Many SoC vendors and ODMs support the use of multiple DTs on a device, enabling one image to power multiple SKUs/configurations. In such cases, the bootloader identifies the hardware and loads the corresponding DT at runtime:
Note: Using multiple DTs is not mandatory.
To add support for multiple DTs to the DTO model, set up a list of main DTs and another list of overlay DTs.
The bootloader should be able to:
- Read the SoC ID and select the corresponding main device tree, and
- Read the board ID and select the set of overlay device trees accordingly.
Only one main DT should be selected for use at runtime. Multiple overlay DTs may be selected but they must be compatible with the chosen main DT. Using multiple overlays can help avoid storing one overlay per board within the DTBO partition and enable the bootloader to determine the subset of required overlays based on the board ID (or possibly by probing the peripherals). For example, Board A may need the devices added by the overlays 1, 3, and 5 while Board B may need the devices added by the overlays 1, 4, and 5.
To partition, determine a bootloader runtime-accessible and trusted location
in flash memory to store the DTBs and DTBOs (bootloader must be able to locate
these files in the matching process). Keep in mind that DTBs and DTBOs can not
exist in the same partition. If your DTBs/DTBOs are in the
dtbo partition, use the table structure and header
format detailed in DTB/DTBO
Running in bootloader
- Identify the SoC and load the corresponding .dtb from storage into memory.
- Identify the board and load the corresponding
.dtbofrom storage into memory.
- Overlay the
.dtboto be a merged DT.
- Start kernel given the memory address of the merged DT.