ACPI: video: Change how we determine if brightness key-presses are handled

Some systems have an ACPI video bus but not ACPI video devices with
backlight capability. On these devices brightness key-presses are
(logically) not reported through the ACPI video bus.

Change how acpi_video_handles_brightness_key_presses() determines if
brightness key-presses are handled by the ACPI video driver to avoid
vendor specific drivers/platform/x86 drivers filtering out their
brightness key-presses even though they are the only ones reporting
these presses.

Fixes: ed83c917 ("platform/x86: panasonic-laptop: Resolve hotkey double trigger bug")
Reported-and-tested-by: Stefan Seyfried <seife+kernel@b1-systems.com>
Reported-and-tested-by: Kenneth Chan <kenneth.t.chan@gmail.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20220624112340.10130-2-hdegoede@redhat.com

drivers/acpi/acpi_video.c

@@ -73,6 +73,7 @@ module_param(device_id_scheme, bool, 0444);
 static int only_lcd = -1;
 module_param(only_lcd, int, 0444);
 
+static bool has_backlight;
 static int register_count;
 static DEFINE_MUTEX(register_count_mutex);
 static DEFINE_MUTEX(video_list_lock);
@@ -1222,6 +1223,9 @@ acpi_video_bus_get_one_device(struct acpi_device *device,
 	acpi_video_device_bind(video, data);
 	acpi_video_device_find_cap(data);
 
+	if (data->cap._BCM && data->cap._BCL)
+		has_backlight = true;
+
 	mutex_lock(&video->device_list_lock);
 	list_add_tail(&data->entry, &video->video_device_list);
 	mutex_unlock(&video->device_list_lock);
@@ -2249,6 +2253,7 @@ void acpi_video_unregister(void)
 	if (register_count) {
 		acpi_bus_unregister_driver(&acpi_video_bus);
 		register_count = 0;
+		has_backlight = false;
 	}
 	mutex_unlock(&register_count_mutex);
 }
@@ -2270,13 +2275,7 @@ void acpi_video_unregister_backlight(void)
 
 bool acpi_video_handles_brightness_key_presses(void)
 {
-	bool have_video_busses;
-
-	mutex_lock(&video_list_lock);
-	have_video_busses = !list_empty(&video_bus_head);
-	mutex_unlock(&video_list_lock);
-
-	return have_video_busses &&
+	return has_backlight &&
 	       (report_key_events & REPORT_BRIGHTNESS_KEY_EVENTS);
 }
 EXPORT_SYMBOL(acpi_video_handles_brightness_key_presses);