specs/bls: Add devicetree-dir key #198
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There are many devices that require booting with devicetree. Until now, most of those devices were of embedded kind where one would be expected to maintain a device-specific OS image. For those devices it makes sense to just hardcode devicetree name in the boot loader entry. In recent years, however, many devices became available that don't fit into such "embedded" category and instead would be more useful with a generic OS, such as Qualcomm WoA laptops. As those laptops can't be used with their terribly disfuncitonal ACPI implementation, distribution have to rely on manually loading appropriate devicetrees. Unfortunately, for rapidly evolving targets like those Qualcomm laptops, it's critical to boot the kernel with matching version of the devicetree, as many features are added rapidly. UKI images already have provisions to support such devices, namely .dtbauto section of the UKI that allows the implementation to include as many DTBs into the same image as one wants. However same can't be said for BLS#1 boot entries. To solve this discrepency, document a new "devicetree-dir" key, that would define path to a directory with devicetree files, letting the bootloader implementation to pick an appropriate devicetree and load it.
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Similarly to #123 this PR introduces provisions for automatic DTB selection, this time for BLS#1 boot entries.
This PR documents a new key - "devicetree-dir" that is supposed to be used by the bootloader to construct a full path to the dtb file.
A generic OS that wishes to target DeviceTree platforms may want to be able to automatically select a correct devicetree for any of supported devices. This may be useful for multiple reasons such as the target device not having any DT loaded (i.e. because it's a Qualcomm WoA device with horrid useless ACPI) or the device having a DT loaded but outdated (i.e. if it's a device with "some" DTB baked into firmware).
Having a "devicetree-dir" key allows the distribution to install a [kernel/initramfs/dtbs dir] combination for each available kernel while making sure "mismatched" dtb/kernel combinations won't be used and the user will always get the appropriate version of the dtb when they pick a boot entry, which is especially important considering many of those devices are actively developed and the usual expectation of both-ways-compatibility of devicetrees may not always hold.
Similar key is already supported in u-boot's extlinux.conf /pxe implementation [1], with names "fdtdir" and "devicetreedir". I've decided that adding a dash to this key makes more sense to be in line with the naming of other keys in the specification.
Reference implementation: systemd/systemd#40077
[1] https://source.denx.de/u-boot/u-boot/-/blob/master/boot/pxe_utils.c#L899-900