[U-Boot-Users] fdt chosen node handling

[U-Boot-Users] fdt chosen node handling

[Stefan Roese]

Hi,

I am wondering how the device tree "chosen" node should be handled, if it 
already exists in the device tree blob given to U-Boot. When this node 
doesn't exist it gets created and everything is fine. But if it already 
exists, it doesn't get overwritten and the defaults from the device tree blob 
most of the time are not what I really want.

So I would like U-Boot to overwrite the existing chosen node. I discovered 
that the fdt_chosen() function already has this "force" parameter, but it is 
not set in the current U-Boot implementation. I would like to set it for my 
board ports and I am wondering how I should handle this. Should this be done 
via a compile switch (#define), or via an env variable, or...?

Any ideas/comments on this?

Thanks.

Best regards,
Stefan

=====================================================================
DENX Software Engineering GmbH,     MD: Wolfgang Denk & Detlev Zundel
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Phone: +49-8142-66989-0 Fax: +49-8142-66989-80  Email: office at denx.de
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[U-Boot-Users] fdt chosen node handling

[Jerry Van Baren]

Stefan Roese wrote:
> Hi,
> 
> I am wondering how the device tree "chosen" node should be handled, if it 
> already exists in the device tree blob given to U-Boot. When this node 
> doesn't exist it gets created and everything is fine. But if it already 
> exists, it doesn't get overwritten and the defaults from the device tree blob 
> most of the time are not what I really want.
> 
> So I would like U-Boot to overwrite the existing chosen node. I discovered 
> that the fdt_chosen() function already has this "force" parameter, but it is 
> not set in the current U-Boot implementation. I would like to set it for my 
> board ports and I am wondering how I should handle this. Should this be done 
> via a compile switch (#define), or via an env variable, or...?
> 
> Any ideas/comments on this?
> 
> Thanks.
> 
> Best regards,
> Stefan

Hi Stefan,

Disclaimer: I am not a OF expert, I have not read the OF documentation 
cover to cover and do not understand many implications of what I've 
read, much less what I have _not_ read.

1) I believe the /chosen node should be auto-created.

2) As you point out, if the /chosen node already exists the current code 
bails out and doesn't change anything.  This was done as a very badly 
misguided attempt to make libfdt things work like bd_t things.  The 
granularity should be on the individual properties, not on /chosen.  I 
consider this a bug and have it on my "todo" list to fix (actually, I 
had created a patch, but I need to rebase and publish it).

<http://www.denx.de/wiki/view/UBoot/UBootFdtInfo#Outstanding_Patches>

"Unpublished patch of JerryVanBaren, implementing a suggestion by Scott 
Wood to make the /chosen handling finer grained: if the /chosen node 
exists, currently u-boot-fdt bails out. A better methodology is to not 
overwrite pre-existing properties on a per-property basis, so if /chosen 
exists but a necessary /chosen/property doesn't, it gets created. Good 
idea, but not published and poorly tested at the moment."

Best regards,
gvb

[U-Boot-Users] fdt chosen node handling

[Stefan Roese]

Hi Jerry,

On Wednesday 19 December 2007, Jerry Van Baren wrote:
> Disclaimer: I am not a OF expert, I have not read the OF documentation
> cover to cover and do not understand many implications of what I've
> read, much less what I have _not_ read.
>
> 1) I believe the /chosen node should be auto-created.

Ufff. :)

> 2) As you point out, if the /chosen node already exists the current code
> bails out and doesn't change anything.  This was done as a very badly
> misguided attempt to make libfdt things work like bd_t things.  The
> granularity should be on the individual properties, not on /chosen.  I
> consider this a bug and have it on my "todo" list to fix (actually, I
> had created a patch, but I need to rebase and publish it).
>
> <http://www.denx.de/wiki/view/UBoot/UBootFdtInfo#Outstanding_Patches>
>
> "Unpublished patch of JerryVanBaren, implementing a suggestion by Scott
> Wood to make the /chosen handling finer grained: if the /chosen node
> exists, currently u-boot-fdt bails out. A better methodology is to not
> overwrite pre-existing properties on a per-property basis, so if /chosen
> exists but a necessary /chosen/property doesn't, it gets created. Good
> idea, but not published and poorly tested at the moment."

OK, thanks for these infos.

Best regards,
Stefan

=====================================================================
DENX Software Engineering GmbH,     MD: Wolfgang Denk & Detlev Zundel
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Phone: +49-8142-66989-0 Fax: +49-8142-66989-80  Email: office at denx.de
=====================================================================