DtbLoader is an EFI "driver" intended to support booting a devicetree based kernel on ACPI based hardware, in particular windows ARM based laptops. It provides a way to determine which dtb tables to load, and apply appropriate dtb overlays based on the hw configuration, as determined by SMBIOS table values, etc.
The motivation is to support linux support for Snapdragon(TM) ARM laptops, although it's utility is not limited to a particular architecture or OS. DtbLoader runs before the OS starts, in order to support any EBBR compatible OS or distro.
Why not ACPI
ACPI provides a lot of advantages from the perspective on the operating system provider (OSV), in particular it provides enough of a description of the hardware (in tables and methods) to boot an operating system which was released before the hardware existed. That may not provide full functionality of all the various accelerators (GPUs, etc), but it gets the user far enough along that they can install, boot, and then install OS updates to get support for the newest, shiniest, features in their hardware.
But, the windows snapdragon laptops rely on Platform Extension Plugins (PEPs) for power control for more advanced platform hardware. In particular, device power state transitions which previously would have been handled entirely via ACPI methods, are instead handled by a PEP driver which can control the necessary clocks, power domains, interconnect/memory scaling required for a given device power state, for more precise power management. Linux currently has no equivalent mechanism. So while ACPI boot is sufficient to get basic functionality and run an an OS installer, it is not sufficient for more advanced support (display support beyond efifb, gpu, video acceleration, audio, etc).
How it works
Once installed, DtbLoader.efi runs before the OS bootloader. It looks for a matching dtb file installed in the ESP, based on the machines CHID ($ESP\dtb\$CHID.dtb), and if found, it will load and install the devicetree table as a EFI config table. If necessary, it will apply any needed dtb overlays first. If no matching dtb is found, DtbLoader will not change/adjust anything.
In particular, it is common for a given laptop SKU to use one of several
possible different LCD panels. (They may have the same resolution and
otherwise appear identical to the user, but could have different panel
timings, etc.) For the snapdragon arm laptops, DtbLoader reads the
UEFIDisplayInfo EFI variable to determine the panel-id, and applies the
appropriate dt overlay. The dtb overlay is loaded from $ESP\dtb\qcom-panels\$panel-id.dtb.
ComputerHardwareIds are a way to generate a UUID from fields in the SMBIOS tables. This gives a convenient way to generate a file path without having to worry about escaping any random characters a hw vendor might stuff into SMBIOS, or having to worry about file path length. There are actually multiple CHIDs for a given machine, and DtbLoader will search from most- specific to least, to provide a way to overload dtb table matching in case some particular variant of a given device needs specific workarounds.
For more details, see:
In particular, DtbLoader looks for the following sequence of CHIDs, in priority order:
CHID_3- Manufacturer + Family + ProductName + ProductSku + BaseboardManufacturer + BaseboardProduct
CHID_4- Manufacturer + Family + ProductName + ProductSku
CHID_5- Manufacturer + Family + ProductName
CHID_6- Manufacturer + ProductSku + BaseboardManufacturer + BaseboardProduct
CHID_7- Manufacturer + ProductSku
CHID_8- Manufacturer + ProductName + BaseboardManufacturer + BaseboardProduct
CHID_9- Manufacturer + ProductName
When adding support for a new device, pick the least specific possible CHID,
and leave the more specific options for overriding the less specific options.
CHID_9 should be sufficient in most/all cases.
For example, the Lenovo C630 laptop has Manufacturer "LENOVO" and ProductName "81JL", which can be seen in the linux kernel bootlog:
[ 0.057874] DMI: LENOVO 81JL/LNVNB161216, BIOS 9UCN23WW(V1.06) 10/25/2018
Which translates to CHID_9 value of
therefore DtbLoader will load the dtb file from $ESP\dtb\30b031c0-9de7-5d31-a61c-dee772871b7d.dtb.
export GCC5_AARCH64_PREFIX=aarch64-linux-gnu- source edksetup.sh build -b DEBUG -a AARCH64 -t GCC5 -p EmbeddedPkg/EmbeddedPkg.dsc -m EmbeddedPkg/Application/ConfigTableLoader/DtbLoader.inf build -b DEBUG -a AARCH64 -t GCC5 -p ShellPkg/ShellPkg.dsc -m ShellPkg/Application/Shell/Shell.inf
(the last step is option but useful if you need a Shell.efi for installation)
The result will be ./Build/Embedded/DEBUG_GCC5/AARCH64/DtbLoader.efi
Copy DtbLoader.efi to ESP, and boot to Shell.efi, and:
Shell> fs2: <== or whichever partition your ESP is on FS:\> bcfg driver add 1 DtbLoader.efi "dtb loader" FS:\> reset
As the upstream kernel is not using a dtb naming scheme conducive to automated
choosing of appropriate dtb, EmbeddedPkg/Application/ConfigTableLoader/install-dtbs.py
provides a way to copy dtb files from a linux kernel build, translating
the linux kernel dtb name into an appropriate CHID based name. To add
support for new laptops, add additional entries to the
smoother installation process.. In particular,
bcfgwill use the full EFI device-path to *DtbLoader.efi`, meaning that if it is an external usb drive, and the user plugs it in to a different USB port (or hub topology changes, etc) on the next boot, DtbLoader.efi won't be found and loaded.
update process.. rough idea is to write a fwupd plugin and use the LVFS mechanism to distribute updated and new dtb files to users. Fwupd already has the logic to find and mount the ESP partition in order to install new/updated dtb files. By the time the devicetree files land in upstream kernel, they should always be backwards compatible, so booting an older. distro kernel with newer dtb files should be a thing that always works.
Note than this could also be accomplished via normal distro update mechanisms, but that is likely to cause problems if a user is booting multiple different distros/OSs from different partitions. It is better to have a distro-agnosting way to deliver updated dtb to users.
find a better home than my personal github for DtbLoader