leds: trigger: netdev: Add core support for hw not supporting fallback to LED sw control

If hw doesn't support sw control of the LED and we switch to a mode
not supported by hw, currently we get lots of errors because neither
brigthness_set() nor brithness_set_blocking() is set.
Deal with this by not falling back to sw control, and return
-EOPNOTSUPP to the user. Note that we still store the new mode.
This is needed in case an intermediate unsupported mode is necessary
to switch from one supported mode to another.

Add a comment explaining how a driver for such hw is supposed to behave.

Signed-off-by: Heiner Kallweit <[e-mail]>
Reviewed-by: Andrew Lunn <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Lee Jones <[e-mail]>

drivers/leds/trigger/ledtrig-netdev.c

···
  * tx -  LED blinks on transmitted data
  * rx -  LED blinks on receive data
  *
+ * Note: If the user selects a mode that is not supported by hw, default
+ * behavior is to fall back to software control of the LED. However not every
+ * hw supports software control. LED callbacks brightness_set() and
+ * brightness_set_blocking() are NULL in this case. hw_control_is_supported()
+ * should use available means supported by hw to inform the user that selected
+ * mode isn't supported by hw. This could be switching off the LED or any
+ * hw blink mode. If software control fallback isn't possible, we return
+ * -EOPNOTSUPP to the user, but still store the selected mode. This is needed
+ * in case an intermediate unsupported mode is necessary to switch from one
+ * supported mode to another.
  */
 
 struct led_netdev_data {
···
 				     size_t size, enum led_trigger_netdev_modes attr)
 {
 	struct led_netdev_data *trigger_data = led_trigger_get_drvdata(dev);
+	struct led_classdev *led_cdev = trigger_data->led_cdev;
 	unsigned long state, mode = trigger_data->mode;
 	int ret;
 	int bit;
···
 
 	trigger_data->mode = mode;
 	trigger_data->hw_control = can_hw_control(trigger_data);
+
+	if (!led_cdev->brightness_set && !led_cdev->brightness_set_blocking &&
+	    !trigger_data->hw_control)
+		return -EOPNOTSUPP;
 
 	set_baseline_state(trigger_data);