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 12:16:52 -0700
From:   Frank Rowand <frowand.list@...il.com>
To:     Jan Kiszka <jan.kiszka@....de>, 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>,
        Frank Rowand <frowand.list@...il.com>
Subject: Re: [PATCH v7 2/5] of: change overlay apply input data from
 unflattened to FDT

On 04/05/18 00:22, Jan Kiszka wrote:
> On 2018-04-05 02:55, 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.
>>
>> 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.

For short term out of tree usage, exporting of_overlay_apply() is ok.

Yes, for in-tree, exporting it again defeats the attempted process to
solve the overlay issues to make them acceptable in main line.


>>
>> 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?

It might work in some specific cases, but the result is undefined.


> 
> Jan
>