Re: [PATCH 1/2] kernel-fitimage: add support for custom ITS file override

[kavinaya@qti.qualcomm.com]

On Tue, Sep 23, 2025 at 02:28 AM, Adrian Freihofer wrote:

>
> Could you clarify the specific need for this custom ITS file? Perhaps
> the desired outcome could be achieved by leveraging the existing
> KERNEL_DEVICETREE_* variables without needing to introduce a separate
> custom file that overrides the generated one.
> 
> I would like to understand:
> * What specific limitations or requirements does the generated ITS
> file not meet for your use case?
> * Could you share an example of how your custom ITS file differs from
> what's generated?
>
Hi Adrian,

1.The compatible string format required by our UEFI implementation is different from what dynamic generation allows. The format we rely on is:
   compatible = "qcom,<SoC>[-<soc_version>][-<foundry_id>]-<board>[/<subtype>][-<pmic>][-<board_version>]"
   This structure enables precise identification of the appropriate DTB for each board variant.

2.As in the upstream kernel Makefile for QCOM platforms (https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/arm64/boot/dts/qcom/Makefile ), overlays are used to generate composite DTBs:
 
   dtb-$(CONFIG_ARCH_QCOM)	+= qcs6490-rb3gen2.dtb
   qcs6490-rb3gen2-vision-mezzanine-dtbs := qcs6490-rb3gen2.dtb qcs6490-rb3gen2-vision-mezzanine.dtbo
   qcs6490-rb3gen2-industrial-mezzanine-dtbs := qcs6490-rb3gen2.dtb qcs6490-rb3gen2-industrial-mezzanine.dtbo

   In the above cases, the base DTB remains the same, but the overlays differ. Without manual .its file support, we cannot assign unique compatible strings to these variants.

The ITS file generated dynamically will look like:
             configurations {
                default = "conf-qcs6490-rb3gen2.dtb";
                conf-qcs6490-rb3gen2.dtb {
                        description = "1 Linux kernel, FDT blob";
                        kernel = "kernel-1";
                        fdt = "fdt-qcs6490-rb3gen2.dtb";
                        compatible = "qcom,qcs6490-rb3gen2", "qcom,qcm6490";
                        hash-1 {
                                algo = "sha256";
                        };
                };
                conf-qcs6490-rb3gen2-vision-mezzanine.dtb {
                        description = "0 Linux kernel, FDT blob";
                        kernel = "kernel-1";
                        fdt = "fdt-qcs6490-rb3gen2-vision-mezzanine.dtb";
                        compatible = "qcom,qcs6490-rb3gen2", "qcom,qcm6490";
                        hash-1 {
                                algo = "sha256";
                        };
                };
                conf-qcs6490-rb3gen2-industrial-mezzanine.dtb {
                        description = "0 Linux kernel, FDT blob";
                        kernel = "kernel-1";
                        fdt = "fdt-qcs6490-rb3gen2-industrial-mezzanine.dtb";
                        compatible = "qcom,qcs6490-rb3gen2", "qcom,qcm6490";
                        hash-1 {
                                algo = "sha256";
                };
               };




The ITS file that would meet our requirements will look like:
        configurations {
                default = "conf-qcs6490-rb3gen2.dtb";
                conf-qcs6490-rb3gen2.dtb {
                        description = "1 Linux kernel, FDT blob";
                        kernel = "kernel-1";
                        fdt = "fdt-qcs6490-rb3gen2.dtb";
                        compatible = "qcom,qcs6490-rb3gen2", "qcom,qcm6490";
                        hash-1 {
                                algo = "sha256";
                        };
                };
                conf-qcs6490-rb3gen2-vision-mezzanine.dtb {
                        description = "0 Linux kernel, FDT blob";
                        kernel = "kernel-1";
                        fdt = "fdt-qcs6490-rb3gen2-vision-mezzanine.dtb";
                        compatible = "qcom,qcs6490-rb-subtype5";
                        hash-1 {
                                algo = "sha256";
                        };
                };
                conf-qcs6490-rb3gen2-industrial-mezzanine.dtb {
                        description = "0 Linux kernel, FDT blob";
                        kernel = "kernel-1";
                        fdt = "fdt-qcs6490-rb3gen2-industrial-mezzanine.dtb";
                        compatible = "qcom,qcs6490-rb-subtype2";
                        hash-1 {
                                algo = "sha256";
                };
               };
Please look into the difference in the compatible string in both cases.

Thanks,
Kavinaya