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

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Message-ID: <a21dbee8-6e4e-9ceb-c798-dda43c2eae2d@web.de>
Date:   Thu, 5 Apr 2018 21:13:09 +0200
From:   Jan Kiszka <jan.kiszka@....de>
To:     Frank Rowand <frowand.list@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        pantelis.antoniou@...sulko.com,
        Pantelis Antoniou <panto@...oniou-consulting.com>
Cc:     devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        geert@...ux-m68k.org, 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 20:59, Frank Rowand wrote:
> Hi Jan,
> 
> On 04/04/18 15:35, Jan Kiszka 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?
> 
> Thank you for the pointer to the driver - that makes it much easier to
> understand the use case and consider solutions.
> 
> If you can make the changes directly on the FDT instead of on the
> expanded devicetree, then you could move to the new API.

Are there some examples/references on how to edit FDTs in-place in the
kernel? I'd like to avoid writing the n-th FDT parser/generator.

> 
> Looking at the driver, I see one potential issue with that approach.
> The property "interrupt-map" is added directly to the changeset
> instead of being an existing property in the overlay.  Is it possible
> to have this property in the overlay when needed?

Well, the size of that property has a runtime dependency on the gic's
#address-cells. If that is easy to account for depends a bit on the
available FDT manipulation services. Or it would take multiple templates
to handle the different cases (0, 1, or 2 IIRC).

> 
> I'll also reply to other comments in this thread.

TIA!

Jan