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

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Date:   Wed, 11 Apr 2018 07:42:13 +0200
From:   Jan Kiszka <jan.kiszka@....de>
To:     Rob Herring <robh+dt@...nel.org>,
        Frank Rowand <frowand.list@...il.com>
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 23:12, Rob Herring wrote:
> On Thu, Apr 5, 2018 at 2:28 PM, Frank Rowand <frowand.list@...il.com> wrote:
>> On 04/05/18 12:13, Jan Kiszka wrote:
>>> 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.
>>
>> I don't know of any existing in-kernel edits of the FDT (but they might
>> exist).  The functions to access an FDT are in libfdt, which is in
>> scripts/dtc/libfdt/.
> 
> Let's please not go down that route of doing FDT modifications. There
> is little reason to other than for early boot changes. And it is much
> easier to work on unflattened trees.

I just briefly looked into libfdt, and it would have meant building it
into the module as there are no library functions exported by the kernel
either. Another reason to drop that.

What's apparently working now is the pattern I initially suggested:
Register template with status = "disabled" as overlay, then prepare and
apply changeset that contains all needed modifications and sets the
status to "ok". I might be leaking additional resources, but to find
that out, I will now finally have to resolve clean unbinding of the
generic PCI host controller [1] first.

Thanks,
Jan

[1] http://lkml.iu.edu/hypermail/linux/kernel/1606.3/00072.html