Introduce a mechanism for drivers to extend the properties implemented
by a power supply.

Motivation
----------

Various drivers, mostly in platform/x86 extend the ACPI battery driver
with additional sysfs attributes to implement more UAPIs than are
exposed through ACPI by using various side-channels, like WMI,
nonstandard ACPI or EC communication.

While the created sysfs attributes look similar to the attributes
provided by the powersupply core, there are various deficiencies:

* They don't show up in uevent payload.
* They can't be queried with the standard in-kernel APIs.
* They don't work with triggers.
* The extending driver has to reimplement all of the parsing,
  formatting and sysfs display logic.
* Writing a extension driver is completely different from writing a
  normal power supply driver.
* ~Properties can not be properly overriden.~
  (Overriding is now explicitly forbidden)

The proposed extension API avoids all of these issues.
An extension is just a "struct power_supply_ext" with the same kind of
callbacks as in a normal "struct power_supply_desc".

The API is meant to be used via battery_hook_register(), the same way as
the current extensions.

When testing, please enable lockdep to make sure the locking is correct.

Contents
--------

* Patch 1 to 6 are preparation patches.
* Patch 7 implements the extension API itself.
* Patch 8 adds extension support to test_power.c
* Patch 9 converts the in-tree cros_charge-control driver to the
  extension API.

Open issues
-----------

* As this is only useful with the hooks of CONFIG_ACPI_BATTERY, should
  it also be gated behind this or another config?
* Is an rw_semaphore acceptable?

[0] https://lore.kernel.org/lkml/20240528-cros_ec-charge-cont...

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
---
Changes in v4:
- Drop RFC state
- Integrate locking commit
- Reregister hwmon device
- Link to v3: https://lore.kernel.org/r/20240904-power-supply-extension...

Changes in v3:
- Make naming more consistent
- Readd locking
- Allow multiple active extensions
- Allow passing a "void *ext_data" when registering
- Switch example driver from system76 to cros_charge-control
- Link to v2: https://lore.kernel.org/r/20240608-power-supply-extension...

Changes in v2:
- Drop locking patch, let's figure out the API first
- Allow registration of multiple extensions
- Pass extension to extension callbacks as parameter
- Disallow property overlap between extensions and core psy
- Drop system76/pdx86 maintainers, as the system76 changes are only RFC
  state anyways
- Link to v1: https://lore.kernel.org/r/20240606-power-supply-extension...

---
Thomas Weißschuh (9):
      power: supply: sysfs: print single value in uevent for POWER_SUPPLY_PROP_CHARGE_BEHAVIOUR
      power: supply: core: rename psy_has_property() to psy_desc_has_property()
      power: supply: core: introduce power_supply_has_property()
      power: supply: hwmon: prepare for power supply extensions
      power: supply: sysfs: prepare for power supply extensions
      power: supply: sysfs: rework uevent property loop
      power: supply: core: implement extension API
      power: supply: test-power: implement a power supply extension
      power: supply: cros_charge-control: use power_supply extensions

 drivers/power/supply/cros_charge-control.c | 205 +++++++++++------------------
 drivers/power/supply/power_supply.h        |  16 +++
 drivers/power/supply/power_supply_core.c   | 175 ++++++++++++++++++++++--
 drivers/power/supply/power_supply_hwmon.c  |  50 ++++---
 drivers/power/supply/power_supply_sysfs.c  |  85 ++++++------
 drivers/power/supply/test_power.c          | 102 ++++++++++++++
 include/linux/power_supply.h               |  32 +++++
 7 files changed, 456 insertions(+), 209 deletions(-)
---
base-commit: 83bce34420eaf91506957703bf9a31d8581ed6cb
change-id: 20240602-power-supply-extensions-07d949f509d9

Best regards,
-- 
Thomas Weißschuh <linux@weissschuh.net>