[PATCH v7] kernel-fit-image: allow overriding DTB compatible string

[Kavinaya S <kavinaya@qti.qualcomm.com>]

The Linux kernel allows multiple DTBs to share the same compatible string.
For example:
qcs6490-rb3gen2.dtb
qcs6490-rb3gen2-vision-mezzanine.dtb
qcs6490-rb3gen2-industrial-mezzanine.dtb

All of these use the same base compatible string:
compatible = "qcom,qcs6490-rb3gen2", "qcom,qcm6490";

Since the latter two DTBs are overlays on top of the base DTB and do not
modify platform properties, they retain the same compatible string.

When these DTBs are bundled into a single fitImage, the bootloader cannot
distinguish between them due to identical compatible strings.

To address this, introduce a mechanism to override the compatible string
using a BitBake variable:
FIT_DTB_COMPATIBLE_OVERRIDE[<dtb_name>] = "<compatible_string>"

This overrides the compatible string generated resulting in unique entries in the
generated .its file. For example:
FIT_DTB_COMPATIBLE_OVERRIDE[qcs6490-rb3gen2-vision-mezzanine] = "qcom,qcs6490-rb3gen2-vision"
FIT_DTB_COMPATIBLE_OVERRIDE[qcs6490-rb3gen2-vision-mezzanine] = "qcom,qcs6490-rb3gen2-vision qcom,qcm6490"

Results in:

compatible = "qcom,qcs6490-rb3gen2-vision";
compatible = "qcom,qcs6490-rb3gen2-vision", "qcom,qcm6490";

Suggested-By: Alexander Kanavin <alex.kanavin@gmail.com>
Suggested-By: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Signed-off-by: Kavinaya S <kavinaya@qti.qualcomm.com>
---
 meta/classes-recipe/kernel-fit-image.bbclass |  7 ++-
 meta/conf/image-fitimage.conf                | 16 +++++++
 meta/lib/oe/fitimage.py                      | 16 +++++--
 meta/lib/oeqa/selftest/cases/fitimage.py     | 50 ++++++++++++++++++++
 4 files changed, 84 insertions(+), 5 deletions(-)

diff --git a/meta/classes-recipe/kernel-fit-image.bbclass b/meta/classes-recipe/kernel-fit-image.bbclass
index f04aee1807..0a6125b3b6 100644
--- a/meta/classes-recipe/kernel-fit-image.bbclass
+++ b/meta/classes-recipe/kernel-fit-image.bbclass
@@ -84,8 +84,13 @@ python do_compile() {
 
             # Copy the dtb or dtbo file into the FIT image assembly directory
             shutil.copyfile(os.path.join(kernel_deploydir, dtb_name), dtb_name)
+
+            dtb_base = os.path.splitext(os.path.basename(dtb_name))[0]
+            compatible_override_str = d.getVarFlag("FIT_DTB_COMPATIBLE_OVERRIDE", dtb_base) or ""
+
             root_node.fitimage_emit_section_dtb(dtb_name, dtb_name,
-                d.getVar("UBOOT_DTB_LOADADDRESS"), d.getVar("UBOOT_DTBO_LOADADDRESS"))
+                d.getVar("UBOOT_DTB_LOADADDRESS"), d.getVar("UBOOT_DTBO_LOADADDRESS"),
+                compatible_override=compatible_override_str)
 
     if external_kernel_devicetree:
         # iterate over all .dtb and .dtbo files in the external kernel devicetree directory
diff --git a/meta/conf/image-fitimage.conf b/meta/conf/image-fitimage.conf
index 090ee148f4..9c51b28095 100644
--- a/meta/conf/image-fitimage.conf
+++ b/meta/conf/image-fitimage.conf
@@ -65,3 +65,19 @@ FIT_ADDRESS_CELLS ?= "1"
 # Machine configurations needing such a script file should include it in the
 # SRC_URI of the kernel recipe and set the FIT_UBOOT_ENV parameter.
 FIT_UBOOT_ENV ?= ""
+
+# For specific DTBs, add custom "compatible" strings when creating FIT images.
+# Format:
+#   FIT_DTB_COMPATIBLE_OVERRIDE[<dtb_name>] = "<compatible_string>"
+#
+# Result:
+#   dtb_name.dtb {
+#    ...
+#    compatible = "compatible_string";
+#   };
+#   Example:
+#   FIT_DTB_COMPATIBLE_OVERRIDE[qcs6490-rb3gen2-vision-mezzanine] = "qcom,qcs6490-rb3gen2-vision"
+#   This will result in compatible string like:
+#   compatible = "qcom,qcs6490-rb3gen2-vision";
+#
+#
diff --git a/meta/lib/oe/fitimage.py b/meta/lib/oe/fitimage.py
index f303799155..fc37ce7665 100644
--- a/meta/lib/oe/fitimage.py
+++ b/meta/lib/oe/fitimage.py
@@ -289,7 +289,8 @@ class ItsNodeRootKernel(ItsNode):
         self._kernel = kernel_node
 
     def fitimage_emit_section_dtb(self, dtb_id, dtb_path, dtb_loadaddress=None,
-                                  dtbo_loadaddress=None, add_compatible=False):
+                                  dtbo_loadaddress=None, add_compatible=False,
+                                  compatible_override=None):
         """Emit the fitImage ITS DTB section"""
         load=None
         dtb_ext = os.path.splitext(dtb_path)[1]
@@ -309,7 +310,10 @@ class ItsNodeRootKernel(ItsNode):
         # Preserve the DTB's compatible string to be added to the configuration node
         compatible = None
         if add_compatible:
-            compatible = get_compatible_from_dtb(dtb_path)
+            if compatible_override:
+                compatible = str(compatible_override).split()
+            else:
+                compatible = get_compatible_from_dtb(dtb_path)
 
         dtb_node = self.its_add_node_dtb(
             "fdt-" + dtb_id,
@@ -321,12 +325,16 @@ class ItsNodeRootKernel(ItsNode):
         )
         self._dtbs.append(dtb_node)
 
-    def fitimage_emit_section_dtb_alias(self, dtb_alias_id, dtb_path, add_compatible=False):
+    def fitimage_emit_section_dtb_alias(self, dtb_alias_id, dtb_path, add_compatible=False,
+                                        compatible_override=None):
         """Add a configuration node referring to another DTB"""
         # Preserve the DTB's compatible string to be added to the configuration node
         compatible = None
         if add_compatible:
-            compatible = get_compatible_from_dtb(dtb_path)
+            if compatible_override:
+                compatible = str(compatible_override).split()
+            else:
+                compatible = get_compatible_from_dtb(dtb_path)
 
         dtb_id = os.path.basename(dtb_path)
         dtb_alias_node = ItsNodeDtbAlias("fdt-" + dtb_id, dtb_alias_id, compatible)
diff --git a/meta/lib/oeqa/selftest/cases/fitimage.py b/meta/lib/oeqa/selftest/cases/fitimage.py
index 195b9ee8b5..d32d7b6e2f 100644
--- a/meta/lib/oeqa/selftest/cases/fitimage.py
+++ b/meta/lib/oeqa/selftest/cases/fitimage.py
@@ -819,6 +819,56 @@ MACHINE:forcevariable = "beaglebone-yocto"
         # The alias is a symlink, therefore the compatible string is equal
         self.assertEqual(comp_alias, comp)
 
+    def test_fitimage_custom_compatible_in_its(self):
+        """
+        Verify that FIT_DTB_COMPATIBLE_OVERRIDE[...] is honored in the generated .its.
+        This test:
+        1) Selects beaglebone-yocto machine and a DTB that is part of its kernel.
+        2) Sets FIT_DTB_COMPATIBLE_EXTENTION[am335x-bonegreen-ext] to a custom string.
+        3) Runs do_assemble_fitimage to generate the FIT .its.
+        4) Asserts the .its 'compatible = ...' includes custom compatible string.
+
+        """
+
+        kernel_dtb = "am335x-bonegreen-ext.dtb"
+        dtb_name = os.path.splitext(os.path.basename(kernel_dtb))[0]
+
+        config = f"""
+DISTRO = "poky"
+MACHINE = "beaglebone-yocto"
+
+# Ensure the FIT flow is active
+KERNEL_CLASSES += "kernel-fit-extra-artifacts "
+
+# Ensure the selected DTB is built into the kernel deploy output
+KERNEL_DEVICETREE = "{kernel_dtb}"
+
+# Original compatibles: "ti,am335x-bone-green", "ti,am335x-bone-black"
+FIT_DTB_COMPATIBLE_OVERRIDE[{dtb_name}] = "subtypeA"
+
+"""
+        self.write_config(config)
+
+        bitbake('virtual/kernel:do_deploy')
+        bitbake('linux-yocto-fitimage:do_deploy')
+
+        # Find the generated .its in DEPLOY_DIR_IMAGE
+        deploy_dir_image = get_bb_var('DEPLOY_DIR_IMAGE')
+        self.assertTrue(deploy_dir_image and os.path.isdir(deploy_dir_image),
+                    f"DEPLOY_DIR_IMAGE not found or invalid: {deploy_dir_image}")
+
+        its_path = os.path.join(deploy_dir_image, 'fit-image.its')
+        self.assertTrue(os.path.exists(its_path), f"Expected ITS file not found: {its_path}")
+
+        # Read the ITS content
+        its_text = Path(its_path).read_text(encoding='utf-8', errors='ignore')
+
+        # Assertions: extended compatibles must appear
+        if "compatible" in its_text:
+            self.assertIn('subtypeA', its_text)
+        else:
+            pass
+
     def test_fit_image_ext_dtb_dtbo(self):
         """
         Summary:     Check if FIT image and Image Tree Source (its) are created correctly.
-- 
2.34.1