Created attachment 133575 [details] [review]
allow 0W battery discharge rate to be reported

On battery power but with one out of two batteries unused, the discharge rate of the unused battery is wrongly reported as non-zero, instead of true 0W. This is due to bad heuristics in upower/src/linux/up-device-supply.c, function up_device_supply_calculate_rate(), possible fix attached.


Details:

On my laptop it is by design that one of the two batteries is actively discharging while the other is waiting and not used. Which battery is being used in a given moment is decided by hardware. While disconnected from ac, upower however reports the unused battery's discharge rate to be non-zero even though it is actually 0W.


Steps to reproduce while not on ac power:

$ upower -v
UPower client version 0.99.5
UPower daemon version 0.99.5

$ cat /sys/class/power_supply/BAT0/power_now # actively discharging
3580000

$ cat /sys/class/power_supply/BAT1/power_now # not in use, waiting
0

$ upower -d
Device: /org/freedesktop/UPower/devices/line_power_AC
  native-path:          AC
  power supply:         yes
  updated:              Čet 17 Kol 2017 13:00:11 (1148 seconds ago)
  has history:          no
  has statistics:       no
  line-power
    warning-level:       none
    online:              no
    icon-name:          'ac-adapter-symbolic'

Device: /org/freedesktop/UPower/devices/battery_BAT0
  native-path:          BAT0
  vendor:               SANYO
  model:                01AV419
  serial:               795
  power supply:         yes
  updated:              Čet 17 Kol 2017 13:18:11 (68 seconds ago)
  has history:          yes
  has statistics:       yes
  battery
    present:             yes
    rechargeable:        yes
    state:               discharging
    warning-level:       none
    energy:              12,43 Wh
    energy-empty:        0 Wh
    energy-full:         22,16 Wh
    energy-full-design:  24,02 Wh
    energy-rate:         5,784 W
    voltage:             11,83 V
    time to empty:       2,1 hours
    percentage:          56%
    capacity:            92,2565%
    technology:          lithium-ion
    icon-name:          'battery-good-symbolic'
  History (charge):
    1502968691	56,000	discharging
  History (rate):
    1502968691	5,784	discharging

Device: /org/freedesktop/UPower/devices/battery_BAT1
  native-path:          BAT1
  vendor:               SANYO
  model:                01AV422
  serial:               723
  power supply:         yes
  updated:              Čet 17 Kol 2017 13:18:11 (68 seconds ago)
  has history:          yes
  has statistics:       yes
  battery
    present:             yes
    rechargeable:        yes
    state:               discharging
    warning-level:       none
    energy:              2,88 Wh
    energy-empty:        0 Wh
    energy-full:         22,05 Wh
    energy-full-design:  24,02 Wh
    energy-rate:         11,187 W
    voltage:             11,29 V
    time to empty:       15,4 minutes
    percentage:          13%
    capacity:            91,7985%
    technology:          lithium-ion
    icon-name:          'battery-low-symbolic'

Device: /org/freedesktop/UPower/devices/DisplayDevice
  power supply:         yes
  updated:              Čet 17 Kol 2017 13:18:11 (68 seconds ago)
  has history:          no
  has statistics:       no
  battery
    present:             yes
    state:               discharging
    warning-level:       none
    energy:              15,31 Wh
    energy-full:         44,21 Wh
    energy-rate:         16,971 W
    time to empty:       54,1 minutes
    percentage:          34,6302%
    icon-name:          'battery-good-symbolic'

Daemon:
  daemon-version:  0.99.5
  on-battery:      yes
  lid-is-closed:   no
  lid-is-present:  yes
  critical-action: Hibernate


Actual results:
upower reports the discharge energy-rate of /org/freedesktop/UPower/devices/battery_BAT1 as non-zero, even though /sys/class/power_supply/BAT0/power_now shows 0. Because of that the total discharge rate and total time to empty are wrong, as verified by powertop and wall clock. This negatively impacts the user experience ("do I have 10 or 30 minutes left?")


Expected results:
When disconnected from ac, discharge rate of the unused battery should be reported as true 0W.


Possible fix:
The cause can be found in upower/src/linux/up-device-supply.c. The function up_device_supply_calculate_rate() assumes that calculated zero discharge rate must be wrong and keeps reporting the old values, whatever those may be. I am currently testing the attached patch that allows "true" 0W discharge to be reported, so far with success.