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

From: Pavel Machek <pavel@ucw.cz>
To: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Cc: linux-pm@vger.kernel.org, sre@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.
Date: Sat, 24 Nov 2018 00:22:04 +0100	[thread overview]
Message-ID: <20181123232203.GA3852@amd> (raw)
In-Reply-To: <20181122101119.29194-1-enric.balletbo@collabora.com>

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Hi!

> We have a problem with USBPD chargers which under certain conditions
> can result in system overheating if the voltage provided by the USBPD
> port is too high. While the preferred means to control this would be
> through devicetree or ACPI settings, this is not always possible, and
> we need to have a means to set a voltage limit.
> 
> This patch exposes a new property, similar to input current limit, to
> re-configure the maximum voltage from the external supply at runtime
> based on system-level knowledge or user input.

First, this should really be handled by dt / ACPI. If it is broken,
that's a hardware bug, and we can do DMI-based blacklists in kernel.

How are you supposed to fsck a system, for example?

> +What:		/sys/class/power_supply/<supply_name>/input_voltage_limit
> +Date:		Nov 2018
> +Contact:	linux-pm@vger.kernel.org
> +Description:
> +		Details the incoming VBUS voltage limit currently set in the
> +		supply. Normally this is configured based on the type of
> +		connection made.

"Details"?

Who can write to this value and when? What is the limit? If USB
charger is plugged in, should it show 5.0V (because that's nominal on
the USB) or 5.25V (because that is the real limit)?

Who can write to this and when. And what happens on write? What
happens if I write value that charger can't provide there?

Does it set the voltage power supply should produce? Or maximal it can
choose to produce?

This really needs better documentation.

								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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