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

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Message-ID: <6d48095d-59b1-4439-8e2a-927aa1aa1b55@beagleboard.org>
Date: Wed, 8 Jan 2025 13:06:03 +0530
From: Ayush Singh <ayush@...gleboard.org>
To: Herve Codina <herve.codina@...tlin.com>
Cc: Andrew Davis <afd@...com>, Geert Uytterhoeven <geert@...ux-m68k.org>,
 Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Conor Dooley <conor+dt@...nel.org>, Arnd Bergmann <arnd@...db.de>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Saravana Kannan <saravanak@...gle.com>, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, Luca Ceresoli <luca.ceresoli@...tlin.com>,
 Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
 David Gibson <david@...son.dropbear.id.au>
Subject: Re: [PATCH 0/7] of: overlay: Add support for export-symbols node
 feature



On 10/12/24 16:25, Herve Codina wrote:
> Hi Ayush,
> 
> On Tue, 10 Dec 2024 15:26:44 +0530
> Ayush Singh <ayush@...gleboard.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@...gleboard.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@...00/pinctrl@...00/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.

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