Date:   Fri, 23 Sep 2022 20:00:55 +0200
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Ulf Hansson <ulf.hansson@...aro.org>,
        Francesco Dolcini <francesco.dolcini@...adex.com>
Cc:     Mark Brown <broonie@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Robin Murphy <robin.murphy@....com>,
        Max Krummenacher <max.oss.09@...il.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Max Krummenacher <max.krummenacher@...adex.com>,
        Linux PM list <linux-pm@...r.kernel.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Kevin Hilman <khilman@...nel.org>,
        Andrejs Cainikovs <andrejs.cainikovs@...adex.com>,
        Biju Das <biju.das.jz@...renesas.com>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Fabio Estevam <festevam@...il.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Marcel Ziswiler <marcel.ziswiler@...adex.com>,
        NXP Linux Team <linux-imx@....com>,
        Pengutronix Kernel Team <kernel@...gutronix.de>,
        Rob Herring <robh+dt@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Shawn Guo <shawnguo@...nel.org>, Vinod Koul <vkoul@...nel.org>,
        Will Deacon <will@...nel.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v1 0/5] power: domain: Add driver for a PM domain provider
 which controls

On 22/09/2022 15:49, Ulf Hansson wrote:
> On Fri, 9 Sept 2022 at 16:22, Francesco Dolcini
> <francesco.dolcini@...adex.com> wrote:
>>
>> Hello Ulf,
>>
>> On Fri, Aug 26, 2022 at 03:50:46PM +0200, Ulf Hansson wrote:
>>> On Thu, 28 Jul 2022 at 13:21, Francesco Dolcini
>>> <francesco.dolcini@...adex.com> wrote:
>>>>
>>>> On Thu, Jul 28, 2022 at 11:37:07AM +0200, Ulf Hansson wrote:
>>>>> On Tue, 26 Jul 2022 at 18:03, Francesco Dolcini
>>>>> <francesco.dolcini@...adex.com> wrote:
>>>>>>
>>>>>> Hello Ulf and everybody,
>>>>>>
>>>>>> On Wed, Jul 13, 2022 at 01:43:28PM +0200, Ulf Hansson wrote:
>>>>>>> On Thu, 23 Jun 2022 at 18:14, Max Krummenacher <max.oss.09@...il.com> wrote:
>>>>>>>> So our plan is to explicitly handle a (shared) regulator in every
>>>>>>>> driver involved, adding that regulator capability for drivers not
>>>>>>>> already having one.
>>>>>>>
>>>>>>> Please don't! I have recently rejected a similar approach for Tegra
>>>>>>> platforms, which now have been converted into using the power domain
>>>>>>> approach.
>>>>>>
>>>>>> Just to quickly re-iterate how our hardware design looks like, we do
>>>>>> have a single gpio that control the power of a whole board area that is
>>>>>> supposed to be powered-off in suspend mode, this area could contains
>>>>>> devices that have a proper Linux driver and some passive driver-less
>>>>>> components (e.g. level shifter) - the exact mix varies.
>>>>>>
>>>>>> Our proposal in this series was to model this as a power domain that
>>>>>> could be controlled with a regulator. Krzysztof, Robin and others
>>>>>> clearly argued against this idea.
>>>>>
>>>>> Well, historically we haven't modelled these kinds of power-rails
>>>>> other than through power-domains. And this is exactly what genpd and
>>>>> PM domains in Linux are there to help us with.
>>>>>
>>>>> Moreover, on another SoC/platform, maybe the power-rails are deployed
>>>>> differently and maybe those have the ability to scale performance too.
>>>>> Then it doesn't really fit well with the regulator model anymore.
>>>>>
>>>>> If we want to continue to keep drivers portable, I don't see any
>>>>> better option than continuing to model these power-rails as
>>>>> power-domains.
>>>>>
>>>>>>
>>>>>> The other approach would be to have a single regulator shared with the
>>>>>> multiple devices we have there (still not clear how that would work in
>>>>>> case we have only driver-less passive components). This is just a
>>>>>> device-tree matter, maybe we would need to add support for a supply to
>>>>>> some device drivers.
>>>>>>
>>>>>> Honestly my conclusion from this discussion is that the only viable
>>>>>> option is this second one, do I miss something?
>>>>>
>>>>> No thanks!
>>>>>
>>>>> Well, unless you can convince me there are benefits to this approach
>>>>> over the power-domain approach.
>>>>
>>>> I'm fine with our current power-domain proposal here, I do not need to
>>>> convince you, I have the other problem to convince someone to merge
>>>> it :-)
>>>>
>>>> Maybe Krzysztof, Robin or Mark can comment again after you explained
>>>> your view on this topic.
>>>
>>> To move things forward, I suggest you re-start with the power domain approach.
>>>
>>> Moreover, to avoid any churns, just implement it as another new SoC
>>> specific genpd provider and let the provider deal with the regulator.
>> I'm sorry, but I was not able to understand what you mean, can you
>> provide some additional hint on the topic? Some reference driver we can
>> look at?
> 
> Typically, "git grep pm_genpd_init" will find genpd providers.
> 
> There are a couple of examples where a regulator (among other things)
> is being controlled from the genpd's ->power_on|off() callbacks, such
> as:
> 
> drivers/soc/mediatek/mtk-pm-domains.c
> drivers/soc/imx/gpc.c
> 
>>
>> The driver we implemented and proposed with this patch is just
>> connecting a power-domain to a regulator, it's something at the board
>> level, not at the SoC one.
>> We do not have a (existing) SoC driver were we could add the power
>> domain provider as an additional functionality.
> 
> Right, so you need to add a new SoC/platform driver for this.
> 
>>
>>> In this way, you don't need to invent any new types of DT bindings,
>>> but can re-use existing ones.
>> The only new binding would be a new "compatible" to have a place to
>> tie the regulator instance used in the device tree, but I do not think
>> that this is an issue at all.
> 
> Yes, I agree.
> 
>>
>> The main concern that was raised on this topic was that we have to
>> somehow link the power-domain to the specific peripherals (the power
>> domain consumer) in the device tree.
> 
> Yes, that is needed. Although, I don't see how that is a concern?
> 
> We already have the valid bindings to use for this, see more below.
> 
>>
>> Adding the power-domain property there will trigger validation errors
>> unless we do explicitly add the power-domains to the schema for each
>> peripheral we need this. To me this does not really work, but maybe I'm
>> not understanding something.
>>
>> This is what Rob wrote on the topic [1]:
>>   > No. For 'power-domains' bindings have to define how many there are and
>>   > what each one is.
>>
>> Just as an example from patch [2]:
>>
>>   can1: can@0 {
>>     compatible = "microchip,mcp251xfd";
>>     power-domains = <&pd_sleep_moci>;
>>   };
>>
>> leads to:
>>
>>   imx8mm-verdin-nonwifi-dahlia.dtb: can@0: 'power-domains' does not match any of the regexes: 'pinctrl-[0-9]+'
>>           From schema: .../bindings/net/can/microchip,mcp251xfd.yaml
> 
> I think it should be fine to just add the below line to the DT
> bindings, for each peripheral device to fix the above problem.
> 
> power-domains: true

Again, as Rob said, no, because it must be strictly defined. So for
example: "maxItems: 1" for simple cases. But what if device is then part
of two power domains?

> 
> That should be okay, right?

Adding it to each peripheral scales poorly. Especially that literally
any device can be part of such power domain.

If we are going with power domain approach, then it should be applicable
basically to every device or to every device of some class (e.g. I2C,
SPI). This means it should be added to respective core schema in
dtschema repo, in a way it does not interfere with other power-domains
properties (existing ones).


Best regards,
Krzysztof

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