linux-kernel - Re: [PATCH] arm64: dts: qcom: sa8775p: add symbols to dtb

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Date:   Tue, 14 Mar 2023 10:59:41 +0100
From:   Konrad Dybcio <konrad.dybcio@...aro.org>
To:     Prasad Sodagudi <quic_psodagud@...cinc.com>,
        Eric Chanudet <echanude@...hat.com>,
        Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>
Cc:     linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: dts: qcom: sa8775p: add symbols to dtb



On 14.03.2023 05:48, Prasad Sodagudi wrote:
> 
> 
> On 3/9/2023 3:47 PM, Konrad Dybcio wrote:
>>
>>
>> On 10.03.2023 00:39, Eric Chanudet wrote:
>>> ABL uses the __symbols__ section to process the DTB before passing it
>>> forward. Without it, the bootstrap is interrupted.
>>>
>>> Signed-off-by: Eric Chanudet <echanude@...hat.com>
>>> ---
>> Fix your ABL.
> Hi Konrad,
> 
> Apps boot-loader need __symbols__ for dynamic overlay operation. Qualcomm firmware passes an overlay file to apps boot-loader to overlay some of the nodes based on firmware configuration. Without __symbols__ apps boot-loader is not able to overlay.
Yes/no. There are a plenty of libfdt functions that let you do that,
especially if you know something about the node.

> 
> Qualcomm hypervisor/gunyah would like to overlay arch timer node with always-on property, So adding __symbols__ helps boot-loader to overlay.
For the arch timer, you can simply iterate over the top-level nodes (or
in the worst case scenario, over all nodes which would not take very
long on cortex-a / cortex-x cores) and look for the specific timer
compatible that has been with us for like 10 years at this point and
use libfdt's fdt_add_property without overlays.

> 
> I think, commit text is misleading here and I will request Eric to fix the commit text.
Sort of, but then the design you explained is very error-prone as it's
dependent on the device tree always satisfying your hypervisor's needs.

Konrad
> 
> -Thanks, Prasad
> 
>>
>> Konrad
>>> Depends on initial sa8775p-ride.dts:
>>> https://lore.kernel.org/all/20230214092713.211054-3-brgl@bgdev.pl/
>>>
>>>   arch/arm64/boot/dts/qcom/Makefile | 4 ++++
>>>   1 file changed, 4 insertions(+)
>>>
>>> diff --git a/arch/arm64/boot/dts/qcom/Makefile b/arch/arm64/boot/dts/qcom/Makefile
>>> index b63cd1861e68..72e85ab31d74 100644
>>> --- a/arch/arm64/boot/dts/qcom/Makefile
>>> +++ b/arch/arm64/boot/dts/qcom/Makefile
>>> @@ -1,4 +1,8 @@
>>>   # SPDX-License-Identifier: GPL-2.0
>>> +
>>> +# Enable support for device-tree overlays required on sa8775p-ride.
>>> +DTC_FLAGS_sa8775p-ride := -@
>>> +
>>>   dtb-$(CONFIG_ARCH_QCOM)    += apq8016-sbc.dtb
>>>   dtb-$(CONFIG_ARCH_QCOM)    += apq8094-sony-xperia-kitakami-karin_windy.dtb
>>>   dtb-$(CONFIG_ARCH_QCOM)    += apq8096-db820c.dtb
>>
>>