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

[Frank Rowand <frowand.list@gmail.com>]

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@gmail.com wrote:
>>>> From: Frank Rowand <frank.rowand@sony.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/.


>>
>> 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.


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