Re: [PATCH 2/2] HID: input: Add HID_BATTERY_QUIRK_DYNAMIC for Elan touchscreens

[Dmitry Torokhov <dmitry.torokhov@gmail.com>]

Hi Hans,

On Sat, Feb 28, 2026 at 03:52:58PM +0100, Hans de Goede wrote:
> Elan touchscreens have a HID-battery device for the stylus which is always
> there even if there is no stylus.
> 
> This is causing upower to report an empty battery for the stylus and some
> desktop-environments will show a notification about this, which is quite
> annoying.
> 
> Because of this the HID-battery is being ignored on all Elan I2c and USB
> touchscreens, but this causes there to be no battery reporting for
> the stylus at all.
> 
> This adds a new HID_BATTERY_QUIRK_DYNAMIC and uses these for the Elan
> touchscreens.
> 
> This new quirks causes the present value of the battery to start at 0,
> which will make userspace ignore it and only sets present to 1 after
> receiving a battery input report which only happens when the stylus
> gets in range.

My understanding was that "present" attribute is to be used for
removable batteries (i.e. when a battery can be extracted from either
from the system or from a peripheral) and we want to notify userspace of
that fact). For example, if a laptop can have an additional battery UI
can show if it is installed or not, and it should not simply ignore such
power supplies.

On the other hand the "Unknown" operating status signals that we
actually do not know the state of the battery, and fits better in our
situation.

Anyway, I think we need Sebastian's input here.

> 
> Reported-by: ggrundik@gmail.com
> Closes: https://bugzilla.kernel.org/show_bug.cgi?id=221118
> Signed-off-by: Hans de Goede <johannes.goede@oss.qualcomm.com>
> ---
>  drivers/hid/hid-input.c | 14 +++++++++++---
>  include/linux/hid.h     |  1 +
>  2 files changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/hid/hid-input.c b/drivers/hid/hid-input.c
> index 67ca1e88ce13..8fc20df99b97 100644
> --- a/drivers/hid/hid-input.c
> +++ b/drivers/hid/hid-input.c
> @@ -354,6 +354,7 @@ static enum power_supply_property hidinput_battery_props[] = {
>  #define HID_BATTERY_QUIRK_FEATURE	(1 << 1) /* ask for feature report */
>  #define HID_BATTERY_QUIRK_IGNORE	(1 << 2) /* completely ignore the battery */
>  #define HID_BATTERY_QUIRK_AVOID_QUERY	(1 << 3) /* do not query the battery */
> +#define HID_BATTERY_QUIRK_DYNAMIC	(1 << 4) /* report present only after life signs */
>  
>  static const struct hid_device_id hid_battery_quirks[] = {
>  	{ HID_BLUETOOTH_DEVICE(USB_VENDOR_ID_APPLE,
> @@ -398,8 +399,8 @@ static const struct hid_device_id hid_battery_quirks[] = {
>  	 * Elan HID touchscreens seem to all report a non present battery,
>  	 * set HID_BATTERY_QUIRK_IGNORE for all Elan I2C and USB HID devices.
>  	 */
> -	{ HID_I2C_DEVICE(USB_VENDOR_ID_ELAN, HID_ANY_ID), HID_BATTERY_QUIRK_IGNORE },
> -	{ HID_USB_DEVICE(USB_VENDOR_ID_ELAN, HID_ANY_ID), HID_BATTERY_QUIRK_IGNORE },
> +	{ HID_I2C_DEVICE(USB_VENDOR_ID_ELAN, HID_ANY_ID), HID_BATTERY_QUIRK_DYNAMIC },
> +	{ HID_USB_DEVICE(USB_VENDOR_ID_ELAN, HID_ANY_ID), HID_BATTERY_QUIRK_DYNAMIC },
>  	{}
>  };
>  
> @@ -456,11 +457,14 @@ static int hidinput_get_battery_property(struct power_supply *psy,
>  	int ret = 0;
>  
>  	switch (prop) {
> -	case POWER_SUPPLY_PROP_PRESENT:
>  	case POWER_SUPPLY_PROP_ONLINE:
>  		val->intval = 1;
>  		break;
>  
> +	case POWER_SUPPLY_PROP_PRESENT:
> +		val->intval = dev->battery_present;
> +		break;
> +
>  	case POWER_SUPPLY_PROP_CAPACITY:
>  		if (dev->battery_status != HID_BATTERY_REPORTED &&
>  		    !dev->battery_avoid_query) {
> @@ -573,6 +577,8 @@ static int hidinput_setup_battery(struct hid_device *dev, unsigned report_type,
>  	if (quirks & HID_BATTERY_QUIRK_AVOID_QUERY)
>  		dev->battery_avoid_query = true;
>  
> +	dev->battery_present = (quirks & HID_BATTERY_QUIRK_DYNAMIC) ? false : true;
> +
>  	dev->battery = power_supply_register(&dev->dev, psy_desc, &psy_cfg);
>  	if (IS_ERR(dev->battery)) {
>  		error = PTR_ERR(dev->battery);
> @@ -628,6 +634,7 @@ static void hidinput_update_battery(struct hid_device *dev, unsigned int usage,
>  		return;
>  
>  	if (hidinput_update_battery_charge_status(dev, usage, value)) {
> +		dev->battery_present = true;
>  		power_supply_changed(dev->battery);
>  		return;
>  	}
> @@ -643,6 +650,7 @@ static void hidinput_update_battery(struct hid_device *dev, unsigned int usage,
>  	if (dev->battery_status != HID_BATTERY_REPORTED ||
>  	    capacity != dev->battery_capacity ||
>  	    ktime_after(ktime_get_coarse(), dev->battery_ratelimit_time)) {
> +		dev->battery_present = true;
>  		dev->battery_capacity = capacity;
>  		dev->battery_status = HID_BATTERY_REPORTED;
>  		dev->battery_ratelimit_time =
> diff --git a/include/linux/hid.h b/include/linux/hid.h
> index dce862cafbbd..d9b54f0e8671 100644
> --- a/include/linux/hid.h
> +++ b/include/linux/hid.h
> @@ -682,6 +682,7 @@ struct hid_device {
>  	__s32 battery_charge_status;
>  	enum hid_battery_status battery_status;
>  	bool battery_avoid_query;
> +	bool battery_present;
>  	ktime_t battery_ratelimit_time;
>  #endif
>  

Thanks.

-- 
Dmitry