[RFC PATCH 1/1] platform/x86: thinkpad_acpi: Add support for auto keyboard backlight

[Qiling Zhu <kmatzkaczor244@gmail.com>]

Recent ThinkPads with an ambient light sensor (ALS) support an auto mode
for the keyboard backlight: the EC controls the backlight according to
ambient light. The mode is EC keyboard backlight level 3, in addition to
the two fixed brightness levels currently exposed by the driver, and is
currently reachable only through the Fn+Space shortcut, which is handled
directly by the firmware.

Wire the auto mode up as a private hardware control trigger, named
"tpacpi-kbd-auto", on top of the LED hw control trigger framework:

  - selecting the trigger writes level 3 (activate); the offloaded state
    is derived from the current EC level
  - when Fn+Space cycles into or out of auto mode, the driver notifies
    the LED core about the hardware-initiated hardware control
    transition, so the trigger state (shown bracketed in the trigger
    sysfs attribute) follows along
  - if the hardware is already in auto mode at driver initialization,
    the trigger is set as the default one

Deliberately unchanged:

  - max_brightness stays 2, auto mode is not multiplexed into the
    brightness value
  - writing 3 to /proc/acpi/ibm/kbdlight keeps returning -EINVAL, while
    reads keep reporting the current EC level verbatim (including 3)
  - module unload still turns the keyboard backlight off

Signed-off-by: Qiling Zhu <kmatzkaczor244@gmail.com>
---

Note: Documentation/admin-guide/laptops/thinkpad-acpi.rst has no kbdlight
section at all, so there is no in-tree documentation to update; the
userspace-facing semantics of hardware control triggers are documented
in Documentation/leds/leds-class.rst.

Open questions (details in the cover letter): whether a MLCG capability
bit indicates auto mode support, and the EC behavior on ALS-less models
when asked for level 3.

 drivers/platform/x86/lenovo/Kconfig         |   1 +
 drivers/platform/x86/lenovo/thinkpad_acpi.c | 104 ++++++++++++++++----
 2 files changed, 88 insertions(+), 17 deletions(-)

diff --git a/drivers/platform/x86/lenovo/Kconfig b/drivers/platform/x86/lenovo/Kconfig
index 516594993..74fc66a44 100644
--- a/drivers/platform/x86/lenovo/Kconfig
+++ b/drivers/platform/x86/lenovo/Kconfig
@@ -75,6 +75,7 @@ config THINKPAD_ACPI
 	depends on RFKILL || RFKILL = n
 	depends on ACPI_VIDEO || ACPI_VIDEO = n
 	depends on BACKLIGHT_CLASS_DEVICE
+	depends on LEDS_TRIGGERS_HW_CHANGED
 	depends on I2C
 	depends on DRM
 	select ACPI_PLATFORM_PROFILE
diff --git a/drivers/platform/x86/lenovo/thinkpad_acpi.c b/drivers/platform/x86/lenovo/thinkpad_acpi.c
index 4f81855ad..cd5c17d5f 100644
--- a/drivers/platform/x86/lenovo/thinkpad_acpi.c
+++ b/drivers/platform/x86/lenovo/thinkpad_acpi.c
@@ -5102,6 +5102,9 @@ static struct ibm_struct video_driver_data = {
  * Keyboard backlight subdriver
  */
 
+/* EC level that selects ALS-controlled auto mode for the keyboard backlight */
+#define TPACPI_KBD_LIGHT_AUTO_LEVEL	3
+
 static enum led_brightness kbdlight_brightness;
 static DEFINE_MUTEX(kbdlight_mutex);
 
@@ -5197,13 +5200,33 @@ static enum led_brightness kbdlight_sysfs_get(struct led_classdev *led_cdev)
 	return level;
 }
 
+static struct led_hw_trigger_type kbdlight_auto_trigger_type;
+
+static int kbdlight_auto_trigger_activate(struct led_classdev *led_cdev)
+{
+	return kbdlight_set_level(TPACPI_KBD_LIGHT_AUTO_LEVEL);
+}
+
+static bool kbdlight_auto_trigger_hw_offloaded(struct led_classdev *led_cdev)
+{
+	return kbdlight_get_level() == TPACPI_KBD_LIGHT_AUTO_LEVEL;
+}
+
+static struct led_trigger kbdlight_auto_trigger = {
+	.name		= "tpacpi-kbd-auto",
+	.trigger_type	= &kbdlight_auto_trigger_type,
+	.activate	= kbdlight_auto_trigger_activate,
+	.hw_offloaded	= kbdlight_auto_trigger_hw_offloaded,
+};
+
 static struct tpacpi_led_classdev tpacpi_led_kbdlight = {
 	.led_classdev = {
 		.name		= "tpacpi::kbd_backlight",
 		.max_brightness	= 2,
-		.flags		= LED_BRIGHT_HW_CHANGED,
+		.flags		= LED_BRIGHT_HW_CHANGED | LED_TRIG_HW_CHANGED,
 		.brightness_set_blocking = &kbdlight_sysfs_set,
 		.brightness_get	= &kbdlight_sysfs_get,
+		.trigger_type	= &kbdlight_auto_trigger_type,
 	}
 };
 
@@ -5224,6 +5247,26 @@ static int __init kbdlight_init(struct ibm_init_struct *iibm)
 	kbdlight_brightness = kbdlight_sysfs_get(NULL);
 	tp_features.kbdlight = 1;
 
+	/*
+	 * Register the auto-mode trigger before the LED classdev: if the
+	 * hardware is already in auto mode, the trigger can then be set as
+	 * the default one and gets activated during LED classdev
+	 * registration.
+	 */
+	rc = devm_led_trigger_register(&tpacpi_pdev->dev,
+				       &kbdlight_auto_trigger);
+	if (rc) {
+		tp_features.kbdlight = 0;
+		return rc;
+	}
+
+	tpacpi_led_kbdlight.led_classdev.hw_control_trigger =
+		kbdlight_auto_trigger.name;
+
+	if (kbdlight_brightness == TPACPI_KBD_LIGHT_AUTO_LEVEL)
+		tpacpi_led_kbdlight.led_classdev.default_trigger =
+			kbdlight_auto_trigger.name;
+
 	rc = led_classdev_register(&tpacpi_pdev->dev,
 				   &tpacpi_led_kbdlight.led_classdev);
 	if (rc < 0) {
@@ -5313,6 +5356,47 @@ static void kbdlight_resume(void)
 	led_classdev_resume(&tpacpi_led_kbdlight.led_classdev);
 }
 
+/* Notify the LED core when the hardware cycles into or out of auto mode. */
+static void kbdlight_notify_hw_control(struct led_classdev *led_cdev,
+				       int old_level, int new_level)
+{
+	bool was_auto = old_level == TPACPI_KBD_LIGHT_AUTO_LEVEL;
+	bool is_auto = new_level == TPACPI_KBD_LIGHT_AUTO_LEVEL;
+
+	if (is_auto != was_auto)
+		led_trigger_notify_hw_control_changed(led_cdev, is_auto);
+}
+
+/*
+ * The EC cycles the keyboard backlight level (and auto mode) itself on
+ * Fn+Space; check the current level and update the LED core state.
+ */
+static void kbdlight_notify_change(void)
+{
+	struct led_classdev *led_cdev = &tpacpi_led_kbdlight.led_classdev;
+	enum led_brightness brightness;
+
+	guard(mutex)(&kbdlight_mutex);
+
+	/*
+	 * Check the brightness actually changed, setting the brightness
+	 * through kbdlight_set_level() also triggers this event.
+	 */
+	brightness = kbdlight_sysfs_get(NULL);
+	if (kbdlight_brightness == brightness)
+		return;
+
+	/*
+	 * Calling kbdlight_notify_hw_control() is safe while holding
+	 * kbdlight_mutex: the notification is only queued, and the worker
+	 * re-acquires the mutex (if at all) after this critical section has
+	 * been left.
+	 */
+	kbdlight_notify_hw_control(led_cdev, kbdlight_brightness, brightness);
+	kbdlight_brightness = brightness;
+	led_classdev_notify_brightness_hw_changed(led_cdev, brightness);
+}
+
 static struct ibm_struct kbdlight_driver_data = {
 	.name = "kbdlight",
 	.read = kbdlight_read,
@@ -11581,22 +11665,8 @@ static bool tpacpi_driver_event(const unsigned int hkey_event)
 		/* Key events are suppressed by default hotkey_user_mask */
 		return false;
 	case TP_HKEY_EV_KBD_LIGHT:
-		if (tp_features.kbdlight) {
-			enum led_brightness brightness;
-
-			guard(mutex)(&kbdlight_mutex);
-
-			/*
-			 * Check the brightness actually changed, setting the brightness
-			 * through kbdlight_set_level() also triggers this event.
-			 */
-			brightness = kbdlight_sysfs_get(NULL);
-			if (kbdlight_brightness != brightness) {
-				kbdlight_brightness = brightness;
-				led_classdev_notify_brightness_hw_changed(
-					&tpacpi_led_kbdlight.led_classdev, brightness);
-			}
-		}
+		if (tp_features.kbdlight)
+			kbdlight_notify_change();
 		/* Key events are suppressed by default hotkey_user_mask */
 		return false;
 	case TP_HKEY_EV_DFR_CHANGE_ROW:
-- 
2.56.0


base-commit: 7649ba73f9e7e1e04d0b46213e39c79ae09dd55c