[RFC PATCH 0/9] leds: Add support for hw initiated hw control trigger transition

[Rong Zhang]

Hi all,

Some laptops can tune their keyboard backlight according to ambient
light sensors (auto mode). This capability is essentially a hw control
trigger. Meanwhile, such laptops also offer a shrotcut for cycling
through brightness levels and auto mode. For example, on ThinkBook,
pressing Fn+Space cycles keyboard backlight levels in the following
sequence:

  1 => 2 => 0 => auto => 1 ...

Recent ThinkPad models should have similar sequence too.

However, there are some issues preventing us from using hw control
trigger:

1. We want a mechanism to tell userspace which trigger is the hw control
   trigger, so that userspace can determine if auto mode is on/off or
   turing it on/off programmatically without obtaining the hw control
   trigger's name via other channels
2. Turing on/off auto mode via the shortcut cannot activate/deactivate
   the hw control trigger, making the software state out-of-sync
3. Even with #1 resolved, deactivating the hw control trigger after
   receiving the event indicating "auto => 1" has a side effect of
   emitting LED_OFF, breaking the shortcut cycle

This RFC series tries to demonstrate a path on solving these issues:

- Introduce an attribute called trigger_may_offload, so that userspace
   can determine:
   - if the LED device supports hw control (supported => visible)
   - which trigger is the hw control trigger
   - if the hw control trigger is selected
   - if the hw control trigger is in hw control (i.e., offloaded)
     - A callback offloaded() is added so that LED triggers can report
       their hw control state
- Add led_trigger_notify_hw_control_changed() interface, so that LED
  drivers can notify the LED core about hardware initiated hw control
  state transitions. The LED core will then determine if the transition
  is allowed and turning on/off the hw control trigger accordingly
- Tune the logic of trigger deactivation so that it won't emit LED_OFF
  when the deactivation is triggered by hardware

The last two patches are included into the RFC series to demonstrate how
to utilize these interfaces to add support for auto keyboard backlight
to ThinkBook. They will be submitted separately once the dust settles.

Currently no Kconfig entry is provided to disable either interface. If
needed, I will add one later.

[ Summary of other approaches ]

< custom attribute >

Pros:
- simplicity, KISS
- no need to touch the LED core
- extensible as long as it has a sensor-neutral name
  - a sensor-related name could potentially lead to a mess if a future
    device implements auto mode based on multiple different sensors

Cons:
- must have zero influence on brightness_set[_blocking] callbacks
  in order not to break triggers
  - potential interference with triggers and the brightness attribute
- weird semantic (an attribute other than "brightness" and "trigger"
  changes the brightness)

< hw control trigger (this series) >

Pros:
- mutually exclusive with other triggers (hence less chaos)
- semantic correctness
- acts as an aggregate switch to turn on/off auto mode even a future
  device implements auto mode based on multiple different sensors
  - extensibility (through trigger attributes)

Cons:
- complexity

[ Previous discussion threads ]

https://lore.kernel.org/r/08580ec5-1d7b-4612-8a3f-75bc2f40aad2@xxxxxxxxxxxxxxxx

https://lore.kernel.org/r/1dbfcf656cdb4af0299f90d7426d2ec7e2b8ac9e.camel@xxxxxxxx

Thanks,
Rong

Rong Zhang (9):
  leds: Load trigger modules on-demand if used as hw control trigger
  leds: Add callback offloaded() to query the state of hw control
    trigger
  leds: cros_ec: Implement offloaded() callback for trigger
  leds: turris-omnia: Implement offloaded() callback for trigger
  leds: trigger: netdev: Implement offloaded() callback
  leds: Add trigger_may_offload attribute
  leds: trigger: Add led_trigger_notify_hw_control_changed() interface
  platform/x86: ideapad-laptop: Decouple HW & cdev brightness for kbd
    backlight
  platform/x86: ideapad-laptop: Fully support auto kbd backlight

 .../obsolete/sysfs-class-led-trigger-netdev   |  15 ++
 Documentation/ABI/testing/sysfs-class-led     |  22 ++
 .../testing/sysfs-class-led-trigger-netdev    |  13 --
 Documentation/leds/leds-class.rst             |  72 ++++++-
 drivers/leds/led-class.c                      |  23 +++
 drivers/leds/led-triggers.c                   | 176 +++++++++++++++-
 drivers/leds/leds-cros_ec.c                   |   6 +
 drivers/leds/leds-turris-omnia.c              |   7 +
 drivers/leds/leds.h                           |   3 +
 drivers/leds/trigger/ledtrig-netdev.c         |  10 +
 drivers/platform/x86/lenovo/Kconfig           |   1 +
 drivers/platform/x86/lenovo/ideapad-laptop.c  | 194 ++++++++++++++----
 include/linux/leds.h                          |   6 +
 13 files changed, 492 insertions(+), 56 deletions(-)
 create mode 100644 Documentation/ABI/obsolete/sysfs-class-led-trigger-netdev


base-commit: a75cb869a8ccc88b0bc7a44e1597d9c7995c56e5
-- 
2.51.0