linux-kernel - Re: [PATCH 0/2] {vision/navigation}-mezzanine: Fix overlay root node

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Message-ID: <62a53d3222dfc516accd8dcd5e1adca0@trvn.ru>
Date: Thu, 27 Feb 2025 14:01:41 +0500
From: Nikita Travkin <nikita@...n.ru>
To: Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Cc: Bjorn Andersson <andersson@...nel.org>, Konrad Dybcio
 <konradybcio@...nel.org>, Rob Herring <robh@...nel.org>, Krzysztof Kozlowski
 <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, Bryan O'Donoghue
 <bryan.odonoghue@...aro.org>, Konrad Dybcio
 <konrad.dybcio@....qualcomm.com>, linux-arm-msm@...r.kernel.org,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] {vision/navigation}-mezzanine: Fix overlay root node

Dmitry Baryshkov писал(а) 27.02.2025 09:16:
> On Wed, Feb 26, 2025 at 07:29:54PM +0500, Nikita Travkin wrote:
>> While considering to propose WoA EL2 dt overlays upstream I was looking
>> at existing overlays and noticed that some of them are broken: they put
>> seemingly meaningful fixups into the overlay's "/" node, which places
>> them into the overlay "metadata" itself, not into a fixup fragment to be
>> applied to the actual dtb. This series fixes those two by changing to
>> full path "&{/}" which should work as it was initially intended.
>> 
>> See demonstration of the problem below:
>> 
[...]
>> $ dtc extra.dtbo
>> /dts-v1/;
>> 
>> / {
>> 	foo;
>> 
>> 	bar {
>> 		baz;
>> 	};
> 
> Is this behaviour documented somewhere? I'd say, it would be a surprise
> to me.
> 

According to dtc docs [1],

   3.b) The Device Tree fragments must be compiled with the same option but they
   must also have a tag (/plugin/) that allows undefined references to nodes
   that are not present at compilation time to be recorded so that the runtime
   loader can fix them.

so per my understanding "plugin" directive only changes the meaning of
references (i.e. stuff with "&"), to generate fragments/fixups, which
are the only way libfdt combines overlays into the base dtb.

I suppose the old way of "manually" writing fragments (and thus writing
to / as raw nodes) was kept because phandle/path based updates were
added later to dtc and many overlays were still defining raw fragments...

"/" also allows one to write "raw" nodes into the overlay, which is
sometimes used by downstreams. (i.e. they put custom extensions to the
overlay format [2] or add metadata into / of the dtbo like "compatible"
values to reject incompatible overlays from applying. [3]) This is
actually why I started looking here in the first place as for woa el2
overlays I was asked to add compatible metadata as, apparently, NixOS
tooling requires it to validate the overlays [4].

[1] https://web.git.kernel.org/pub/scm/utils/dtc/dtc.git/tree/Documentation/dt-object-internal.txt#n120
[2] https://github.com/raspberrypi/linux/blob/rpi-6.6.y/arch/arm/boot/dts/overlays/adafruit-st7735r-overlay.dts#L73
[3] https://github.com/radxa-pkg/radxa-overlays/blob/main/arch/arm64/boot/dts/rockchip/overlays/rk3588-i2c0-m1.dts#L5
[4] https://github.com/TravMurav/slbounce/blob/main/dtbo/x1e-el2.dtso#L12

>> 
>> 	fragment@0 {
>> 		target-path = "/";
>> 
>> 		__overlay__ {
>> 			whatever-comes-next-after-baz;
>> 		};
>> 	};
>> };
>> 
>> $ dtc combine.dtb
>> /dts-v1/;
>> 
>> / {
>> 	whatever-comes-next-after-baz;
>> 	compatible = "fake,board";
>> 	fake,value = <0x2a>;
>> };
>> 
>> In the resulting dtb foo bar and baz are missing.
>> 
>> Signed-off-by: Nikita Travkin <nikita@...n.ru>
>> ---
>> Nikita Travkin (2):
>>       arm64: dts: qcom: qrb5165-rb5-vision-mezzanine: Fix broken overlay root
>>       arm64: dts: qcom: sdm845-db845c-navigation-mezzanine: Fix the overlay root
>> 
>>  arch/arm64/boot/dts/qcom/qrb5165-rb5-vision-mezzanine.dtso       | 2 +-
>>  arch/arm64/boot/dts/qcom/sdm845-db845c-navigation-mezzanine.dtso | 2 +-
>>  2 files changed, 2 insertions(+), 2 deletions(-)
>> ---
>> base-commit: 8433c776e1eb1371f5cd40b5fd3a61f9c7b7f3ad
>> change-id: 20250226-qcom-nonroot-overlays-bfe21d33be8c
>> 
>> Best regards,
>> -- 
>> Nikita Travkin <nikita@...n.ru>
>>