linux-kernel - Re: [PATCH 2/2] platform/x86: ideapad-laptop: Fully support auto kbd backlight

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Message-Id: <08580ec5-1d7b-4612-8a3f-75bc2f40aad2@app.fastmail.com>
Date: Wed, 06 Aug 2025 15:02:25 -0400
From: "Mark Pearson" <mpearson-lenovo@...ebb.ca>
To: "Rong Zhang" <i@...g.moe>, "Ike Panhc" <ikepanhc@...il.com>,
 "Derek J . Clark" <derekjohn.clark@...il.com>,
 Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
 "Hans de Goede" <hansg@...nel.org>
Cc: 
 "platform-driver-x86@...r.kernel.org" <platform-driver-x86@...r.kernel.org>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] platform/x86: ideapad-laptop: Fully support auto kbd backlight

Hi Rong,

On Tue, Aug 5, 2025, at 10:01 AM, Rong Zhang wrote:
> Currently, the auto brightness mode of keyboard backlight maps to
> brightness=0 in LED classdev. The only method to switch to such a mode
> is by pressing the manufacturer-defined shortcut (Fn+Space). However, 0
> is a multiplexed brightness value; writing 0 simply results in the
> backlight being turned off.
>
> With brightness processing code decoupled from LED classdev, we can now
> fully support the auto brightness mode. In this mode, the keyboard
> backlight is controlled by the EC according to the ambient light sensor
> (ALS).
>
> To utilize this, a sysfs node is exposed to the userspace:
> /sys/class/leds/platform::kbd_backlight/als_enabled. The name is chosen
> to align with dell-laptop, which provides a similar feature.
>
> Signed-off-by: Rong Zhang <i@...g.moe>
> ---
>  .../ABI/testing/sysfs-platform-ideapad-laptop | 12 ++++
>  drivers/platform/x86/lenovo/ideapad-laptop.c  | 65 ++++++++++++++++++-
>  2 files changed, 75 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/ABI/testing/sysfs-platform-ideapad-laptop 
> b/Documentation/ABI/testing/sysfs-platform-ideapad-laptop
> index 5ec0dee9e707..a2b78aa60aaa 100644
> --- a/Documentation/ABI/testing/sysfs-platform-ideapad-laptop
> +++ b/Documentation/ABI/testing/sysfs-platform-ideapad-laptop
> @@ -50,3 +50,15 @@ Description:
>  		Controls whether the "always on USB charging" feature is
>  		enabled or not. This feature enables charging USB devices
>  		even if the computer is not turned on.
> +
> +What:		/sys/class/leds/platform::kbd_backlight/als_enabled
> +Date:		July 2025
> +KernelVersion:	6.17
> +Contact:	platform-driver-x86@...r.kernel.org
> +Description:
> +		This file allows to control the automatic keyboard
> +		illumination mode on some systems that have an ambient
> +		light sensor. Write 1 to this file to enable the auto
> +		mode, 0 to disable it. In this mode, the actual
> +		brightness level is not available and reading the
> +		"brightness" file always returns 0.
> diff --git a/drivers/platform/x86/lenovo/ideapad-laptop.c 
> b/drivers/platform/x86/lenovo/ideapad-laptop.c
> index 5014c1d0b633..49f2fc68add4 100644
> --- a/drivers/platform/x86/lenovo/ideapad-laptop.c
> +++ b/drivers/platform/x86/lenovo/ideapad-laptop.c
> @@ -1712,6 +1712,57 @@ static void ideapad_kbd_bl_notify(struct 
> ideapad_private *priv)
>  	ideapad_kbd_bl_notify_known(priv, brightness);
>  }
> 
> +static ssize_t als_enabled_show(struct device *dev,
> +				struct device_attribute *attr,
> +				char *buf)
> +{
> +	struct led_classdev *led_cdev = dev_get_drvdata(dev);
> +	struct ideapad_private *priv = container_of(led_cdev, struct 
> ideapad_private, kbd_bl.led);
> +	int hw_brightness;
> +
> +	hw_brightness = ideapad_kbd_bl_hw_brightness_get(priv);
> +	if (hw_brightness < 0)
> +		return hw_brightness;
> +
> +	return sysfs_emit(buf, "%d\n", hw_brightness == 
> KBD_BL_AUTO_MODE_HW_BRIGHTNESS);
> +}
> +
> +static ssize_t als_enabled_store(struct device *dev,
> +				 struct device_attribute *attr,
> +				 const char *buf, size_t count)
> +{
> +	struct led_classdev *led_cdev = dev_get_drvdata(dev);
> +	struct ideapad_private *priv = container_of(led_cdev, struct 
> ideapad_private, kbd_bl.led);
> +	bool state;
> +	int err;
> +
> +	err = kstrtobool(buf, &state);
> +	if (err)
> +		return err;
> +
> +	/*
> +	 * Auto (ALS) mode uses a predefined HW brightness value. It is
> +	 * impossible to disable it without setting another brightness value.
> +	 * Set the brightness to 0 when disabling is requested.
> +	 */
> +	err = ideapad_kbd_bl_hw_brightness_set(priv, state ? 
> KBD_BL_AUTO_MODE_HW_BRIGHTNESS : 0);
> +	if (err)
> +		return err;
> +
> +	/* Both HW brightness values map to 0 in the LED classdev. */
> +	ideapad_kbd_bl_notify_known(priv, 0);
> +
> +	return count;
> +}
> +
> +static DEVICE_ATTR_RW(als_enabled);
> +
> +static struct attribute *ideapad_kbd_bl_als_attrs[] = {
> +	&dev_attr_als_enabled.attr,
> +	NULL,
> +};
> +ATTRIBUTE_GROUPS(ideapad_kbd_bl_als);
> +
>  static int ideapad_kbd_bl_init(struct ideapad_private *priv)
>  {
>  	int brightness, err;
> @@ -1722,10 +1773,20 @@ static int ideapad_kbd_bl_init(struct 
> ideapad_private *priv)
>  	if (WARN_ON(priv->kbd_bl.initialized))
>  		return -EEXIST;
> 
> -	if (ideapad_kbd_bl_check_tristate(priv->kbd_bl.type)) {
> +	switch (priv->kbd_bl.type) {
> +	case KBD_BL_TRISTATE_AUTO:
> +		/* The sysfs node will be 
> /sys/class/leds/platform::kbd_backlight/als_enabled */
> +		priv->kbd_bl.led.groups = ideapad_kbd_bl_als_groups;
> +		fallthrough;
> +	case KBD_BL_TRISTATE:
>  		priv->kbd_bl.led.max_brightness = 2;
> -	} else {
> +		break;
> +	case KBD_BL_STANDARD:
>  		priv->kbd_bl.led.max_brightness = 1;
> +		break;
> +	default:
> +		/* This has already been validated by ideapad_check_features(). */
> +		unreachable();
>  	}
> 
>  	brightness = ideapad_kbd_bl_brightness_get(priv);
> -- 
> 2.50.1

We're looking to implement this feature on the Thinkpads, so this change is timely :)

I did wonder if we should be making changes at the LED class level? Something similar to LED_BRIGHT_HW_CHANGED maybe as a way to advertise that auto mode is supported and some hooks to support that in sysfs?
I know it would be more work, but I'm guessing this is going to be a common feature across multiple vendors it might need doing at a common layer.

As a note - on the Thinkpads we've had to look at getting the correct Intel ISH firmware loaded (and we're working on getting that upstream to linux-firmware). Is that needed on the Ideapads for the feature to work well or not?

Mark