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Subject: Re: [PATCH v7 2/5] of: change overlay apply input data from
 unflattened to FDT
To:     Rob Herring <robh+dt@kernel.org>
Cc:     Frank Rowand <frowand.list@gmail.com>,
        Pantelis Antoniou <pantelis.antoniou@konsulko.com>,
        Pantelis Antoniou <panto@antoniou-consulting.com>,
        devicetree@vger.kernel.org,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>,
        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 09:22:11 +0200
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On 2018-04-05 02:55, Rob Herring wrote:
> On Wed, Apr 4, 2018 at 5:35 PM, Jan Kiszka <jan.kiszka@web.de> wrote:
>> 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?
> 
> No. I don't think that will work.
> 
> The of_overlay_apply() function is still there, but static. We can
> export it again if the need arises.

That would be the simplest solution from our perspective, but I'm not
sure if that is in the original spirit of this change.

> 
> Another option is there is a notifier callback OF_OVERLAY_PRE_APPLY,
> but I'm not sure we want to make that be the normal interface to make
> modifications.

And would calling modification functions from that callback be legal at all?

Jan