Introduce support for overriding system's DeviceTree

While experimenting with OS images on development boards where either
the bootloader doesn't load a Flattened DeviceTree Blob, or if one wants
to replace the one provided by the bootloader with a specific/new one
for development/experimentation/testing purposes, it's convenient to use
the OS-loaders ability to load a specific DeviceTree blob.

Add a new option to mkosi, to cause a specified DeviceTree blob to be
baked into the UKI, or copied into /boot and added to the systemd loader
entry.

As different distributions store the dtb files in different locations
the requested file is searched for in the locations spotted in Debian,
Fedora, Arch Linux packages (both from distro and kernel build system -
as these differs as well).

Resolves #2439
Based on initial effort by Manuel Traut.

Signed-off-by: Bjorn Andersson <bjorn.andersson@oss.qualcomm.com>

mkosi/__init__.py

@@ -1460,6 +1460,21 @@ def build_kernel_modules_initrd(context: Context, kver: str) -> Path:
     return kmods
 
 
+def find_requested_devicetree(context: Context, kver: str) -> Path:
+    assert context.config.devicetree
+
+    for f in (
+        context.root / f"lib/firmware/{kver}/device-tree",
+        context.root / f"usr/lib/modules/{kver}/dtb",
+        context.root / f"usr/lib/linux-image-{kver}",
+    ):
+        dtb = f / context.config.devicetree
+        if dtb.exists():
+            return dtb
+
+    die(f"Requested devicetree {context.config.devicetree} not found")
+
+
 def join_initrds(initrds: Sequence[Path], output: Path) -> Path:
     assert initrds
 
@@ -1609,6 +1624,11 @@ def build_uki(
         *flatten(["--ro-bind", os.fspath(profile), os.fspath(workdir(profile))] for profile in profiles),
     ]  # fmt: skip
 
+    if context.config.devicetree:
+        dtb = find_requested_devicetree(context, kver)
+        arguments += ["--devicetree", workdir(dtb)]
+        options += ["--ro-bind", dtb, workdir(dtb)]
+
     if context.config.secure_boot:
         assert context.config.secure_boot_key
         assert context.config.secure_boot_certificate
@@ -1814,6 +1834,14 @@ def install_type1(
 
     kmods = build_kernel_modules_initrd(context, kver)
 
+    dtb = None
+    if context.config.devicetree:
+        dtb_src = find_requested_devicetree(context, kver)
+
+        dtb = dst / context.config.devicetree
+        dtb.parent.mkdir(parents=True, exist_ok=True)
+        shutil.copy2(dtb_src, dtb)
+
     with umask(~0o600):
         if (
             want_efi(context.config)
@@ -1846,6 +1874,9 @@ def install_type1(
             for initrd in initrds:
                 f.write(f"initrd /{initrd.relative_to(context.root / 'boot')}\n")
 
+            if dtb:
+                f.write(f"devicetree /{dtb.relative_to(context.root / 'boot')}\n")
+
     if want_grub_efi(context) or want_grub_bios(context, partitions):
         config = prepare_grub_config(context)
         assert config

mkosi/config.py

@@ -1789,6 +1789,7 @@ class Config:
     initrd_packages: list[str]
     initrd_volatile_packages: list[str]
     microcode_host: bool
+    devicetree: Optional[Path]
     kernel_command_line: list[str]
     kernel_modules_include: list[str]
     kernel_modules_exclude: list[str]
@@ -2757,6 +2758,12 @@ def parse_ini(path: Path, only_sections: Collection[str] = ()) -> Iterator[tuple
         parse=config_make_list_parser(delimiter=","),
         help="Packages to install in the initrd that are not cached",
     ),
+    ConfigSetting(
+        dest="devicetree",
+        section="Content",
+        parse=config_parse_string,
+        help="Devicetree to be used by the booting kernel",
+    ),
     ConfigSetting(
         dest="kernel_command_line",
         metavar="OPTIONS",
@@ -4816,6 +4823,7 @@ def summary(config: Config) -> str:
                             Initrds: {line_join_list(config.initrds)}
                     Initrd Packages: {line_join_list(config.initrd_packages)}
            Initrd Volatile Packages: {line_join_list(config.initrd_volatile_packages)}
+                         Devicetree: {config.devicetree}
                 Kernel Command Line: {line_join_list(config.kernel_command_line)}
              Kernel Modules Include: {line_join_list(config.kernel_modules_include)}
              Kernel Modules Exclude: {line_join_list(config.kernel_modules_exclude)}

tests/test_json.py

@@ -139,6 +139,7 @@ def test_config() -> None:
             "Dependencies": [
                 "dep1"
             ],
+            "Devicetree": "freescale/imx8mm-verdin-nonwifi-dev.dtb",
             "Distribution": "fedora",
             "Drives": [
                 {
@@ -480,6 +481,7 @@ def test_config() -> None:
         make_initrd=False,
         manifest_format=[ManifestFormat.json, ManifestFormat.changelog],
         microcode_host=True,
+        devicetree=Path("freescale/imx8mm-verdin-nonwifi-dev.dtb"),
         minimum_version=GenericVersion("123"),
         mirror=None,
         nspawn_settings=None,