document booting with EFI vs DT only

[orangecms]

trafficstars

Roughly, regardless of EFI or DT, register conventions are used to pass information to the OS.

The respective register would hold a memory address, and that would point to either

• an FDT, so the OS can check for the d00dfeed magic
• an EFI System Table, so the OS can check for the 0x5453595320494249 (TSYS IBI) signature

From there, the OS traverses to find more information.

In the case of (U)EFI, it will find references (pointers) to methods in memory to call into. Note that UEFI technically does not have to be coupled with ACPI, however, de facto it mostly is. Some firmware implementations for SoC platforms offer EFI + ACPI + DT.

With Device Tree, the OS will read static information from specific nodes to identify the platform, take the cmdline and memory to use, and learn about the buses and peripherals. The design of DTs may differ between SoCs, in part because SoCs also differ in their design.