[PATCH 2/2] platform/x86: x86-android-tablets: Create LED device for Xiaomi Pad 2 bottom bezel touch buttons

[Hans de Goede <hdegoede@redhat.com>]

The Xiaomi [Mi]Pad 2 has 3 menu / home / back capacitive touch-buttons
on its bottom bezel. These are backlit by LEDs attached to a TPS61158 LED
controller which is controlled by the "pwm_soc_lpss_2" PWM output.

Create a LED class device for this, using the new input-events trigger
as default trigger so that the buttons automatically light up on any
input activity.

Note alternatively a "leds_pwm" platform device could be created together
with the necessary fwnode_s_ and a fwnode link to the PWM controller.
There are 2 downsides to this approach:

1. The code would still need to pwm_get() the PWM controller to get/attach
a fwnode for the PWM controller fwnode link and setting up the necessary
fwnodes is non trivial. So this would likely require more code then simply
registering the LED class device directly.

2. Currently the leds_pwm driver and its devicetree bindings do not support
limiting the maximum dutycycle to less then 100% which is required in this
case (the leds_pwm driver can probably  be extended to allow this).

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
---
 .../platform/x86/x86-android-tablets/other.c  | 44 +++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/drivers/platform/x86/x86-android-tablets/other.c b/drivers/platform/x86/x86-android-tablets/other.c
index ff7b1d0abaa3..aa95645f3f24 100644
--- a/drivers/platform/x86/x86-android-tablets/other.c
+++ b/drivers/platform/x86/x86-android-tablets/other.c
@@ -11,7 +11,9 @@
 #include <linux/acpi.h>
 #include <linux/gpio/machine.h>
 #include <linux/input.h>
+#include <linux/leds.h>
 #include <linux/platform_device.h>
+#include <linux/pwm.h>
 
 #include <dt-bindings/leds/common.h>
 
@@ -662,8 +664,50 @@ static const struct software_node *ktd2026_node_group[] = {
 	NULL
 };
 
+/*
+ * For the LEDs which backlight the menu / home / back capacitive buttons on
+ * the bottom bezel. These are attached to a TPS61158 LED controller which
+ * is controlled by the "pwm_soc_lpss_2" PWM output.
+ */
+#define XIAOMI_MIPAD2_LED_PERIOD_NS		19200
+#define XIAOMI_MIPAD2_LED_MAX_DUTY		 6000 /* From Android kernel */
+
+static struct pwm_device *xiaomi_mipad2_led_pwm;
+
+static int xiaomi_mipad2_brightness_set(struct led_classdev *led_cdev,
+					enum led_brightness val)
+{
+	struct pwm_state state = {
+		.period = XIAOMI_MIPAD2_LED_PERIOD_NS,
+		.duty_cycle = val,
+		.enabled = !!val,
+	};
+
+	return pwm_apply_might_sleep(xiaomi_mipad2_led_pwm, &state);
+}
+
 static int __init xiaomi_mipad2_init(struct device *dev)
 {
+	struct led_classdev *led_cdev;
+	int ret;
+
+	xiaomi_mipad2_led_pwm = devm_pwm_get(dev, "pwm_soc_lpss_2");
+	if (IS_ERR(xiaomi_mipad2_led_pwm))
+		return dev_err_probe(dev, PTR_ERR(xiaomi_mipad2_led_pwm), "getting pwm\n");
+
+	led_cdev = devm_kzalloc(dev, sizeof(*led_cdev), GFP_KERNEL);
+	if (!led_cdev)
+		return -ENOMEM;
+
+	led_cdev->name = "mipad2:white:touch-buttons-backlight";
+	led_cdev->max_brightness = XIAOMI_MIPAD2_LED_MAX_DUTY;
+	led_cdev->brightness_set_blocking = xiaomi_mipad2_brightness_set;
+	led_cdev->default_trigger = "input-events";
+
+	ret = devm_led_classdev_register(dev, led_cdev);
+	if (ret)
+		return dev_err_probe(dev, ret, "registering LED\n");
+
 	return software_node_register_node_group(ktd2026_node_group);
 }
 
-- 
2.44.0