[RESEND] power: supply: qcom_battmgr: abs() on POWER_NOW property

From: Anthony Ruhier <aruhier@mailbox.org>

The value for the POWER_NOW property is by default negative when the
battery is discharging, positive when charging.

However on x1e laptops it breaks several userland tools that give a
prediction of the battery run time (such as the acpi command, powertop
or the waybar battery module), as these tools do not expect a negative
value for /sys/class/power_supply/qcom-battmgr-bat/power_now. They
estimate the battery run time by dividing the value of energy_full by
power_now. The battery percentage is calculated by dividing energy_full
by energy_now, therefore it is not impacted.

While having a negative number during discharge makes sense, it is not
standard with how other battery drivers expose it. Instead, it seems
standard to have a positive value for power_now, and rely on the status
file instead to know if the battery is charging or discharging. It is
what other x86 laptops do.

Without the patch:
    $ acpi
    Battery 0: Discharging, 98%, discharging at zero rate - will never fully discharge.

With the patch:
    $ acpi
    Battery 0: Discharging, 97%, 10:18:27 remaining

---
Signed-off-by: Anthony Ruhier <aruhier@mailbox.org>
---
 drivers/power/supply/qcom_battmgr.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)


---
base-commit: ffd294d346d185b70e28b1a28abe367bbfe53c04
change-id: 20250128-patch-qcomm-bat-uint-power-5793f3638c56

Best regards,

On Sat, Feb 15, 2025 at 04:08:25AM +0100, Sebastian Reichel wrote:
> Hi,
>
> There are other drivers reporting negative values as documented.
> Most of the embedded ones do this actually and there surely are
> (embedded) userspace programs relying on this by now. But the
> most used driver - generic ACPI battery - does not. That's why
> quite a few userspace tools handle it wrong without anyone
> noticing for quite some time. Fixing it to follow the ABI would
> obviously end up in a bunch of regression reports, so things are
> a bit messy :(
>
> > I think it is a problem of the 'acpi' tool. At least 'upower -d' uses
> > fabs internally since the initial commit in 2008.
>
> It's definitely sensible to fix the userspace tools. We can't change
> the documented ABI for current_now after that many years and while
> documentation for power_now is missing, it would be quite unexpected
> to have it behave differently than current_now. Also userspace
> tooling needs to handle current_now and power_now anyways. And we
> surely can't change the behaviour for all drivers reporting signed
> data. So let's keep qcom_battmgr as is. It follows the documented
> ABI and hopefully helps giving this more exposure (I'm typing this
> on a X1E laptop right now and can see your problem with waybar).
>
> But we should document the power_now property. It somehow fell
> through the cracks :)
>
> -- Sebastian

Hi,
As an update around this topic, I sent some patches in the different tools I'm
using to correctly handle negative values in current_now and power_now:

  * Waybar (included in release 0.12.0): https://github.com/Alexays/Waybar/pull/3942
  * Powertop (merged): https://github.com/fenrus75/powertop/pull/173
  * acpi-client (included in release 1.8): https://sourceforge.net/p/acpiclient/code/merge-requests/1/

It was quicker to get this merged than what I expected, which is good news!

There's probably other tools to fix, I just fixed the tools I'm using. I
encounter the issue on other tools, I'll send a patch.

--
Anthony Ruhier

Hi,

On Fri, Feb 28, 2025 at 04:25:47PM +0100, Anthony Ruhier wrote:
> On Sat, Feb 15, 2025 at 04:08:25AM +0100, Sebastian Reichel wrote:
> > There are other drivers reporting negative values as documented.
> > Most of the embedded ones do this actually and there surely are
> > (embedded) userspace programs relying on this by now. But the
> > most used driver - generic ACPI battery - does not. That's why
> > quite a few userspace tools handle it wrong without anyone
> > noticing for quite some time. Fixing it to follow the ABI would
> > obviously end up in a bunch of regression reports, so things are
> > a bit messy :(
> >
> > > I think it is a problem of the 'acpi' tool. At least 'upower -d' uses
> > > fabs internally since the initial commit in 2008.
> >
> > It's definitely sensible to fix the userspace tools. We can't change
> > the documented ABI for current_now after that many years and while
> > documentation for power_now is missing, it would be quite unexpected
> > to have it behave differently than current_now. Also userspace
> > tooling needs to handle current_now and power_now anyways. And we
> > surely can't change the behaviour for all drivers reporting signed
> > data. So let's keep qcom_battmgr as is. It follows the documented
> > ABI and hopefully helps giving this more exposure (I'm typing this
> > on a X1E laptop right now and can see your problem with waybar).
> >
> > But we should document the power_now property. It somehow fell
> > through the cracks :)
> >
> > -- Sebastian
> 
> Hi,
> As an update around this topic, I sent some patches in the different tools I'm
> using to correctly handle negative values in current_now and power_now:
> 
>   * Waybar (included in release 0.12.0): https://github.com/Alexays/Waybar/pull/3942
>   * Powertop (merged): https://github.com/fenrus75/powertop/pull/173
>   * acpi-client (included in release 1.8): https://sourceforge.net/p/acpiclient/code/merge-requests/1/
> 
> It was quicker to get this merged than what I expected, which is good news!
> 
> There's probably other tools to fix, I just fixed the tools I'm using. I
> encounter the issue on other tools, I'll send a patch.

Thanks, appreciated.

Greetings,

-- Sebastian