Re: [Buildroot] Adding Device Tree Overlays for MIPI-CSI2 Camera and libcamera to a buildroot Build

[buildroot@cairnwater.com]

Hi Fiona,

Thank you very much!

> It's expected to match the structure of the per-arch dts directory in 
> the kernel source, with vendor directories. The 
> polyhex_debix_model_a_defconfig uses the freescale/imx8mp-debix-model-a 
> DTS, so inside your BR2_LINUX_KERNEL_CUSTOM_DTS_DIR you'll probably 
> need something like freescale/my-camera.dtso (whatever you want to call 
> your DTSOs, the DTBOs will match the source names).

How do I work out where the BR2_LINUX_KERNEL_CUSTOM_DTS_DIR is?

I had a look in the defconfig here:

https://github.com/buildroot/buildroot/blob/master/configs/polyhex_debix_model_a_defconfig

It seems to specify other parameters related to BR2_LINUX_KERNEL but not 
the BR2_LINUX_KERNEL_CUSTOM_DTS_DIR so I got a bit lost trying to work 
out where the BR2_LINUX_KERNEL_CUSTOM_DTS_DIR is and whether it's a 
directory that already exists or a new directory that I should create 
myself?

Is BR2_LINUX_KERNEL_CUSTOM_DTS_DIR something that's not already in the 
defconfig and that I'd have to add to the defconfig?

BR2_LINUX_KERNEL=y
BR2_LINUX_KERNEL_CUSTOM_VERSION=y
BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="6.12.14"
BR2_LINUX_KERNEL_USE_ARCH_DEFAULT_CONFIG=y
BR2_LINUX_KERNEL_DTS_SUPPORT=y
BR2_LINUX_KERNEL_INTREE_DTS_NAME="freescale/imx8mp-debix-model-a"
BR2_LINUX_KERNEL_DTB_KEEP_DIRNAME=y
BR2_LINUX_KERNEL_INSTALL_TARGET=y
BR2_LINUX_KERNEL_NEEDS_HOST_OPENSSL=y

> On some boards (like RPi) the firmware can also do it (only useful if 
> U-Boot & kernel then use that FDT), last time I
> checked mainline Linux cannot, but some patched kernels can.

Yes - that's my understanding as well - my knowledge of this is very 
limited but I think maybe RPi can do this and maybe NXP's i.MX Linux can 
do this but I'm not sure whether mainline Linux can do it at the moment.

> There are a number of options with U-Boot, which one to pick
> depends on how you configure the boot (at a glance the defconfig seems 
> to use extlinux.conf):

I'd probably want to stick as close as I can to what's there at the 
moment.

> Either way you may have to enable overlay support in the U-Boot build 
> config (if it isn't by default).

I looked in

https://github.com/buildroot/buildroot/blob/master/configs/polyhex_debix_model_a_defconfig

but couldn't work out how to enable overlay support. Is that usually an 
existing line in the defconfig that should be amended or a new line that 
should be added?

Thank you very much for all your help!

Will

On 2025-05-08 08:46, buildroot@cairnwater.com wrote:
> Hi Fiona,
> 
> Thank you very much - that's an enormous help - I'd have got very 
> confused trying to work that out.
> 
> I'll study what you said and the links in detail and take more time to 
> think about it.
> 
> Thank you very much again!
> 
> Will
> 
> On 2025-05-07 21:06, Fiona Klute wrote:
>> Am 07.05.25 um 20:31 schrieb buildroot@cairnwater.com:
>>> Hi Fiona,
>>> 
>>> Thank you very much! That's a big help to know to use 
>>> BR2_LINUX_KERNEL_CUSTOM_DTS_DIR for that.
>>> 
>>> What directory structure is needed for the overlays? Just the DTO 
>>> files in a directory or something more complicated?
>> 
>> It's expected to match the structure of the per-arch dts directory in 
>> the kernel source, with vendor directories. The 
>> polyhex_debix_model_a_defconfig uses the 
>> freescale/imx8mp-debix-model-a DTS, so inside your 
>> BR2_LINUX_KERNEL_CUSTOM_DTS_DIR you'll probably need something like 
>> freescale/my-camera.dtso (whatever you want to call your DTSOs, the 
>> DTBOs will match the source names).
>> 
>>>> From there you'll need to adjust the boot process for your board to 
>>>> actually apply the overlay. If you want to send a patch, the 
>>>> BR2_LINUX_KERNEL_CUSTOM_DTS_DIR should be inside the board 
>>>> directory.
>>> 
>>> Thank you very much. I don't know how to do that. What's the best way 
>>> to get the boot process to apply the overlay? I'd heard that it can 
>>> be done by the kernel or by U-Boot but I'm doing this for the first 
>>> time so I'm not sure what I'm doing.
>> 
>> I'm not familiar with the board, but usually U-Boot will be the right 
>> place. On some boards (like RPi) the firmware can also do it (only 
>> useful if U-Boot & kernel then use that FDT), last time I checked 
>> mainline Linux cannot, but some patched kernels can. There are a 
>> number of options with U-Boot, which one to pick depends on how you 
>> configure the boot (at a glance the defconfig seems to use 
>> extlinux.conf):
>> 
>> * If you use extlinux.conf, use the "devicetree-overlay" option.
>> * If you use a boot script, load the overlay and apply it before 
>> starting the kernel.
>> * If you use a FIT image, add the overlay to it and the relevant boot 
>> config defined in it.
>> 
>> See also:
>> https://docs.u-boot.org/en/latest/develop/distro.html (for 
>> extlinux.conf)
>> https://docs.u-boot.org/en/latest/usage/fdt_overlays.html (for the 
>> latter 2 options)
>> 
>> Either way you may have to enable overlay support in the U-Boot build 
>> config (if it isn't by default).
>> 
>> On the Buildroot side you need to make sure that the DTBOs and 
>> adjusted config get included in the image where U-Boot expects them 
>> (depending on the boot setup you use), if you need to make adjustments 
>> there it'll probably be in a post-build or post-image script, or in 
>> the genimage config.
>> 
>> The defconfig seems to put everything into the root partition image, 
>> so it might just work, as long as you use the right path in the U-Boot 
>> config. Maybe just set BR2_LINUX_KERNEL_CUSTOM_DTS_DIR, build, and see 
>> if you end up with a DTBO somewhere in your /boot dir. If yes, 
>> adjusting the U-Boot config to use it should be enough. ;-)
>> 
>> Best regards,
>> Fiona
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