[yocto] [meta-raspberrypi] Adding an additional device tree overlay in the SDCard

[Herve Jourdain]

Hi François,

 

Well, if the .dtb might not get removed, like you describe, then changing the sdcard_image-rpi.bbclass the way I do might for sure create issues at some point.

It works for me – obviously – but definitely can’t be generalized.

Thanks for the insight :)

 

Best regards,

 

Herve

 

From: Francois Muller [mailto:forlorn18 at gmail.com] 
Sent: vendredi 31 juillet 2015 17:16
To: Petter Mabäcker <petter at technux.se>; Herve Jourdain <herve.jourdain at neuf.fr>; Andrei Gherzan <andrei at gherzan.ro>
Cc: yocto at yoctoproject.org
Subject: Re: [yocto] [meta-raspberrypi] Adding an additional device tree overlay in the SDCard

 

Hi,

 

My comment below.

 

Best regards,

François

 

Le vendredi 31 juillet 2015 à 15:29 +0200, Petter Mabäcker a écrit :

Hi Herve,

See inline answers.

BR,
Petter

On 07/30/2015 05:27 AM, Herve Jourdain wrote:

Hello,

 

I’m trying to add a specific .dts file to the raspberrypi.

Therefore, in my own layer, I have made a linux-raspberrypi_3.%.bbappend, that applies a patch to the arch/arm/boot/dts makefile to add my own layer, and a new rule that copies my specific .dts file to the kernel source directory.

And, initially, I had also defined KERNEL_DEVICETREE += “myown-overlay.dtb” in there.

 

It definitely compiles well, installation process is smooth, I get the Image-myown-overlay.dtb in tmp/deploy/raspberrypi2.

 

BUT when I flash my SDCard, it does NOT appear in the “overlays” directory of the DOS partition!

 

If I add it manually, everything works fine, so I know my overlay has been correctly generated and is working.

 

What I discovered is that, the sdcard_image_rpi class takes the list of .dtb to be considered from split_overlays(), which is defined in linux-raspberrypi-base class.

And split_overlays() basically  gets the value of KERNEL_DEVICETREE (dts = d.getVar("KERNEL_DEVICETREE", True)).

 

But when I set KERNEL_DEVICETREE in linux-raspberrypi_3.%.bbappend, it seems the additional value it defines is NOT taken into account when linux-raspberry-class evaluates the value of KERNEL_DEVICETREE.

I’ve checked in the create_image.rpi-sdimg script created, it does not appear in DTS nor DT_OVERLAYS.

 

But when I define KERNEL_DEVICETREE in my distro.conf, THEN it is taken into account…

 

My goal would be to add that overlay by NOT modifying distro.conf (distros are not necessarily RaspberryPi centric) nor in the raspberry machine files (or local.conf).

This leads me to several questions:

#1 Is there a specific reason why my addition to KERNEL_DEVICETREE gets lost when creating the image? I mean, is it normal, or are there some steps I missed there?

Yes, in the current solution KERNEL_DEVICETREE is needed in both the kernel recipe and image creation context. When appending KERNEL_DEVICETREE in linux-raspberrypi_3.%.bbappend your new overlay will only be visible within the linux-raspberrypi scope and NOT within the image class.




#2 Is there a way that I could achieve adding my .dtb overlay in the sdcard image only from my linux kernel bbappend?


If I remember the code correct within the kernel scope we are only interesting if device tree should be used. At least in theory (assuming you are using a >= 3.18 kernel since then KERNEL_DEVICETREE will bve set by default) you should be able to skip the KERNEL_DEVICETREE += code in linux-raspberrypi_3.%.bbappend (since it will be set any way and the overlays themselves isn't really used in here..) then append the KERNEL_DEVICETREE within the image scope. If you don't want to put this into the layer.conf or similiar in your own layer I guess it should work to "prepend" for example the IMAGE_CMD. But in the current solution I guess local.conf, layer.conf (in your own layer) or rpi-base.inc (in meta-raspberrypi) is the easiest options..

 

If modifying *.inc / *.conf files is not an option, then I would give a try to Petter's suggestion to create a new image recipe (or bbappending the current one). I see no reason this shouldn't do the trick, while still being easy.

 





#3 On a more general note, wouldn’t it be simpler (better?) to just add all the .dtb that are found in the deploy/images/ directory to the DOS partition of the SDCard? After all, the .dtb need to be compiled from a .dts, so all the .dtb that appear in deploy/images are “legit”.


That might perhaps be a better solution. Have you tried it out? Andrei or Francois might have some comments about this?

 

As far as I can remember, deploy/images/ links are only updated (meaning created if not previously present, updated if a new artefact is available) but never removed. This implies that if you simply copy 'deploy/images/*-overlay.dtb'  to the sdcard, your image may end up with some devicetree overlays that are potentially no longer wanted or (even worse) not to be used with the specific kernel that will boot up the board...

That's the reason why I think choosing consciously the devicetree (overlay) set to pack in the sdcard may be important.

 

 

Herve





 

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