linux-kernel - Re: [PATCH 1/4] arm64: dts: mediatek: mt8183-kukui-jacuzzi: Drop pp3300

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Message-ID: <CAGXv+5F2SvGZL+XKi+6wHf0ejgsxMMMSNnXQMU6T51OTdD2SBg@mail.gmail.com>
Date: Mon, 23 Dec 2024 18:08:34 +0800
From: Chen-Yu Tsai <wenst@...omium.org>
To: AngeloGioacchino Del Regno <angelogioacchino.delregno@...labora.com>
Cc: Matthias Brugger <matthias.bgg@...il.com>, devicetree@...r.kernel.org, 
	linux-mediatek@...ts.infradead.org, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH 1/4] arm64: dts: mediatek: mt8183-kukui-jacuzzi: Drop
 pp3300_panel voltage settings

On Tue, Dec 10, 2024 at 4:40 PM Chen-Yu Tsai <wenst@...omium.org> wrote:
>
> On Tue, Dec 10, 2024 at 4:27 PM AngeloGioacchino Del Regno
> <angelogioacchino.delregno@...labora.com> wrote:
> >
> > Il 04/11/24 14:47, Chen-Yu Tsai ha scritto:
> > > On Mon, Nov 4, 2024 at 9:19 PM AngeloGioacchino Del Regno
> > > <angelogioacchino.delregno@...labora.com> wrote:
> > >>
> > >> Il 04/11/24 14:00, Chen-Yu Tsai ha scritto:
> > >>> On Wed, Oct 30, 2024 at 3:02 PM Chen-Yu Tsai <wenst@...omium.org> wrote:
> > >>>>
> > >>>> The pp3300_panel fixed regulator is just a load switch. It does not have
> > >>>> any regulating capabilities. Thus having voltage constraints on it is
> > >>>> wrong.
> > >>>>
> > >>>> Remove the voltage constraints.
> > >>>>
> > >>>> Fixes: cabc71b08eb5 ("arm64: dts: mt8183: Add kukui-jacuzzi-damu board")
> > >>>> Signed-off-by: Chen-Yu Tsai <wenst@...omium.org>
> > >>>
> > >>> I see that the other three patches were merged and included in the pull
> > >>> request, but not this one. Were there any concerns?
> > >>>
> > >>
> > >> Sorry I forgot to actually provide an explanation for that - yes, I do have some
> > >> comment about this one.
> > >>
> > >> Despite this being a load switch, it's still switching power from regulator A to
> > >> target device X, so this is technically still providing 3.3V to device X.
> > >>
> > >> Think about how a "regular" full-fledged regulator works: you can (sometimes) set
> > >> a voltage, and then you can ENABLE the VOUT for said regulator (/rail): this kind
> > >> of "load switch" does exactly the same as the ENABLE switch for a full-fledged
> > >> regulator.
> > >
> > > But it does not provide regulation. One cannot "set" the voltage on a load
> > > switch; one can only set it on its upstream supply, if that supply provides
> > > regulation.
> > >
> > > IIRC Mark said some years ago that if a regulator doesn't regulate the
> > > voltage, then the voltage constraints should not be given. The constraints
> > > are then derived from its upstream supply.
> > >
> > > That's the guideline I've followed for all the regulator related changes
> > > I've done over the years. Does that work for you?
> > >
> >
> > Ok, let's go with that then.
> >
> > There's only one problem - patches 2 to 4 don't apply for whatever reason, is there
> > any dependency?
>
> I believe they were already applied in the previous cycle? At least
> they were dropped from my branch automatically after a recent rebase.

Ping? This one still hasn't been merged.

ChenYu

> ChenYu
>
> > Cheers,
> > Angelo
> >
> > >> So, this is switching on and off a power rail that is derived from a source rail,
> > >> practically creating... well, a "new" rail, with...
> > >>
> > >>    VIN=somewhere-3.3v,
> > >>    VOUT=somewhere-still-3.3v
> > >>
> > >> Any objections/doubts/etc? :-)
> > >
> > > I agree with most of it, except the part that I laid out above about the
> > > load switch not providing regulation.
> > >
> > >> P.S.: I'm writing fast, sorry if anything appears unclear, feel free to shoot more
> > >>         questions in case :-)
> > >
> > > No, it's pretty clear, and I believe one of the common interpretations
> > > I see. Thank you for the quick response.
> > >
> > >
> > > Thanks
> > > ChenYu
> > >
> > >> Cheers,
> > >> Angelo
> > >>
> > >>>
> > >>> ChenYu
> > >>>
> > >>>> ---
> > >>>>    arch/arm64/boot/dts/mediatek/mt8183-kukui-jacuzzi.dtsi | 2 --
> > >>>>    1 file changed, 2 deletions(-)
> > >>>>
> > >>>> diff --git a/arch/arm64/boot/dts/mediatek/mt8183-kukui-jacuzzi.dtsi b/arch/arm64/boot/dts/mediatek/mt8183-kukui-jacuzzi.dtsi
> > >>>> index 783c333107bc..7bbafe926558 100644
> > >>>> --- a/arch/arm64/boot/dts/mediatek/mt8183-kukui-jacuzzi.dtsi
> > >>>> +++ b/arch/arm64/boot/dts/mediatek/mt8183-kukui-jacuzzi.dtsi
> > >>>> @@ -35,8 +35,6 @@ pp1800_mipibrdg: pp1800-mipibrdg {
> > >>>>           pp3300_panel: pp3300-panel {
> > >>>>                   compatible = "regulator-fixed";
> > >>>>                   regulator-name = "pp3300_panel";
> > >>>> -               regulator-min-microvolt = <3300000>;
> > >>>> -               regulator-max-microvolt = <3300000>;
> > >>>>                   pinctrl-names = "default";
> > >>>>                   pinctrl-0 = <&pp3300_panel_pins>;
> > >>>>
> > >>>> --
> > >>>> 2.47.0.163.g1226f6d8fa-goog
> > >>>>
> > >>
> >
> >