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Date:   Wed, 5 Dec 2018 16:47:28 +0100
From:   Sebastian Reichel <sre@collabora.com>
To:     Benson Leung <bleung@google.com>
Cc:     Pavel Machek <pavel@ucw.cz>,
        Enric Balletbo i Serra <enric.balletbo@collabora.com>,
        linux-pm@vger.kernel.org, Sameer Nanda <snanda@chromium.org>,
        gwendal@chromium.org, linux-kernel@vger.kernel.org,
        groeck@chromium.org, Adam.Thomson.Opensource@diasemi.com,
        kernel@collabora.com, bleung@chromium.org,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Len Brown <len.brown@intel.com>
Subject: Re: [PATCH v2 1/2] power: supply: add input voltage limit property.
Message-ID: <20181205154728.3yhxhov5iltb3ncl@earth.universe>
References: <20181122101119.29194-1-enric.balletbo@collabora.com>
 <20181123232203.GA3852@amd>
 <c122a1f9-70d7-7129-f139-4a0449c366a5@collabora.com>
 <20181201150934.GA7052@amd>
 <20181203195845.GA120704@google.com>
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Hi,

On Mon, Dec 03, 2018 at 11:58:45AM -0800, Benson Leung wrote:
> On Sat, Dec 01, 2018 at 04:09:34PM +0100, Pavel Machek wrote:
> > > I think that handle this via dt / ACPI is not possible for our use ca=
se. It can
> > > be a hardware bug or a hardware/user constrain, let me try to explain=
 better
> > > with an example.
> > >=20
> > > On Pixel C's devices, userspace uses this to set a USB input limit of=
 5V when
> > > the screen is on for thermal reasons, but those go away when the scre=
en is
> > > off/system is sleeping, so we allow 9V and 12V levels when sleeping.
> >=20
> > So, on pixel C, what happens if userland ignores the constraint, keeps
> > display on and sets charger to 12V?
>=20
> I was the software tech lead for the Google Pixel C and was involved in t=
his
> particular code change in 2015 before the release of the product.
>=20
> So there's nothing fundamentally broken about the hardware that would cau=
se
> the Pixel C to malfunction if we were charging at 9V or 12V instead of 5V
> when the screen is on, ie if userspace doesn't change this.
>=20
> This is part of the Pixel C's thermal management strategy to effectively
> limit the input power to 5V 3A when the screen is on. When the screen is =
on,
> the display, the CPU, and the GPU all contribute more heat to the system
> than while the screen is off, and we made a tradeoff to throttle the char=
ger
> in order to give more of the thermal budget to those other components.
>=20
> What would happen is that you wouldn't meet Google's skin temperature tar=
gets
> on the system if the charger was allowed to run at 9V or 12V with the scr=
een
> on.
>=20
> For folks hacking on Pixel Cs (which is now outside of Google's official =
support
> window for Android) and customizing their own kernel and userspace
> this would be acceptable, but we wanted to expose this feature in the pow=
er
> supply properties because the feature does exist in the Emedded Controller
> firmware of the Pixel C and all of Google's Chromebooks with USB-C made s=
ince
> 2015 in case someone running an up to date kernel wanted to limit the cha=
rging
> power for thermal or other reasons.

I'm fine with merging this with the above description. I hope
vendors never decide to move safety relevant decisions to userspace.
Enric, can you please integrate the great description from Benson
into the patch description?

Thanks,

-- Sebastian

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