Re: [PATCH RFC v3 00/11] leds: Add support for hardware-initiated hardware control trigger transition

[Rong Zhang <i@rong.moe>]

Hi Lee,

On Wed, 2026-07-22 at 13:39 +0100, Lee Jones wrote:
> Why is this still an RFC?

I plan to make it non-RFC in v4, after resolving concerns from Ilpo and
Sashiko.

Thanks,
Rong

> 
> > 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 ("shortcut") 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. Writing brightness has the side effect of disabling hardware control,
> >    but the hardware control trigger remains active, resulting in the
> >    software and hardware being out of sync. Most LED drivers that
> >    supports hardware control also suffer from the same issue
> > 3. Turing on/off auto mode via the shortcut cannot activate/deactivate
> >    the corresponding hardware control trigger, making the software state
> >    out of sync
> > 4. Even with #3 solved, deactivating the hardware control trigger has
> >    the side effect of emitting LED_OFF, breaking the shortcut cycle,
> >    especially "auto => 1"
> > 
> > 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
> > - Remove hardware control trigger when writing brightness
> > - 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,
> >     can't solve #2
> > - weird semantic (an attribute other than "brightness" and "trigger"
> >   changes the brightness)
> > 
> > < private hardware control trigger (this series) >
> > 
> > Pros:
> > - mutually exclusive with other triggers and the brightness attribute
> >   (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@app.fastmail.com
> > https://lore.kernel.org/r/1dbfcf656cdb4af0299f90d7426d2ec7e2b8ac9e.camel@rong.moe
> > 
> > Signed-off-by: Rong Zhang <i@rong.moe>
> > ---
> > Changes in v3:
> > - Integrate https://lore.kernel.org/all/20260712-leds-hw-control-brightness-set-v1-1-1de593b09d26@rong.moe/
> >   into the series
> >   - Adopt __led_trigger_is_hw_controlled() in the rest of the series
> > - Rearrange the series so that the code using the offloaded() callback is
> >   introduced before the driver implementation (thanks Thomas Weißschuh)
> > - Reword documentations and commit messages (ditto)
> > - Adopt guard() and lockdep (ditto)
> > - Address concerns from Sashiko
> >   - Fix a race condition in ideapad_kbd_bl_led_cdev_brightness_set()
> >   - Fix trigger re-registration of ideapad_kbd_bl_auto_trigger
> >   - https://sashiko.dev/#/patchset/20260618-leds-trigger-hw-changed-v2-0-c28c44053cf3%40rong.moe
> > - Make registration failures of ideapad_kbd_bl_auto_trigger non-fatal
> > - Link to v2: https://patch.msgid.link/20260618-leds-trigger-hw-changed-v2-0-c28c44053cf3@rong.moe
> > 
> > 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@rong.moe
> > 
> > ---
> > Rong Zhang (11):
> >       leds: Move led_trigger_is_hw_controlled() to the right place
> >       leds: class: Remove hardware control trigger when writing brightness
> >       leds: trigger: Add offloaded() callback and provide trigger_may_offload attribute
> >       leds: cros_ec: trigger: Implement offloaded() callback
> >       leds: turris-omnia: trigger: Implement offloaded() and declare hw_control_trigger
> >       leds: trigger: netdev: Implement offloaded() callback
> >       leds: trigger: Enforce strict checks in led_trigger_is_hw_controlled()
> >       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
> > 
> >  Documentation/ABI/testing/sysfs-class-led          |  25 ++
> >  .../ABI/testing/sysfs-class-led-trigger-netdev     |   3 +
> >  Documentation/leds/leds-class.rst                  |  72 ++++++
> >  drivers/leds/led-class.c                           |  35 ++-
> >  drivers/leds/led-triggers.c                        | 146 +++++++++++-
> >  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              |   8 +
> >  drivers/platform/x86/lenovo/Kconfig                |   1 +
> >  drivers/platform/x86/lenovo/ideapad-laptop.c       | 264 ++++++++++++++++-----
> >  include/linux/leds.h                               |  19 ++
> >  13 files changed, 532 insertions(+), 65 deletions(-)
> > ---
> > base-commit: 1229e2e57a5c2980ccd457b9b53ea0eed5a22ab3
> > change-id: 20260506-leds-trigger-hw-changed-96a62188cbdf
> > 
> > Thanks,
> > Rong
> >