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

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Date:   Thu, 5 Apr 2018 21:17:25 +0200
From:   Jan Kiszka <jan.kiszka@....de>
To:     Frank Rowand <frowand.list@...il.com>,
        Rob Herring <robh+dt@...nel.org>
Cc:     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 2018-04-05 21:13, Frank Rowand wrote:
> On 04/04/18 17:55, Rob Herring wrote:
>> 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.
> 
> I don't think it will work either.  The drivers won't be probed if the
> nodes are disabled.
> 

That would be the purpose of registering it disabled. The question is:
Would it be probed at the point when we enabled it while being applied?
If not, strike this.

Jan