Building uImage with appended DTB

Building uImage with appended DTB

[rgovostes]

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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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Re: Building uImage with appended DTB

[Ryan Govostes]

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A solution that seems to work is to create a new .bbclass inherited from my layer's linux .bbappend file that implements uboot_prep_kimage:append() to append the DTB to linux.bin. This code would run at the beginning of do_uboot_mkimage() before the uImage is created.

There are a couple of prerequisites such as setting KEEPUIMAGE = "yes" and KERNEL_DEVICETREE_BUNDLE = "0".

https://gist.github.com/rgov/0628785685ab858a99c5bfca626c1d8f

I would still be curious to know why KERNEL_DEVICETREE_BUNDLE doesn't work as expected if that is supposed to be the supported method of achieving this, but my solution at least lets me move on.

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Re: [yocto] Building uImage with appended DTB

[Nicolas Jeker]

> On 17 Jan 2022, at 06:41, rgovostes@gmail.com wrote:
> 
> 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.

Just a short hint if you didn’t discover it yourself, there is a (very) old BSP available from PHYTEC [1]. The downloads don’t seem to work, but they’re available on a probably forgotten FTP server [2].

[1]: https://web.archive.org/web/20120501212117/http://www.phytec.com/products/linux/bsp-LPC3180.html
[2]: ftp://ftp.phytec.com/temp/PCM-031_phyCORE-LPC3180

> 
> 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
>