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

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Message-ID: <20250108090750.45685a50@bootlin.com>
Date: Wed, 8 Jan 2025 09:07:50 +0100
From: Herve Codina <herve.codina@...tlin.com>
To: Ayush Singh <ayush@...gleboard.org>
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

Hi Ayush,

On Wed, 8 Jan 2025 13:06:03 +0530
Ayush Singh <ayush@...gleboard.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@...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.

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.

> 
> 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