linux-kernel - Re: [PATCH 2/2] ARM: dts: imx6qdl: support child mfd cells for the reset controller

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Message-ID: <e492a90c-147c-a50b-80aa-86f47306cff0@pengutronix.de>
Date:   Mon, 16 Jan 2023 10:26:35 +0100
From:   Ahmad Fatoum <a.fatoum@...gutronix.de>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Bastian Krause <bst@...gutronix.de>,
        Philipp Zabel <p.zabel@...gutronix.de>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Fabio Estevam <festevam@...il.com>,
        NXP Linux Team <linux-imx@....com>
Cc:     devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] ARM: dts: imx6qdl: support child mfd cells for the
 reset controller

On 16.01.23 10:14, Krzysztof Kozlowski wrote:
> On 16/01/2023 09:27, Ahmad Fatoum wrote:
>> Hello Krzysztof,
>>
>> On 16.01.23 09:20, Krzysztof Kozlowski wrote:
>>> On 13/01/2023 18:32, Bastian Krause wrote:
>>>> The actual syscon-reboot-mode child node can be added by a board
>>>> device-tree or fixed up by the bootloader. For the child node to be
>>>> probed, the compatible needs to include simple-mfd. The binding now
>>>> specifies this, so have the SoC dtsi adhere to it.
>>>>
>>>> Suggested-by: Ahmad Fatoum <a.fatoum@...gutronix.de>
>>>> Signed-off-by: Bastian Krause <bst@...gutronix.de>
>>>> ---
>>>>  arch/arm/boot/dts/imx6qdl.dtsi | 3 ++-
>>>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/arch/arm/boot/dts/imx6qdl.dtsi b/arch/arm/boot/dts/imx6qdl.dtsi
>>>> index ff1e0173b39be..b16be39458aa6 100644
>>>> --- a/arch/arm/boot/dts/imx6qdl.dtsi
>>>> +++ b/arch/arm/boot/dts/imx6qdl.dtsi
>>>> @@ -865,7 +865,8 @@ epit2: epit@...4000 { /* EPIT2 */
>>>>  			};
>>>>  
>>>>  			src: reset-controller@...8000 {
>>>> -				compatible = "fsl,imx6q-src", "fsl,imx51-src";
>>>> +				compatible = "fsl,imx6q-src", "fsl,imx51-src",
>>>> +					     "syscon", "simple-mfd";
>>>
>>> You need children here. Otherwise simple-mfd does not make sense. If you
>>> expect something else to add children (I don't understand why, usually
>>> reboot capability is fixed per SoC and only sometimes extended with some
>>> other means), then this "else" will also change compatible.
>>
>> It's about syscon-reboot-mode, not syscon-reboot. Such modes are board-
>> not soc-specific. 
> 
> syscon-reboot-mode is also mostly SoC specific. What exactly would
> differ on different boards? Register offsets of SoC component? Register
> values used by SoC power management unit?

The modes supported. Let's say you want a bootloader mode that drops
the board's bootloader into a fastboot gadget mode. You'd add a
syscon-reboot-mode pointing at one of the non-volatile registers and
you would define a magic value to indicate fastboot, both in the
bootloader and Linux.

In theory, the reboot mode could also talk to the bootrom[1] to change
the bootsource. This is also not board-agnostic, because it may not
make sense to have a spinor reboot mode if your board doesn't have one.

We have this scheme for STM32MP1 already and that's why I suggested
Bastian to do it likewise for i.MX as he needs this functionality:
https://lore.kernel.org/all/20201021102855.18026-1-a.fatoum@pengutronix.de/
https://elixir.bootlin.com/barebox/latest/source/arch/arm/dts/stm32mp151.dtsi#L44

Here's the barebox documentation for reboot modes btw:
https://www.barebox.org/doc/latest/user/reboot-mode.html

[1]: The i.MX bootrom samples two registers to determine boot mode
     override. The upstream syscon-reboot-mode binding supports only
     one 32-bit value and thus can be used as-is only for communicating
     with the "user" bootloader.


> Anyway, the binding is then not correct - it does not allow
> syscon-reboot-mode child. I'll comment there.
> 
> 
> Best regards,
> Krzysztof
> 
> 

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