Re: [RFC] add standardized attributes for force_discharge and inhibit_charge

[Hans de Goede <hdegoede@redhat.com>]

Hi,

On 9/28/21 10:11 PM, Nicolò Piazzalunga wrote:
> Hi Sebastian,
> 
> this is a proposal to introduce separate and standardized attributes
> for force_discharge and inhibit_charge of a battery.
> These are simpler than using status from a user-space perspective,
> as discussed on the platform-driver-x86 mail list.

To clarify things a bit here, the reasons for not using status for
this are:

1. This would require adding new status-es which so far have never
been seen by userspace, which will likely cause confusion of e.g.
upower. IOW I believe that adding new status-es for this would
be a userspace ABI break.

2. The devices where we currently want to use this functionality
use the ACPI battery interface, which is standardized between
vendors and dealt with by drivers/apci/battery.c  but this kinda
extra functionality is vendor specific. The drivers/apci/battery.c
has code allowing vendor drivers to "hook" ACPI batteries and get
add/remove calls for them. Then in these calls currently the
vendor drivers do:

	device_add_groups(&battery->dev, my_prop_group))

Which allows them to register extra sysfs_attributes for
for example charge_control_start_threshold and
charge_control_end_threshold.

This works well, but having vendor drivers somehow intercept /
muck with the status handling in drivers/apci/battery.c is a
non trival problem. Where as with new separate attributes
this is already a solved problem.

> ---
>  Documentation/ABI/testing/sysfs-class-power | 27 +++++++++++++++++++++
>  drivers/power/supply/power_supply_sysfs.c   |  2 ++
>  include/linux/power_supply.h                |  2 ++
>  3 files changed, 31 insertions(+)
> 
> diff --git a/Documentation/ABI/testing/sysfs-class-power b/Documentation/ABI/testing/sysfs-class-power
> index ca830c6cd809..2c5f48f49273 100644
> --- a/Documentation/ABI/testing/sysfs-class-power
> +++ b/Documentation/ABI/testing/sysfs-class-power
> @@ -455,6 +455,33 @@ Description:
>  			      "Unknown", "Charging", "Discharging",
>  			      "Not charging", "Full"
>  
> +What:		/sys/class/power_supply/<supply_name>/force_discharge
> +Date:		September 2021
> +Contact:	linux-pm@vger.kernel.org
> +Description:
> +		Represents the forced discharging status of the battery.
> +
> +		Access: Read, Write
> +
> +		Valid values:
> +			== ====================================
> +			0: Force discharge while AC is attached
> +			1: Terminate forced discharging
> +

I think you have 0 and 1 swapped here? I would expect 1 to be enable forced
discharging and 0 be normal operation, iow only discharge when not on AC.

> +What:		/sys/class/power_supply/<supply_name>/inhibit_charge
> +Date:		September 2021
> +Contact:	linux-pm@vger.kernel.org
> +Description:
> +		Represents the presence of a manual override over the threshold
> +		attributes of the battery, thus inhibiting battery charge.
> +
> +		Access: Read, Write
> +
> +		Valid values:
> +			== ======================
> +			1: Stop charging
> +			0: Terminate the override
> +
>  What:		/sys/class/power_supply/<supply_name>/technology
>  Date:		May 2007
>  Contact:	linux-pm@vger.kernel.org
> diff --git a/drivers/power/supply/power_supply_sysfs.c b/drivers/power/supply/power_supply_sysfs.c
> index c3d7cbcd4fad..6e7303935810 100644
> --- a/drivers/power/supply/power_supply_sysfs.c
> +++ b/drivers/power/supply/power_supply_sysfs.c
> @@ -136,6 +136,8 @@ static const char * const POWER_SUPPLY_SCOPE_TEXT[] = {
>  static struct power_supply_attr power_supply_attrs[] = {
>  	/* Properties of type `int' */
>  	POWER_SUPPLY_ENUM_ATTR(STATUS),
> +	POWER_SUPPLY_ENUM_ATTR(FORCE_DISCHARGE),
> +	POWER_SUPPLY_ENUM_ATTR(INHIBIT_CHARGE),
>  	POWER_SUPPLY_ENUM_ATTR(CHARGE_TYPE),
>  	POWER_SUPPLY_ENUM_ATTR(HEALTH),
>  	POWER_SUPPLY_ATTR(PRESENT),
> diff --git a/include/linux/power_supply.h b/include/linux/power_supply.h
> index 9ca1f120a211..4340fe65df4d 100644
> --- a/include/linux/power_supply.h
> +++ b/include/linux/power_supply.h
> @@ -96,6 +96,8 @@ enum {
>  enum power_supply_property {
>  	/* Properties of type `int' */
>  	POWER_SUPPLY_PROP_STATUS = 0,
> +	POWER_SUPPLY_PROP_FORCE_DISCHARGE,
> +	POWER_SUPPLY_PROP_INHIBIT_CHARGE,
>  	POWER_SUPPLY_PROP_CHARGE_TYPE,
>  	POWER_SUPPLY_PROP_HEALTH,
>  	POWER_SUPPLY_PROP_PRESENT,
> 

Regards,

Hans