linux-kernel - Re: [PATCH 2/2] ARM: dts: Add bcm2711-rpi-4-b-7inch-ts-dsi.dts

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Date:   Wed, 9 Feb 2022 21:31:40 +0100
From:   Stefan Wahren <stefan.wahren@...e.com>
To:     Detlev Casanova <detlev.casanova@...labora.com>,
        linux-kernel@...r.kernel.org
Cc:     Arnd Bergmann <arnd@...db.de>, Olof Johansson <olof@...om.net>,
        "maintainer:ARM AND ARM64 SoC SUB-ARCHITECTURES (COMMON PARTS)" 
        <soc@...nel.org>, Rob Herring <robh+dt@...nel.org>,
        Nicolas Saenz Julienne <nsaenz@...nel.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        Ray Jui <rjui@...adcom.com>,
        Scott Branden <sbranden@...adcom.com>,
        "maintainer:BROADCOM BCM281XX/BCM11XXX/BCM216XX ARM ARCHITE..." 
        <bcm-kernel-feedback-list@...adcom.com>,
        "moderated list:ARM AND ARM64 SoC SUB-ARCHITECTURES (COMMON PARTS)" 
        <linux-arm-kernel@...ts.infradead.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        "moderated list:BROADCOM BCM2711/BCM2835 ARM ARCHITECTURE" 
        <linux-rpi-kernel@...ts.infradead.org>
Subject: Re: [PATCH 2/2] ARM: dts: Add bcm2711-rpi-4-b-7inch-ts-dsi.dts

Hi Detlev,

Am 09.02.22 um 20:44 schrieb Detlev Casanova:
> Hi Stefan,
>
> On Wednesday, February 9, 2022 12:10:12 P.M. EST Stefan Wahren wrote:
>> Hi Detlev,
>>
>> Am 09.02.22 um 17:25 schrieb Detlev Casanova:
>>> Add a device tree to support the official Raspberrypi 7" touchscreen for
>>> the Raspberry Pi 4 B
>>>
>>> The panel is connected on the DSI 1 port and uses the simple-panel
>>> driver.
>>>
>>> The device tree also makes sure to activate:
>>>  * dvp: bcm2711 clock driver
>>>  * hvs: Hardware Video Scaler
>>>  * pixelvalve[0-4]: CRTC modules
>>>  * txp: CRTC Writeback
>>>
>>> Signed-off-by: Detlev Casanova <detlev.casanova@...labora.com>
>>> ---
>>>
>>>  arch/arm/boot/dts/Makefile                    |   1 +
>>>  .../boot/dts/bcm2711-rpi-4-b-7inch-ts-dsi.dts | 129 ++++++++++++++++++
>>>  arch/arm64/boot/dts/broadcom/Makefile         |   1 +
>>>  .../broadcom/bcm2711-rpi-4-b-7inch-ts-dsi.dts |   2 +
>>>  4 files changed, 133 insertions(+)
>>>  create mode 100644 arch/arm/boot/dts/bcm2711-rpi-4-b-7inch-ts-dsi.dts
>>>  create mode 100644
>>>  arch/arm64/boot/dts/broadcom/bcm2711-rpi-4-b-7inch-ts-dsi.dts> 
>>> diff --git a/arch/arm/boot/dts/Makefile b/arch/arm/boot/dts/Makefile
>>> index 0de64f237cd8..b46daf2df4ce 100644
>>> --- a/arch/arm/boot/dts/Makefile
>>> +++ b/arch/arm/boot/dts/Makefile
>>> @@ -94,6 +94,7 @@ dtb-$(CONFIG_ARCH_BCM2835) += \
>>>
>>>  	bcm2837-rpi-cm3-io3.dtb \
>>>  	bcm2711-rpi-400.dtb \
>>>  	bcm2711-rpi-4-b.dtb \
>>>
>>> +	bcm2711-rpi-4-b-7inch-ts-dsi.dtb \
>>>
>>>  	bcm2711-rpi-cm4-io.dtb \
>>>  	bcm2835-rpi-zero.dtb \
>>>  	bcm2835-rpi-zero-w.dtb
>>>
>>> diff --git a/arch/arm/boot/dts/bcm2711-rpi-4-b-7inch-ts-dsi.dts
>>> b/arch/arm/boot/dts/bcm2711-rpi-4-b-7inch-ts-dsi.dts new file mode 100644
>>> index 000000000000..62e986358c2a
>>> --- /dev/null
>>> +++ b/arch/arm/boot/dts/bcm2711-rpi-4-b-7inch-ts-dsi.dts
>> i think this should be an overlay because this board can be connected to
>> different boards and we want to avoid copy & paste. Unfortunately i
>> don't know where this should be stored in the kernel tree.
> Yes, that is how it started but the upstream kernel doesn't use them. We 
> thought that a separate device-tree makes sense as it is the official raspberry 
> pi touchscreen.

i'm not an expert here, but this seems not quite correct. AFAIK there is
no full runtime overlay support in the kernel, but the resources that i
found are promising [1], [2], [3]. Unfortunately i never test it by myself.

Please understand that all these possible combination would result in a
unmaintable mess.

Best regards

[1] - https://u-boot.readthedocs.io/en/latest/usage/fdt_overlays.html
[2] - https://www.kernel.org/doc/html/latest/devicetree/overlay-notes.html
[3] -
https://bootlin.com/pub/conferences/2021/lee/maincent-devicetree-overlay-and-uboot-extension-board-management/maincent-devicetree-overlay-and-uboot-extension-board-management.pdf