linux-kernel - Re: [PATCH v2 1/2] power: supply: add input voltage limit property.

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Date:   Wed, 5 Dec 2018 16:47:28 +0100
From:   Sebastian Reichel <sre@...labora.com>
To:     Benson Leung <bleung@...gle.com>
Cc:     Pavel Machek <pavel@....cz>,
        Enric Balletbo i Serra <enric.balletbo@...labora.com>,
        linux-pm@...r.kernel.org, Sameer Nanda <snanda@...omium.org>,
        gwendal@...omium.org, linux-kernel@...r.kernel.org,
        groeck@...omium.org, Adam.Thomson.Opensource@...semi.com,
        kernel@...labora.com, bleung@...omium.org,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <len.brown@...el.com>
Subject: Re: [PATCH v2 1/2] power: supply: add input voltage limit property.

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 case. It can
> > > be a hardware bug or a hardware/user constrain, let me try to explain better
> > > with an example.
> > > 
> > > 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 screen is
> > > off/system is sleeping, so we allow 9V and 12V levels when sleeping.
> > 
> > So, on pixel C, what happens if userland ignores the constraint, keeps
> > display on and sets charger to 12V?
> 
> I was the software tech lead for the Google Pixel C and was involved in this
> particular code change in 2015 before the release of the product.
> 
> So there's nothing fundamentally broken about the hardware that would cause
> 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.
> 
> 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 charger
> in order to give more of the thermal budget to those other components.
> 
> What would happen is that you wouldn't meet Google's skin temperature targets
> on the system if the charger was allowed to run at 9V or 12V with the screen
> on.
> 
> 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 power
> 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 since
> 2015 in case someone running an up to date kernel wanted to limit the charging
> 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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