power: supply: macsmc: add "auto-discharge" charge behaviour for CHLS

[BoiledElectricity]

Lowering the charge limit on CHLS machines always sets CHLS_FORCE_DISCHARGE, so the battery actively drains down to the limit even on AC. Per review feedback, this is now implemented as a new charge_behaviour value instead of a module parameter:

• New auto-discharge value (enum + sysfs string + ABI doc): like auto, but actively discharge down to the charge control end threshold.
• End-threshold writes preserve the current CHLS_FORCE_DISCHARGE bit instead of forcing it on; auto keeps its documented cap-only meaning.
• The selection lives in the CHLS key itself: no driver state, persists across reboots, and systems upgrading from the old behaviour keep force-discharge until they explicitly write auto.

Resolves the in-code TODO. Tested on M1 MacBook Pro (j293), see comments.

[BoiledElectricity]

Tested on Apple MacBook Pro 13" M1 (j293), kernel 6.19.14, on AC at 100% (this firmware uses the CHLS path, has_chls=1):

• force_discharge=N + set 90% limit → battery just caps charging, no discharge (Full, current_now=0)
• force_discharge=Y (default) + set 90% limit → actively discharges (Discharging, ~-360 mA)

force_discharge=N, end_threshold=90:
  status=Full  current_now=0 uA        x4 samples over 8s

force_discharge=Y, end_threshold=90:
  status=Full         current_now=0 uA
  status=Discharging  current_now=-347000 uA
  status=Discharging  current_now=-376000 uA
  status=Discharging  current_now=-361000 uA

Battery restored to 100% / default afterwards.

[jannau]

Using a module option is not a good idea. It most likely will not be accepted upstream and we do not want to carry non-upstreamable patches unless they bring clear benefits.

A possible alternative solution might be to add a charge behaviour to control this. See charge_behaviour in Documentation/ABI/testing/sysfs-class-power and add either auto-discharge or auto-inhibit-discharge. I'm not sure what what the better default is from battery health perspective. Keeping the battery full charged for a long periods is not healthy but constantly charging and discharging the battery from 80% to 100% when the laptop is powered off isn't healthy either.

It would be worth checking if the charge thresholds are now respected while the laptop is powered off now that macOS has charge thresholds in 26.4 / 26.5 and later.

[BoiledElectricity]

Reworked as suggested, I also rebased.

Tested on Apple MacBook Pro 13" M1 (j293), kernel 7.1.5-asahidev+, on AC at 96%, has_chls=1:

threshold=90, behaviour=auto:            status=Not charging  current_now=0 uA        (cap-only, x4 samples)
threshold=90, behaviour=auto-discharge:  status=Discharging   current_now=-330000 uA  (active drain)
threshold 90 -> 85:                      selection survives the threshold write
threshold -> 100 (CHLS=0x100):           no errors, firmware charges normally, selection retained
threshold -> 90 again:                   discharge resumes (-330 mA)

@jannau