[PATCH 0/2] hwmon: (applesmc) add native charge_control_end_threshold support

[Jordan Brough <jordan@brough.org>]

This series adds native charge-limit support to applesmc, so that
generic userspace (UPower, KDE Powerdevil, GNOME Settings) can read
and write a MacBook's charging cutoff through the standard
charge_control_end_threshold sysfs attribute, with no vendor-specific
userspace daemon required.

Patch 1 is a prerequisite: some Macs register their battery through
the ACPI Smart Battery System driver (drivers/acpi/sbs.c) rather than
the ACPI Control Method Battery driver, which is the only one with an
existing hook mechanism for attaching extra power_supply properties.
Patch 1 adds a parallel, purely additive hook mechanism to sbs.c so
patch 2 can support both registration paths symmetrically.

Patch 2 adds the actual charge_control_end_threshold support to
applesmc, using the real SMC BCLM key (not the cosmetic BFCL key,
which only controls the MagSafe LED color and does not affect
charging behavior).

Tested on a MacBookAir6,2 (SBS-registered battery): sysfs attribute
appears, reads/writes round-trip correctly to the real SMC BCLM key,
and this was verified end-to-end through KDE's charge-limit slider
writing directly to sysfs. Also tested on a non-Mac laptop (CmBatt-
registered battery, no BCLM key) to confirm both new hook
registrations are true no-ops there, with no new sysfs attributes and
no behavioral change.

This was previously discussed on this list in 2020, proposed as a
generic sysfs interface for reading/writing arbitrary SMC keys by
name. Guenter raised concerns at the time about that approach adding
non-standard code to the driver:

  Link: https://www.spinics.net/lists/linux-hwmon/msg09839.html

This series takes a different approach: it exposes only the specific,
well-understood BCLM key through the standard power_supply_ext
charge_control_end_threshold property, rather than a generic raw-key
interface, and gates it at runtime on a probe of that specific key so
it stays inert on any hardware that doesn't implement it.

This is my first kernel submission; I'd welcome any correction on
process or style along with the technical review.

Jordan Brough (2):
  ACPI: sbs: add battery hook mechanism for SBS-registered batteries
  hwmon: (applesmc) add charge_control_end_threshold support

 drivers/acpi/sbs.c       | 147 ++++++++++++++++++++++++++++++++++-
 drivers/hwmon/applesmc.c | 164 +++++++++++++++++++++++++++++++++++++--
 include/acpi/battery.h   |  16 ++++
 3 files changed, 321 insertions(+), 6 deletions(-)


base-commit: fd73f4a6659897191fa0d40695fe370925dd3780
-- 
2.55.0