Hi all,
In U-Boot, there are multiple stages which can be loaded from different media each. On Rockchip SoCs, it mostly goes this way, SoC BootROM loads from a medium (storage or over USB) and executes the TPL (or SPL if no TPL supported), the TPL goes back to BootROM and reads the SPL from the same medium and executes it. The SPL will favor the same medium for loading the U-Boot FIT image (containing TF-A BL31, OP-TEE OS BL32 and U-Boot proper image), but in case it fails, falls back to other possible media. In U-Boot we have the storage medium used for loading the U-Boot FIT image stored in the DTB under /chosen/u-boot,spl-boot-device property. But we do not have a property for the medium the BootROM loaded the TPL (or SPL) from. This could be useful information. For example, I have some CI where I want to check whether the BootROM loaded the TPL from the expected location (important when checking that they all work and don't depend on a bootloader flashed in a fallback medium for example). Another usecase could be to detect whenever one loads the TPL from USB (not a USB stick, some USB transfer; see rockusb for Rockchip for example) and decide to do something else in U-Boot or the OS depending on that (could be starting DFU for flashing the device, or fastboot for loading the rest of the system).
Barebox currently does that by setting /chosen/bootsource to the full path of the device the BootROM says it booted from, but that isn't part of the DT spec so before I add support in U-Boot for that, I'm asking whether that's fine or if we should correct course in Barebox and use another name and/or location.
I've also been made aware of other "sources" that are added to the /chosen node in Barebox, reset-source, reset-source-instance and reset-source-device. I am not entirely sure what those exactly are for BUT it seems that this is of interest to U-Boot as well since we do try to report a reset cause as read from the CPU for Rockchip SoCs, c.f. https://elixir.bootlin.com/u-boot/v2024.10/source/arch/arm/mach-rockchip/cpu.... I believe /chosen/reset-source would be the reason, e.g. "POR", "RST", "WDG", "WKE", "THERM", etc. /chosen/reset-source-device contains the node path to the device that triggered that, that could be useful in the event there are more than one thermal trip point (e.g. CPU or GPU), or more than one hardware watchdog (I for example have a system with potentially 4: SoC, PMIC, 2 separate in an external MCU). I guess /chosen/reset-source-instance is for HW blocks that expose the same reset but from two different sources which aren't modeled in Device Tree. Again, this would be useful for the bootloader or OS to decide to do something else based on the reset cause (e.g. have data on why some devices reboot in the field, e.g. because of thermals, power loss, brownout, etc..).
I'm reaching out to the devicetree-spec community before I add code in U-Boot to implement this so we don't add unnecessary backward compatibility code, were the properties or locations to be modified to match the spec.
Does anyone disagree with adding this to the spec? Does anyone have another suggestion for the naming or location of the properties?
Would you prefer a patch to the spec and discuss "code" directly there?
Cheers, Quentin