leds: turris-omnia: Support HW controlled mode via private trigger

Add support for enabling MCU controlled mode of the Turris Omnia LEDs
via a LED private trigger called "omnia-mcu". Recall that private LED
triggers will only be listed in the sysfs trigger file for LEDs that
support them (currently there is no user of this mechanism).

When in MCU controlled mode, the user can still set LED color, but the
blinking is done by MCU, which does different things for different LEDs:
- WAN LED is blinked according to the LED[0] pin of the WAN PHY
- LAN LEDs are blinked according to the LED[0] output of the
corresponding port of the LAN switch
- PCIe LEDs are blinked according to the logical OR of the MiniPCIe port
LED pins

In the future I want to make the netdev trigger to transparently offload
the blinking to the HW if user sets compatible settings for the netdev
trigger (for LEDs associated with network devices).
There was some work on this already, and hopefully we will be able to
complete it sometime, but for now there are still multiple blockers for
this, and even if there weren't, we still would not be able to configure
HW controlled mode for the LEDs associated with MiniPCIe ports.

In the meantime let's support HW controlled mode via the private LED
trigger mechanism. If, in the future, we manage to complete the netdev
trigger offloading, we can still keep this private trigger for backwards
compatibility, if needed.

We also set "omnia-mcu" to cdev->default_trigger, so that the MCU keeps
control until the user first wants to take over it. If a different
default trigger is specified in device-tree via the
'linux,default-trigger' property, LED class will overwrite
cdev->default_trigger, and so the DT property will be respected.

Signed-off-by: Marek Behún <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Lee Jones <[e-mail]>

drivers/leds/Kconfig

···
 	depends on I2C
 	depends on MACH_ARMADA_38X || COMPILE_TEST
 	depends on OF
+	select LEDS_TRIGGERS
 	help
 	  This option enables basic support for the LEDs found on the front
 	  side of CZ.NIC's Turris Omnia router. There are 12 RGB LEDs on the

drivers/leds/leds-turris-omnia.c

···
 	struct led_classdev_mc mc_cdev;
 	struct mc_subled subled_info[OMNIA_LED_NUM_CHANNELS];
 	u8 cached_channels[OMNIA_LED_NUM_CHANNELS];
-	bool on;
+	bool on, hwtrig;
 	int reg;
 };
 
···
 
 	/*
 	 * Only recalculate RGB brightnesses from intensities if brightness is
-	 * non-zero. Otherwise we won't be using them and we can save ourselves
-	 * some software divisions (Omnia's CPU does not implement the division
-	 * instruction).
+	 * non-zero (if it is zero and the LED is in HW blinking mode, we use
+	 * max_brightness as brightness). Otherwise we won't be using them and
+	 * we can save ourselves some software divisions (Omnia's CPU does not
+	 * implement the division instruction).
 	 */
-	if (brightness) {
-		led_mc_calc_color_components(mc_cdev, brightness);
+	if (brightness || led->hwtrig) {
+		led_mc_calc_color_components(mc_cdev, brightness ?:
+						      cdev->max_brightness);
 
 		/*
 		 * Send color command only if brightness is non-zero and the RGB
···
 			err = omnia_led_send_color_cmd(leds->client, led);
 	}
 
-	/* Send on/off state change only if (bool)brightness changed */
-	if (!err && !brightness != !led->on) {
+	/*
+	 * Send on/off state change only if (bool)brightness changed and the LED
+	 * is not being blinked by HW.
+	 */
+	if (!err && !led->hwtrig && !brightness != !led->on) {
 		u8 state = CMD_LED_STATE_LED(led->reg);
 
 		if (brightness)
···
 
 	return err;
 }
+
+static struct led_hw_trigger_type omnia_hw_trigger_type;
+
+static int omnia_hwtrig_activate(struct led_classdev *cdev)
+{
+	struct led_classdev_mc *mc_cdev = lcdev_to_mccdev(cdev);
+	struct omnia_leds *leds = dev_get_drvdata(cdev->dev->parent);
+	struct omnia_led *led = to_omnia_led(mc_cdev);
+	int err = 0;
+
+	mutex_lock(&leds->lock);
+
+	if (!led->on) {
+		/*
+		 * If the LED is off (brightness was set to 0), the last
+		 * configured color was not necessarily sent to the MCU.
+		 * Recompute with max_brightness and send if needed.
+		 */
+		led_mc_calc_color_components(mc_cdev, cdev->max_brightness);
+
+		if (omnia_led_channels_changed(led))
+			err = omnia_led_send_color_cmd(leds->client, led);
+	}
+
+	if (!err) {
+		/* Put the LED into MCU controlled mode */
+		err = omnia_cmd_write_u8(leds->client, CMD_LED_MODE,
+					 CMD_LED_MODE_LED(led->reg));
+		if (!err)
+			led->hwtrig = true;
+	}
+
+	mutex_unlock(&leds->lock);
+
+	return err;
+}
+
+static void omnia_hwtrig_deactivate(struct led_classdev *cdev)
+{
+	struct omnia_leds *leds = dev_get_drvdata(cdev->dev->parent);
+	struct omnia_led *led = to_omnia_led(lcdev_to_mccdev(cdev));
+	int err;
+
+	mutex_lock(&leds->lock);
+
+	led->hwtrig = false;
+
+	/* Put the LED into software mode */
+	err = omnia_cmd_write_u8(leds->client, CMD_LED_MODE,
+				 CMD_LED_MODE_LED(led->reg) |
+				 CMD_LED_MODE_USER);
+
+	mutex_unlock(&leds->lock);
+
+	if (err < 0)
+		dev_err(cdev->dev, "Cannot put LED to software mode: %i\n",
+			err);
+}
+
+static struct led_trigger omnia_hw_trigger = {
+	.name		= "omnia-mcu",
+	.activate	= omnia_hwtrig_activate,
+	.deactivate	= omnia_hwtrig_deactivate,
+	.trigger_type	= &omnia_hw_trigger_type,
+};
 
 static int omnia_led_register(struct i2c_client *client, struct omnia_led *led,
 			      struct device_node *np)
···
 	cdev = &led->mc_cdev.led_cdev;
 	cdev->max_brightness = 255;
 	cdev->brightness_set_blocking = omnia_led_brightness_set_blocking;
+	cdev->trigger_type = &omnia_hw_trigger_type;
+	/*
+	 * Use the omnia-mcu trigger as the default trigger. It may be rewritten
+	 * by LED class from the linux,default-trigger property.
+	 */
+	cdev->default_trigger = omnia_hw_trigger.name;
 
 	/* put the LED into software mode */
 	ret = omnia_cmd_write_u8(client, CMD_LED_MODE,
···
 	i2c_set_clientdata(client, leds);
 
 	mutex_init(&leds->lock);
+
+	ret = devm_led_trigger_register(dev, &omnia_hw_trigger);
+	if (ret < 0) {
+		dev_err(dev, "Cannot register private LED trigger: %d\n", ret);
+		return ret;
+	}
 
 	led = &leds->leds[0];
 	for_each_available_child_of_node(np, child) {