linux-kernel - Re: [PATCH 0/2] arm64: dts: qcom: apq8016-sbc-d3-camera: Convert to DT overlay

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Date:   Fri, 22 Sep 2023 22:13:05 +0100
From:   Bryan O'Donoghue <bryan.odonoghue@...aro.org>
To:     Stephan Gerhold <stephan@...hold.net>
Cc:     Bjorn Andersson <andersson@...nel.org>,
        Andy Gross <agross@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] arm64: dts: qcom: apq8016-sbc-d3-camera: Convert to
 DT overlay

On 22/09/2023 21:58, Stephan Gerhold wrote:
> On Fri, Sep 22, 2023 at 09:47:07PM +0100, Bryan O'Donoghue wrote:
>> On 22/09/2023 16:11, Stephan Gerhold wrote:
>>> Follow the example of the recently added apq8016-sbc-usb-host.dtso and
>>> convert apq8016-sbc-d3-camera-mezzanine.dts to a DT overlay that can be
>>> applied on top of the apq8016-sbc.dtb. This makes it more clear that
>>> this is not a special type of DB410c but just an addon board that can
>>> be added on top.
>>>
>>> I also prepended a patch that cleans up the node names a bit.
>>>
>>> Signed-off-by: Stephan Gerhold <stephan@...hold.net>
>>> ---
>>> Stephan Gerhold (2):
>>>         arm64: dts: qcom: apq8016-sbc-d3-camera: Use more generic node names
>>>         arm64: dts: qcom: apq8016-sbc-d3-camera: Convert to DT overlay
>>>
>>>    arch/arm64/boot/dts/qcom/Makefile                    |  5 +++--
>>>    ...nine.dts => apq8016-sbc-d3-camera-mezzanine.dtso} | 20 ++++++++++++++------
>>>    2 files changed, 17 insertions(+), 8 deletions(-)
>>> ---
>>> base-commit: 7236e86ce5c8198b01c30933c2334d07d877cf48
>>> change-id: 20230922-apq8016-sbc-camera-dtso-f247bea40f99
>>>
>>> Best regards,
>>
>> db410c doesn't ship with a bootloader that is capable of applying a dtbo
>> though, so this conversion mandates an updated or chainloaded bootloader or
>> out-of-tree kernel patch to support.
>>
>> __adding__ is fine but, converting implies imposes a new requirement on the
>> bootchain.
>>
>> Perhaps a middle road solution is to
>>
>> - Add, not convert a standalone dtbo or
>> - Add a dtbo that includes the mezzanine dts but amends it
>>
>> Option 2 for preference but, I'm not sure the dts syntax can be meaningfully
>> made to do that.
>>
> 
> With these patches the apq8016-sbc-d3-camera-mezzanine.dtb is still
> magically built, by running fdtoverlay on apq8016-sbc.dtb and applying
> the dtbo. It's applied during the build process so you don't need a
> bootloader that supports DTBOs.
> 
> There is literally *no change* for you in terms of usage. :-)

Ah I see

+apq8016-sbc-d3-camera-mezzanine-dtbs	:= apq8016-sbc.dtb 
apq8016-sbc-d3-camera-mezzanine.dtbo

I guess I like the idea of having the base board dtb a mezzanine.dtb and 
a dtbo that could be applied as an overlay to the base board dtb, 
optionally.

Our model then for mezzanine enablement wouild be

1. baseboard.dtb
2. baseboard-mezzanine.dtb
3. baseboard-mezzanine.dtbo

With booting #2 or #1+#3 resulting in the same image but, potentially 
allowing for

1. baseboard.dtb
2. baseboard-mezzanine.dtb
3. baseboard-mezzanine.dtbo
4. baseboard-some-other-mezzanine.dtb
5. baseboard-some-other-mezzanine.dtbo

#1 + #3 + #5 which would represent a hypothetical stacking of mezzanine 
boards.

As a model, I think that has merit.

Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@...aro.org>