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

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Message-ID: <20241210115515.1886f73f@bootlin.com>
Date: Tue, 10 Dec 2024 11:55:15 +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>
Subject: Re: [PATCH 0/7] of: overlay: Add support for export-symbols node
 feature

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