Re: Barebox 2020.01.0 and Device Tree from Linux Kernel 5.4

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Hello,

On 1/24/20 4:32 PM, gianluca wrote:
> I was wondering if the device tree from (latest) Linux Kernel can be used when building Barebox 2020.01.0 for iMX6 compatible custom board.
> For sure the include path and other stuff are quite different (kernel and barebox), so I am pretty sure it will fail to build with some sort of "foreign" device-tree.

barebox already regularly imports Linux's device tree directories into its dts/ directory.
v2020.01.0 contains the v5.4-rc7 state.

Another issue, will be the drivers to have full access from Barebox-Point-Of-View. If the device-tree properties and compatible strings are quite different, Barebox will fail to activating/probing/using the internal driver.

barebox driver compatibles should be aligned with the kernel's.
If a barebox driver lacks handling for a property, the driver can be extended.

> So I am asking:
> There is a "official" way to manage those differencies?

I recently added a short section about this in the Documentation
(https://barebox.org/doc/latest/devicetree/index.html):

  "For supporting architectures, barebox device trees are located in arch/$ARCH/dts.
   Usually the barebox board.dts imports the upstream device tree under dts/src/$ARCH
   with #include "$ARCH/board.dts" and then extends it with barebox-specifics like
   Barebox state, environment or boot-time device configuration."

Take a look at arch/arm/dts/imx6q-marsboard.dts to see how that looks in practice.
The kernel device tree is reused as is and extended slightly for barebox use.

> Am I forced to use TWO device-tree dts files (almost identical each other) ???
> 
> It seems it is like re-inventing the wheel...

No, as you can can extend the kernel device tree (see below).

> I think this procedure can be used with ALL supported boards present in barebox source tree.
> 
> Actually I am using a slightly modified device-tree for my boards from kernel, then adapted to be compiled for Barebox.
> Some nodes are "disabled" by default as status, so Barebox will activate them when probing the real-hardware.

I think

#include <arm/imx6q-yourboard.dts>

&device {
	status = "okay";
};

is what you're after.

> I would like to keep my code to activate/deactivate device-tree drivers, but without modifying the Kernel device-tree each time when I do a Kernel update.
> 
> Any hint will be very accepted.

HTH,
Ahmad

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