Re: The zero power level of the HID device in kernel 6.12 is not reported from the kernel to the upper layer.

["José Expósito" <jose.exposito89@gmail.com>]

Hi 卢国宏,

Thanks for reporting this issue.

In the furure, when reporting bugs, it is prefered to send them to the
mailing list (linux-input@vger.kernel.org and linux-kernel@vger.kernel.org)
to discuss them in public.

Let me forward your email to the mailing list and also CC Dmitry, the
author of that code, who might help us understand the problem.

On Tue, Sep 16, 2025 at 12:29:32PM +0000, 卢国宏 wrote:
> Hi, jose!
>
> We encountered a problem where the zero battery level of the HID device
> in kernel 6.12 was not reported from the kernel to the upper layer.
> I checked the HID protocol and it doesn't say that there is no need to
> report the zero power of the HID device. For details, see page 381 of
> the HID protocol, 31.4 Battery Measures. "Absolute State Of Charge DV
> The predicted remaining battery capacity expressed as a percentage of
> design capacity. (Units are %. The value may be greater than 100%.)".
> However, in the file hid-input.c in kernel 6.12, the following code：
> 
> static void hidinput_update_battery(struct hid_device *dev, unsigned int usage,
>                                     int value)
> {
>         int capacity;
> 
>         if (!dev->battery)
>                 return;
> 
>         if (hidinput_update_battery_charge_status(dev, usage, value)) {
>                 power_supply_changed(dev->battery);
>                 return;
>         }
> 
>         if (value == 0 || value < dev->battery_min || value > dev->battery_max)
>                 return;
> 
>         capacity = hidinput_scale_battery_capacity(dev, value);
> 
>          ......
> 
> }
> 
> The parameter value is the power level. When the value is 0, the above code
> returns without reporting.
> Is this a problem?
> We're currently experiencing this issue on Android 16. The upper layer of
> Android needs to receive a zero battery level before it can take appropriate
> action.
> Could you please help me evaluate whether we should remove the behavior of
> returning to zero battery?
>
> Thanks!
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It indeed looks like it could be problematic.

Values are allowed ot be grater than 100, however, I didn't find
any references to negative values. Since it is a percentage, it
make sense to limit it to 0%, i.e., not allowing negative values.

I think that removing the "value == 0" check, or replacing it with
"value < 0" should fix the issue.

By the way, the "Fully Discharged" value (0x00850047), section 31.3.1,
is not handled by the kernel. Do you know if Android handles that
instead or in addition to a 0% battery?

Jose