linux-kernel - Re: [PATCH v1 0/5] power: domain: Add driver for a PM domain provider which controls

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Message-ID: <20220726160337.GA41736@francesco-nb.int.toradex.com>
Date:   Tue, 26 Jul 2022 18:03:37 +0200
From:   Francesco Dolcini <francesco.dolcini@...adex.com>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     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>,
        Francesco Dolcini <francesco.dolcini@...adex.com>,
        Mark Brown <broonie@...nel.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>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        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

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.

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?

> If it's a powerail that is shared between controllers (devices), used
> to keep their registers values for example, that should be modelled as
> a power domain. Moreover for power domains, we can support
> voltage/frequency (performance) scaling, which isn't really applicable
> to a plain regulator.
> 
> However, if the actual powerrail fits well to be modelled as
> regulator, please go ahead. Although, in this case, the regulator must
> only be controlled behind a genpd provider's on/off callback, so you
> still need the power domain approach, rather than using the regulator
> in each driver and for each device.

Francesco