Re: [PATCH 0/7] of: overlay: Add support for export-symbols node feature

[Ayush Singh <ayush@beagleboard.org>]

On 08/01/25 13:37, Herve Codina wrote:
> Hi Ayush,
> 
> On Wed, 8 Jan 2025 13:06:03 +0530
> Ayush Singh <ayush@beagleboard.org> wrote:
> 
>> On 10/12/24 16:25, Herve Codina wrote:
>>> Hi Ayush,
>>>
>>> On Tue, 10 Dec 2024 15:26:44 +0530
>>> Ayush Singh <ayush@beagleboard.org> wrote:
>>>    
>>>> On 10/12/24 15:11, Herve Codina wrote:
>>>>> Hi Ayush,
>>>>>
>>>>> On Tue, 10 Dec 2024 14:52:22 +0530
>>>>> Ayush Singh <ayush@beagleboard.org> wrote:
>>>>>
>>>>> ...
>>>>>>
>>>>>> What is the reason for not using symbols directly as described here [3]?
>>>>>>
>>>>>> I do like this approach since it does not pollute the global symbols.
>>>>>> Just want to know if there are any other reasons for it.
>>>>>>      
>>>>>
>>>>> Modifying the __symbols__ node at runtime (adding / removing properties in
>>>>> it) exposes memory leaks if __symbols__ already exist in the live DT.
>>>>> This __symbols__ node exist if the dtb was compiled with '-@' or if you
>>>>> chain the overlay (i.e. __symbols__ node created by the first overlay).
>>>>
>>>> Yeah, that is a problem, specially in a setup which might involve
>>>> hot-plugging.
>>>>   
>>>>>
>>>>> I think also that some conflicts can appears. What happens if you want to
>>>>> add a new label but this label is already present for some other purpose?
>>>>
>>>> I do not think that actually is a problem. As described in the original
>>>> patch [0], the symbol and connector overlay is supposed to be applied as
>>>> a group (overwriting any conflicting symbols in the process).
>>>>
>>>> The reason why this is not a problem is that `__symbols__` are only used
>>>> to resolve the phandles (overlays do not support path references yet),
>>>> but do not really have a purpose in the livetree (at least far as I
>>>> know, but I can be wrong).
>>>>   
>>>>>
>>>>> Best regards,
>>>>> Hervé
>>>>
>>>> [0]: https://lore.kernel.org/lkml/20240702164403.29067-1-afd@ti.com/
>>>
>>>
>>> Also, in your first overlay (adding symbols in __sympbols__ node), you have
>>> something like:
>>>      GROVE_PIN1_MUX_I2C_SCL = "/bus@f0000/pinctrl@f4000/grove-i2c-pins";
>>>
>>> If I understood correctly, other overlays will have GROVE_PIN1_MUX_I2C_SCL
>>> as unresolved symbols and will use GROVE_PIN1_MUX_I2C_SCL to reference the
>>> grove-i2c-pins node.
>>> This unresolved symbol from the overlay is resolved thanks to the __symbols__
>>> table where you added GROVE_PIN1_MUX_I2C_SCL (first overlay operation).
>>>
>>> In order to work, you need to have a phandle property set in the
>>> grove-i2c-pins node.
>>>
>>> This is done by dtc when you compile the dtb containing the grove-i2c-pins
>>> node (i.e. k3-am625-beagleplay.dts)
>>>
>>> The phandle property will be set only if:
>>> - a label for grove-i2c-pins already exist and -@ option is used
>>> or
>>> - a label for grove-i2c-pins already exist and it is referenced as a phandle
>>>     in the dts (k3-am625-beagleplay.dts).
>>>
>>> Otherwise, dtc will not create the phandle property and without this
>>> property, the symbol resolution will not be correct.
>>>
>>> Best regards,
>>> Hervé
>>>    
>>
>> Hello Hervé
>>
>> Thanks for the clarification. things have changed a bit since the last
>> message and it seems like trying to add path reference support to
>> overlays is not the best way forward [0]. So I would love to help move
>> this approach forward.
>>
>> I do have a question regarding this approach, so here I go:
>>
>> Can the `export-symbols` node be added to devicetree spec and be
>> resolved by the devicetree compiler (and fdtoverlay) instead of being
>> runtime resolution.
> 
> Of course, a solution with fdtoverlay is welcome but it should not fully
> replace the runtime resolution. In our case, we need runtime resolution
> because the overlay is loaded by a driver.
> 
> Both resolutions (fdtoverlay and runtime) should work.

I see, it seems linux does not use libfdt for applying overlays internally.

> 
>>
>> To get some context, I would like to share the addon-board overlays
>> between ZephyrRTOS and Linux kernel. I would be happy to try adding
>> support to dtc compiler for it. I am also tagging David Gibson (dtc
>> maintainer) in this discussion since he also had some ideas regarding
>> the feasibility and pitfalls of adding it to devicetree compiler (and spec).
>>
>>
>> [0]:
>> https://lore.kernel.org/devicetree-compiler/6b2dba90-3c52-4933-88f3-b47f96dc7710@beagleboard.org/T/#m900b5ca13cfc28396d4d46d9c3130a7070fa8c90
>>
>> Best regards,
>> Ayush Singh
>>
> 
> Thanks for your help proposal!
> 
> Best regards,
> Hervé

I will experiment with adding support to dtc and see how things look. 
Hopefully, 2025 is the year of addon board support.

Best regards,
Ayush Singh