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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