U-Boot uses a devicetree for configuration. This includes the devices used by the board, the format of the image created with binman, which UART to use for the console, public keys used for secure boot and many other things.
See Devicetree Control in U-Boot for more information.
Why does U-Boot put <thing> in the devicetree?¶
This question comes up a lot with people new to U-Boot, particular those coming from Linux who are used to quite strict rules about what can go into the devicetree.
U-Boot uses the same devicetree as Linux but adds more things necessary for the bootloader environment (see Adding tweaks for U-Boot).
U-Boot does not have a user space to provide policy and configuration. It cannot do what Linux does and run programs and look up filesystems to figure out how to boot. So configuration and runtime information goes into the devicetree in U-Boot.
Of course it is possible to:
add tables into the rodata section of the U-Boot binary
append some info to the end of U-Boot in a different format
modify the linker script to bring in a file with some info in it
put things in ACPI tables
link in a UEFI hand-off block structure and put things in there
but please don’t. In general, devicetree is the sane place to hold U-Boot’s configuration.
So, please, do NOT ask why U-Boot puts <thing> in the devicetree. It is the only place it can go. It is a highly suitable data structure for just about anything that U-Boot needs to know at runtime.
Note, it is possible to use platdata directly so drivers avoid devicetreee in SPL. But of-platdata is the modern way of avoiding devicetree overhead, so please use that instead.