dts: appending to string property value?

[cxw42]

First, thank you for dtc! I would hate to hex-edit dtb files :) .

Is it possible to append to an existing string property value? If not, could that option be added?

Use case: Nvidia Jetson Xavier, which reads its kernel command line from the dtb, node /chosen/bootargs. At $work we have a common .dtsi file that is #included by individual .dts files for different configurations. I would like to be able to set the bootargs in the dtsi file, then append to it in the dts files. A dummy example follows.

My apologies if this function exists and I have not found it in my searching. Thank you for considering this request!

common.dtsi:

/dts-v1/;
/ { chosen { bootargs = "root=/dev/mmcblk0p1"; }; };

particular.dts:

#include "common.dtsi"
/ { chosen { bootargs += " console=ttyTCU0,115200n8"; }; };

[TheSven73]

If you need this new function just to set the debug console in your particular.dts, consider using stdout-path instead.

[cxw42]

@TheSven73 thanks for the tip! I was trying to provide a minimal example, but in my actual use case we are using custom bootargs read by a bespoke driver. I will keep stdout-path in mind for the future, though!

[TheSven73]

Hmm... instead of letting your driver read bootargs, you could have it look for a specific node included by your particular.dts. Or better still, let particular.dts add to the driver's own node, if it exists.

But I have the feeling you must have considered this already, and found it wanting...

[dgibson]

On Wed, Apr 29, 2020 at 05:39:31PM +0000, Chris White wrote:

First, thank you for dtc --- I would hate to hex-edit dtb files :) .

Is it possible to append to an existing string property value?

I'm afraid not.

If not, could that option be added?

Theoretically, yes. It would be easily to hack in, but much, much harder to implement in a way that has solid semantics that don't conflict with other things.

Use case: Nvidia Jetson Xavier, which reads its kernel command line from the dtb, node /chosen/bootargs. At $work we have a common .dtsi file that is #included by individual .dts files for different configurations. I would like to be able to set the bootargs in the dtsi file, then append to it in the dts files. A dummy example follows.

My apologies if this function exists and I have not found it in my searching. Thank you for considering this request!

common.dtsi:

/dts-v1/;
/ { chosen { bootargs = "root=/dev/mmcblk0p1"; }; };

particular.dts:

#include "common.dtsi"
/ { chosen { bootargs += " console=ttyTCU0,115200n8"; }; };

Honestly, for this simple case, I think a much more feasible approach would be to write a small helper program using libfdt that does the append on an already-compiled .dtb.

-- David Gibson | I'll have my music baroque, and my code david AT gibson.dropbear.id.au | minimalist, thank you. NOT the other | way around! http://www.ozlabs.org/~dgibson

[cxw42]

@dgibson thanks for getting back to me! Since I opened this issue, NVIDIA has changed the Xavier boot path to use U-boot+extlinux, so we can now append command-line args in extlinux.conf. Since there's a viable workaround and appending would be nontrivial to add to dtc and the spec, I will withdraw this request.