Building uImage with appended DTB


Ryan Govostes
 

I'm a complete newcomer to Yocto, trying to translate a manual process of building software for an embedded device based on the PHYTEC phyCORE-LPC3250 SOM. I did not find an existing BSP layer for this board so I am trying to create my own.

When I build a kernel for this device manually, I use the CONFIG_APPENDED_DTB=y option, which instructs the kernel to find the device tree right after the kernel image itself. I build the zImage, then append my DTB and finally create the uImage file:

    cat arch/arm/boot/zImage ./arch/arm/boot/dts/lpc3250-phy3250.dtb > arch/arm/boot/zImage-dtb
    mkimage -A arm -O linux -C none -T kernel -a 0x80008000 -e 0x80008000 -n Linux -d arch/arm/boot/zImage-dtb arch/arm/boot/uImage-dtb

I'm trying to figure out how to properly replicate this process in Yocto.

I see that Poky's meta/classes/kernel-devicetree.bbclass uses the KERNEL_DEVICETREE_BUNDLE variable to do something similar. If I have KERNEL_IMAGETYPE set to "zImage", then it will create a zImage+DTB file.

However, it expects the zImage file to be called "zImage", and the one that gets built is called "zImage-5.14.6-yocto-standard". Since it cannot find "zImage", the build fails.

    cat: .../tmp/work/lpc3250-poky-linux-gnueabi/linux-yocto/5.14.6+gitAUTOINC+4f4ad2c808_7ae156be3b-r0/image/boot/zImage: No such file or directory


I also see that meta-ti has a bundle-devicetree.inc file that has a similar aim, but it deals with uImages instead.

https://git.yoctoproject.org/meta-ti/tree/recipes-kernel/linux/bundle-devicetree.inc

My concern with using this is that it appends the DTB directly to the uImage. However the uImage header structure includes a field for data size and another one for checksum. These fields are only computed correctly if the DTB is already appended before the uImage file is built.


I think if I can figure out how to make KERNEL_DEVICETREE_BUNDLE create the zImage+DTB file, then I can write a do_deploy:append() function that invokes mkimage to produce the uImage. But I'm not sure how to diagnose the problem.

Thanks,
Ryan

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