Date:   Thu, 23 May 2019 12:54:38 -0700
From:   Benson Leung <bleung@...gle.com>
To:     Enric Balletbo i Serra <enric.balletbo@...labora.com>
Cc:     linux-pm@...r.kernel.org, sre@...nel.org,
        Sameer Nanda <snanda@...omium.org>, bleung@...omium.org,
        rjw@...ysocki.net, gwendal@...omium.org,
        linux-kernel@...r.kernel.org, Len Brown <len.brown@...el.com>,
        groeck@...omium.org, Adam.Thomson.Opensource@...semi.com,
        kernel@...labora.com, Pavel Machek <pavel@....cz>
Subject: Re: [PATCH v4 1/2] power: supply: add input power and voltage limit
 properties

Hi Enric,

On Tue, May 07, 2019 at 11:52:47AM +0200, Enric Balletbo i Serra wrote:
> For thermal management strategy you might be interested on limit the
> input power for a power supply. We already have current limit but
> basically what we probably want is to limit power. So, introduce the
> input_power_limit property.
> 
> Although the common use case is limit the input power, in some
> specific cases it is the voltage that is problematic (i.e some regulators
> have different efficiencies at higher voltage resulting in more heat).
> So introduce also the input_voltage_limit property.
> 
> This happens in one Chromebook and is used on 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.
> 
> 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, i.e. if userspace doesn't change this.
> 
> 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.
> 
> 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.
> 
> Signed-off-by: Enric Balletbo i Serra <enric.balletbo@...labora.com>
> Reviewed-by: Guenter Roeck <groeck@...omium.org>
> Acked-by: Adam Thomson <Adam.Thomson.Opensource@...semi.com>

Reviewed-by: Benson Leung <bleung@...omium.org>

> ---
> 
> Changes in v4:
> - Add also input_power_limit.
> 
> Changes in v3:
> - Improve commit log and documentation with Benson comments.
> 
> Changes in v2:
> - Document the new property in ABI/testing/sysfs-class-power.
> - Add the Reviewed-by Guenter Roeck tag.
> 
>  Documentation/ABI/testing/sysfs-class-power | 32 +++++++++++++++++++++
>  Documentation/power/power_supply_class.txt  |  4 +++
>  drivers/power/supply/power_supply_sysfs.c   |  2 ++
>  include/linux/power_supply.h                |  2 ++
>  4 files changed, 40 insertions(+)
> 
> diff --git a/Documentation/ABI/testing/sysfs-class-power b/Documentation/ABI/testing/sysfs-class-power
> index 5e23e22dce1b..962a27a1daf8 100644
> --- a/Documentation/ABI/testing/sysfs-class-power
> +++ b/Documentation/ABI/testing/sysfs-class-power
> @@ -331,10 +331,42 @@ Description:
>  		supply. Normally this is configured based on the type of
>  		connection made (e.g. A configured SDP should output a maximum
>  		of 500mA so the input current limit is set to the same value).
> +		Use preferably input_power_limit, and for problems that can be
> +		solved using power limit use input_current_limit.
>  
>  		Access: Read, Write
>  		Valid values: Represented in microamps
>  
> +What:		/sys/class/power_supply/<supply_name>/input_voltage_limit
> +Date:		May 2019
> +Contact:	linux-pm@...r.kernel.org
> +Description:
> +		This entry configures the incoming VBUS voltage limit currently
> +		set in the supply. Normally this is configured based on
> +		system-level knowledge or user input (e.g. This is part of the
> +		Pixel C's thermal management strategy to effectively limit the
> +		input power to 5V when the screen is on to meet Google's skin
> +		temperature targets). Note that this feature should not be
> +		used for safety critical things.
> +		Use preferably input_power_limit, and for problems that can be
> +		solved using power limit use input_voltage_limit.
> +
> +		Access: Read, Write
> +		Valid values: Represented in microvolts
> +
> +What:		/sys/class/power_supply/<supply_name>/input_power_limit
> +Date:		May 2019
> +Contact:	linux-pm@...r.kernel.org
> +Description:
> +		This entry configures the incoming power limit currently set
> +		in the supply. Normally this is configured based on
> +		system-level knowledge or user input. Use preferably this
> +		feature to limit the incoming power and use current/voltage
> +		limit only for problems that can be solved using power limit.
> +
> +		Access: Read, Write
> +		Valid values: Represented in microwatts
> +
>  What:		/sys/class/power_supply/<supply_name>/online,
>  Date:		May 2007
>  Contact:	linux-pm@...r.kernel.org
> diff --git a/Documentation/power/power_supply_class.txt b/Documentation/power/power_supply_class.txt
> index 300d37896e51..1e3c705111db 100644
> --- a/Documentation/power/power_supply_class.txt
> +++ b/Documentation/power/power_supply_class.txt
> @@ -137,6 +137,10 @@ power supply object.
>  
>  INPUT_CURRENT_LIMIT - input current limit programmed by charger. Indicates
>  the current drawn from a charging source.
> +INPUT_VOLTAGE_LIMIT - input voltage limit programmed by charger. Indicates
> +the voltage limit from a charging source.
> +INPUT_POWER_LIMIT - input power limit programmed by charger. Indicates
> +the power limit from a charging source.
>  
>  CHARGE_CONTROL_LIMIT - current charge control limit setting
>  CHARGE_CONTROL_LIMIT_MAX - maximum charge control limit setting
> diff --git a/drivers/power/supply/power_supply_sysfs.c b/drivers/power/supply/power_supply_sysfs.c
> index 5358a80d854f..860db617d241 100644
> --- a/drivers/power/supply/power_supply_sysfs.c
> +++ b/drivers/power/supply/power_supply_sysfs.c
> @@ -275,6 +275,8 @@ static struct device_attribute power_supply_attrs[] = {
>  	POWER_SUPPLY_ATTR(charge_control_limit),
>  	POWER_SUPPLY_ATTR(charge_control_limit_max),
>  	POWER_SUPPLY_ATTR(input_current_limit),
> +	POWER_SUPPLY_ATTR(input_voltage_limit),
> +	POWER_SUPPLY_ATTR(input_power_limit),
>  	POWER_SUPPLY_ATTR(energy_full_design),
>  	POWER_SUPPLY_ATTR(energy_empty_design),
>  	POWER_SUPPLY_ATTR(energy_full),
> diff --git a/include/linux/power_supply.h b/include/linux/power_supply.h
> index 2f9c201a54d1..ba135a5d8996 100644
> --- a/include/linux/power_supply.h
> +++ b/include/linux/power_supply.h
> @@ -122,6 +122,8 @@ enum power_supply_property {
>  	POWER_SUPPLY_PROP_CHARGE_CONTROL_LIMIT,
>  	POWER_SUPPLY_PROP_CHARGE_CONTROL_LIMIT_MAX,
>  	POWER_SUPPLY_PROP_INPUT_CURRENT_LIMIT,
> +	POWER_SUPPLY_PROP_INPUT_VOLTAGE_LIMIT,
> +	POWER_SUPPLY_PROP_INPUT_POWER_LIMIT,
>  	POWER_SUPPLY_PROP_ENERGY_FULL_DESIGN,
>  	POWER_SUPPLY_PROP_ENERGY_EMPTY_DESIGN,
>  	POWER_SUPPLY_PROP_ENERGY_FULL,
> -- 
> 2.20.1
> 

-- 
Benson Leung
Staff Software Engineer
Chrome OS Kernel
Google Inc.
bleung@...gle.com
Chromium OS Project
bleung@...omium.org

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