted file systems and their formats in List of mounted file systems.

The FAT32 partition in this setup is often referred to as the boot file system.

Listing 5 List of mounted file systems
debian@BeaglePlay:~$ df
Filesystem     1K-blocks     Used Available Use% Mounted on
udev              903276        0    903276   0% /dev
tmpfs             197324     1524    195800   1% /run
/dev/mmcblk0p2  14833640 12144024   1914296  87% /
tmpfs             986608        0    986608   0% /dev/shm
tmpfs               5120        4      5116   1% /run/lock
/dev/mmcblk0p1    130798    53214     77584  41% /boot/firmware
tmpfs             197320       32    197288   1% /run/user/1000
debian@BeaglePlay:~$ lsblk
mmcblk0      179:0    0 14.6G  0 disk
├─mmcblk0p1  179:1    0  128M  0 part /boot/firmware
└─mmcblk0p2  179:2    0 14.5G  0 part /
mmcblk0boot0 179:256  0    4M  1 disk
mmcblk0boot1 179:512  0    4M  1 disk
debian@BeaglePlay:~$ sudo sfdisk -l /dev/mmcblk0
Disk /dev/mmcblk0: 14.6 GiB, 15678308352 bytes, 30621696 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xba67172a

Device         Boot  Start      End  Sectors  Size Id Type
/dev/mmcblk0p1 *      2048   264191   262144  128M  c W95 FAT32 (LBA)
/dev/mmcblk0p2      264192 30621695 30357504 14.5G 83 Linux

To better understand BeaglePlay’s U-Boot Distro Boot, let’s install the kernel image we made in BeaglePlay Kernel Development. To do this, we need to have an uncompressed version of the kernel in the FAT32 file system and a ramdisk image we plan to use. The ramdisk image is utilized to make sure any kernel modules needed are available and to provide a bit of a recovery opportunity in case the root file system is corrupted. You can learn more about initrd on the Debian Initrd Wiki page <> and the Linux kernel documentation admin guide initrd entry <>.

In Copy kernel to FAT32 filesystem, we perform a copy of the kernel that was installed via Install 6.6.0 kernel and reboot and then reverted with …


Put the step into play-kernel-development.rst to revert back to the Beagle kernel.

The contents of the initrd can be read using lsinitramfs /boot/firmware/initrd.img-6.6.0.

Listing 6 Copy kernel to FAT32 filesystem
debian@BeaglePlay:~$ sudo cp /boot/vmlinuz-6.6.0 /boot/firmware/Image-6.6.gz
[sudo] password for debian:
debian@BeaglePlay:~$ sudo gunzip /boot/firmware/Image-6.6.gz
debian@BeaglePlay:~$ sudo cp /boot/initrd.img-6.6.0 /boot/firmware/
Listing 7 Modified /boot/firmware/extlinux/extlinux.conf file
 1menu title Select image to boot
 2timeout 30
 3default Linux 6.6
 5label Linux default
 6    kernel /Image
 7    append root=/dev/mmcblk0p2 ro rootfstype=ext4 rootwait net.ifnames=0 quiet
 8    fdtdir /
 9    #fdtoverlays /overlays/<file>.dtbo
10    initrd /initrd.img
12label Linux 6.6
13    kernel /Image-6.6
14    append root=/dev/mmcblk0p2 ro rootfstype=ext4 rootwait net.ifnames=0 quiet
15    fdtdir /
16    initrd /initrd.img-6.6.0
Listing 8 Reboot into modified kernel
debian@BeaglePlay:~$ sudo shutdown -r now
Connection to closed by remote host.
Connection to closed.
jkridner@slotcar:~$ ssh -Y debian@
Debian GNU/Linux 11 Debian Bullseye Xfce Image 2023-05-18
default username:password is [debian:temppwd]

debian@'s password:

The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/\*/copyright.

Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.
Last login: Tue Dec 12 15:33:21 2023 from
debian@BeaglePlay:~$ uname -a
Linux BeaglePlay 6.6.0 #4 SMP Tue Dec  5 13:50:59 UTC 2023 aarch64 GNU/Linux

Booting U-Boot

Listing 9 Install bootloader to eMMC

if ! id | grep -q root; then
	echo "must be run as root"


if [ -b /dev/mmcblk0 ] ; then
	#mmc extcsd read /dev/mmcblk0
	mmc bootpart enable 1 2 /dev/mmcblk0
	mmc bootbus set single_backward x1 x8 /dev/mmcblk0
	mmc hwreset enable /dev/mmcblk0

	echo "Clearing eMMC boot0"

	echo '0' >> /sys/class/block/mmcblk0boot0/force_ro

	echo "dd if=/dev/zero of=/dev/mmcblk0boot0 count=32 bs=128k"
	dd if=/dev/zero of=/dev/mmcblk0boot0 count=32 bs=128k

	echo "dd if=${wdir}/tiboot3.bin of=/dev/mmcblk0boot0 bs=128k"
	dd if=${wdir}/tiboot3.bin of=/dev/mmcblk0boot0 bs=128k