linux-kernel - Re: [PATCH v7 2/5] of: change overlay apply input data from unflattened to FDT

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Date:   Wed, 4 Apr 2018 19:55:36 -0500
From:   Rob Herring <robh+dt@...nel.org>
To:     Jan Kiszka <jan.kiszka@....de>
Cc:     Frank Rowand <frowand.list@...il.com>,
        Pantelis Antoniou <pantelis.antoniou@...sulko.com>,
        Pantelis Antoniou <panto@...oniou-consulting.com>,
        devicetree@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>,
        Jailhouse <jailhouse-dev@...glegroups.com>
Subject: Re: [PATCH v7 2/5] of: change overlay apply input data from
 unflattened to FDT

On Wed, Apr 4, 2018 at 5:35 PM, Jan Kiszka <jan.kiszka@....de> wrote:
> Hi Frank,
>
> On 2018-03-04 01:17, frowand.list@...il.com wrote:
>> From: Frank Rowand <frank.rowand@...y.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.

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.

Rob