leds: turris-omnia: Notify sysfs on MCU global LEDs brightness change

Recall that on Turris Omnia, the LED controller has a global brightness
property, which allows the user to make the front LED panel dimmer.

There is also a button on the front panel, which by default is
configured so that pressing it changes the global brightness to a lower
value (unless it is at 0%, in which case pressing the button changes the
global brightness to 100%).

Newer versions of the MCU firmware support informing the SOC that the
brightness was changed by button press event via an interrupt.

Now that we have the turris-omnia-mcu driver, which adds support for MCU
interrupts, add the ability to inform the userspace (via a sysfs
notification) that the global brightness was changed.

Signed-off-by: Marek Behún <kabel@kernel.org>
Link: https://lore.kernel.org/r/20241111100355.6978-8-kabel@kernel.org
Signed-off-by: Lee Jones <lee@kernel.org>

drivers/leds/Kconfig

@@ -218,6 +218,7 @@ config LEDS_TURRIS_OMNIA
 	depends on MACH_ARMADA_38X || COMPILE_TEST
 	depends on OF
 	depends on TURRIS_OMNIA_MCU
+	depends on TURRIS_OMNIA_MCU_GPIO
 	select LEDS_TRIGGERS
 	help
 	  This option enables basic support for the LEDs found on the front

drivers/leds/leds-turris-omnia.c

@@ -42,12 +42,15 @@ struct omnia_led {
  * @client:			I2C client device
  * @lock:			mutex to protect cached state
  * @has_gamma_correction:	whether the MCU firmware supports gamma correction
+ * @brightness_knode:		kernel node of the "brightness" device sysfs attribute (this is the
+ *				driver specific global brightness, not the LED classdev brightness)
  * @leds:			flexible array of per-LED data
  */
 struct omnia_leds {
 	struct i2c_client *client;
 	struct mutex lock;
 	bool has_gamma_correction;
+	struct kernfs_node *brightness_knode;
 	struct omnia_led leds[];
 };
 
@@ -372,6 +375,59 @@ static struct attribute *omnia_led_controller_attrs[] = {
 };
 ATTRIBUTE_GROUPS(omnia_led_controller);
 
+static irqreturn_t omnia_brightness_changed_threaded_fn(int irq, void *data)
+{
+	struct omnia_leds *leds = data;
+
+	if (unlikely(!leds->brightness_knode)) {
+		/*
+		 * Note that sysfs_get_dirent() may sleep. This is okay, because we are in threaded
+		 * context.
+		 */
+		leds->brightness_knode = sysfs_get_dirent(leds->client->dev.kobj.sd, "brightness");
+		if (!leds->brightness_knode)
+			return IRQ_NONE;
+	}
+
+	sysfs_notify_dirent(leds->brightness_knode);
+
+	return IRQ_HANDLED;
+}
+
+static void omnia_brightness_knode_put(void *data)
+{
+	struct omnia_leds *leds = data;
+
+	if (leds->brightness_knode)
+		sysfs_put(leds->brightness_knode);
+}
+
+static int omnia_request_brightness_irq(struct omnia_leds *leds)
+{
+	struct device *dev = &leds->client->dev;
+	int ret;
+
+	if (!leds->client->irq) {
+		dev_info(dev,
+			 "Brightness change interrupt supported by MCU firmware but not described in device-tree\n");
+
+		return 0;
+	}
+
+	/*
+	 * Registering the brightness_knode destructor before requesting the IRQ ensures that on
+	 * removal the brightness_knode sysfs node is put only after the IRQ is freed.
+	 * This is needed because the interrupt handler uses the knode.
+	 */
+	ret = devm_add_action(dev, omnia_brightness_knode_put, leds);
+	if (ret < 0)
+		return ret;
+
+	return devm_request_threaded_irq(dev, leds->client->irq, NULL,
+					 omnia_brightness_changed_threaded_fn, IRQF_ONESHOT,
+					 "leds-turris-omnia", leds);
+}
+
 static int omnia_mcu_get_features(const struct i2c_client *mcu_client)
 {
 	u16 reply;
@@ -458,6 +514,12 @@ static int omnia_leds_probe(struct i2c_client *client)
 			 "Consider upgrading MCU firmware with the omnia-mcutool utility.\n");
 	}
 
+	if (ret & OMNIA_FEAT_BRIGHTNESS_INT) {
+		ret = omnia_request_brightness_irq(leds);
+		if (ret < 0)
+			return dev_err_probe(dev, ret, "Cannot request brightness IRQ\n");
+	}
+
 	mutex_init(&leds->lock);
 
 	ret = devm_led_trigger_register(dev, &omnia_hw_trigger);