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Subject: Re: [PATCH v7 2/5] of: change overlay apply input data from
 unflattened to FDT
To:     frowand.list@gmail.com, Rob Herring <robh+dt@kernel.org>,
        pantelis.antoniou@konsulko.com,
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        Jailhouse <jailhouse-dev@googlegroups.com>
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From:   Jan Kiszka <jan.kiszka@web.de>
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Date:   Thu, 5 Apr 2018 00:35:42 +0200
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Hi Frank,

On 2018-03-04 01:17, frowand.list@gmail.com wrote:
> From: Frank Rowand <frank.rowand@sony.com>
> 
> Move duplicating and unflattening of an overlay flattened devicetree
> (FDT) into the overlay application code.  To accomplish this,
> of_overlay_apply() is replaced by of_overlay_fdt_apply().
> 
> The copy of the FDT (aka "duplicate FDT") now belongs to devicetree
> code, which is thus responsible for freeing the duplicate FDT.  The
> caller of of_overlay_fdt_apply() remains responsible for freeing the
> original FDT.
> 
> The unflattened devicetree now belongs to devicetree code, which is
> thus responsible for freeing the unflattened devicetree.
> 
> These ownership changes prevent early freeing of the duplicated FDT
> or the unflattened devicetree, which could result in use after free
> errors.
> 
> of_overlay_fdt_apply() is a private function for the anticipated
> overlay loader.

We are using of_fdt_unflatten_tree + of_overlay_apply in the
(out-of-tree) Jailhouse loader driver in order to register a virtual
device during hypervisor activation with Linux. The DT overlay is
created from a a template but modified prior to application to account
for runtime-specific parameters. See [1] for the current implementation.

I'm now wondering how to model that scenario best with the new API.
Given that the loader lost ownership of the unflattened tree but the
modification API exist only for the that DT state, I'm not yet seeing a
clear solution. Should we apply the template in disabled form (status =
"disabled"), modify it, and then activate it while it is already applied?

Thanks,
Jan

[1]
https://github.com/siemens/jailhouse/blob/11ef401a9064644bf7faf28e35c97f27ececd81d/driver/pci.c#L280