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

[Frank Rowand]

On 04/05/18 12:38, Jan Kiszka wrote:
> 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?

I have not looked at those functions, so I don't know.


> 
> Thanks,
> Jan
>