[PATCH RFC v2 0/9] leds: Add support for hardware-initiated hardware control trigger transition
From: Rong Zhang
Date: Wed Jun 17 2026 - 12:54:37 EST
Some laptops can tune their keyboard backlight according to ambient
light sensors (auto mode). This capability is essentially a hardware
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 a private
hardware control trigger:
1. We want a mechanism to tell userspace which trigger is the hardware
control one, so that userspace can determine if auto mode is on/off,
as well as turing it on/off programmatically without obtaining the
trigger's name via other channels
2. Turing on/off auto mode via the shortcut cannot activate/deactivate
the corresponding hardware control trigger, making the software state
out of sync
3. Even with #1 resolved, deactivating the hardware control trigger has
a side effect of emitting LED_OFF, breaking the shortcut cycle, where
"auto => 1" requires the driver to deactivate the trigger
This RFC series tries to demonstrate a path on solving these issues:
- Introduce an attribute "trigger_may_offload", so that userspace can
determine:
- if the LED device supports hardware control (supported => visible)
- which trigger is the hardware control trigger selected by the LED
device
- if the trigger is selected ("<foo_trigger>")
- if the trigger is offloaded ("[foo_trigger]")
- A callback offloaded() is added so that LED triggers can report
their hardware control state
- Add led_trigger_notify_hw_control_changed() interface, so that LED
drivers can notify the LED core about hardware-initiated hardware
control transitions. The LED core will then determine if the
transition is allowed and switching between "none" (i.e., no trigger)
and the device's private trigger accordingly
- This capability is restricted to the device's private trigger. If
the current trigger is neither the private trigger nor "none", no
transition will be made
- This interface is gated behind Kconfig LEDS_TRIGGERS_HW_CHANGED and
LED device flag LED_TRIG_HW_CHANGED
- Tune the logic of trigger deactivation so that it won't emit LED_OFF
when the deactivation is triggered by hardware
The last three patches are included in the RFC series to demonstrate how
to these interfaces are supposed to be utilized, so that ideapad-laptop
can expose the auto mode of ThinkBook's keyboard backlight. They can be
submitted separately once the dust settles, if preferred.
[ 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)
< private hardware 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
Signed-off-by: Rong Zhang <i@xxxxxxxx>
---
Changes in v2:
- Restrict the led_trigger_notify_hw_control_changed() interface to
private triggers only
- Drop PATCH v1 1/9 ("leds: Load trigger modules on-demand if used as
hw control trigger"), not relavant any more
- Gate the led_trigger_notify_hw_control_changed() interface behind
Kconfig LEDS_TRIGGERS_HW_CHANGED and LED device flag
LED_TRIG_HW_CHANGED
- Fix lock ordering inversion
- ideapad-laptop:
- Only call led_trigger_notify_hw_control_changed() when needed
- Serialize keyboard backlight notifications
- Reword commit messages and documentations
- Link to v1: https://patch.msgid.link/20260227190617.271388-1-i@xxxxxxxx
---
Rong Zhang (9):
leds: Add callback offloaded() to query the state of hardware 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 hardware & classdev brightness for keyboard backlight
platform/x86: ideapad-laptop: Serialize keyboard backlight notifications
platform/x86: ideapad-laptop: Fully support auto keyboard backlight
.../ABI/obsolete/sysfs-class-led-trigger-netdev | 16 ++
Documentation/ABI/testing/sysfs-class-led | 22 +++
.../ABI/testing/sysfs-class-led-trigger-netdev | 13 --
Documentation/leds/leds-class.rst | 74 +++++++
drivers/leds/led-class.c | 23 +++
drivers/leds/led-triggers.c | 131 +++++++++++-
drivers/leds/leds-cros_ec.c | 6 +
drivers/leds/leds-turris-omnia.c | 7 +
drivers/leds/leds.h | 2 +
drivers/leds/trigger/Kconfig | 9 +
drivers/leds/trigger/ledtrig-netdev.c | 10 +
drivers/platform/x86/lenovo/Kconfig | 1 +
drivers/platform/x86/lenovo/ideapad-laptop.c | 219 ++++++++++++++++-----
include/linux/leds.h | 9 +
14 files changed, 481 insertions(+), 61 deletions(-)
---
base-commit: 66affa37cfac0aec061cc4bcf4a065b0c52f7e19
change-id: 20260506-leds-trigger-hw-changed-96a62188cbdf
Thanks,
Rong