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

[Jan Kiszka]

On 2018-04-05 21:28, Frank Rowand 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@xxxxxxxxx wrote:
>>>>> From: Frank Rowand <frank.rowand@xxxxxxxx>
>>>>>
>>>>> 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/.
> 

Ah, libfdt is available for kernel drivers as well. That looks like a
viable path on first sight. I'll try that and come back in case it does
not solve all issues.

> 
>>>
>>> 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).
> 
> If I understand create_vpci_of_overlay() correctly, it is assuming a
> fixed size of 4 cells.  Line 314 is: for (n = 0, cell = 0; n < 4; n++) {
> 
> Off the top of my head, it is theoretically possible to create a property
> that can contain the largest possible size for the property, then shrink
> the size by inserting NOPs at the end of the property to shrink it.

Well, I even find fdt_appendprop which sounds like we could keep adding
that property on the fly.

How does memory management work with libfdt? Do I have to ensure that
the fdt is already backed by an area large enough also for it modified form?

Thanks,
Jan