I got my mox in the mail yesterday, yay! I had forgotten that it needed an sdcard to boot, so after installing the sdcard image via the mox recovery mode, I was ready to explore.
I’m interested in using the pps-gpio kernel module, which requires a device tree change. So I investigated the boot process to see where the device tree was coming from. I wanted to write all this down as I don’t think it’s documented anywhere.
I see an 8MB SPI flash stores the bootloader/uboot:
root@mox:~# dmesg | grep m25 [ 3.712172] m25p80 spi0.0: w25q64dw (8192 Kbytes) root@mox:~# cat /proc/mtd dev: size erasesize name mtd0: 00020000 00010000 "secure-firmware" mtd1: 00160000 00010000 "u-boot" mtd2: 00010000 00010000 "u-boot-env" mtd3: 00660000 00010000 "Rescue system" mtd4: 00010000 00010000 "dtb"
I’m guessing mtd0 contains the first bootloader, which then loads uboot. The u-boot-env can be examined with the command fw_printenv. I assume u-boot starts with the command bootcmd:
root@mox:~# fw_printenv bootcmd bootcmd=run mox_boot
mox_boot then decides to run through the rescue process or the normal process.
The rescue process runs the script rescue_bootcmd, which loads the kernel and initramfs from mtd3 and boots that. The initramfs file /init is a shell script that decides which type of recovery to do. It does not need an sdcard to work.
The regular boot process starts at mox_distro_bootcmd, and has a few options for how things are booted. When booting from the sdcard, the order of scripts currently in use is: distro_bootcmd > bootcmd_mmc0 > mmc_boot > scan_dev_for_boot_part > scan_dev_for_boot > scan_dev_for_scripts > boot_a_script
This loads /boot/boot.scr from the sdcard and runs it as an uboot script. boot.scr loads /boot/armada-3720-turris-mox.dtb as the device tree, /boot/Image as the kernel, sets the boot arguments, and then boots the kernel.
So if I want to change the device tree, it should just be a change to /boot/armada-3720-turris-mox.dtb. I assume that file will be overwritten with OS updates, but that’s good enough to start with.
Hopefully this is helpful to someone. Please let me know if I got any details wrong.