acpi: video: enhance the quirk detect logic of _BQC

Currently we decide if the _BQC is using index by first setting the
level to maximum, and then check if _BQC returned maximum; if not, we
say it is using index.

This is not true for some buggy systems, where the _BQC method will
always return a constant value(e.g. 0 or 100 for the two broken system)
and thus break the current logic. So this patch tries to enhance the
quirk detect logic for _BQC: we do this by picking a test_level, it can
be the maximum level or the mininum one based on some condition. And we
don't make the assumption that if _BQC returned a value that is not what
we just set, it must be using an index. Instead, we will compare the
value returned from _BQC and if it doesn't match, see if the returned
value is an index. And if still no, clear the capability of _BQC.

References: https://bugzilla.kernel.org/show_bug.cgi?id=42861
References: https://bugzilla.kernel.org/show_bug.cgi?id=56011
Reported-and-tested-by: Artem Savkov <artem.savkov@gmail.com>
Reported-by: Luis Medinas <lmedinas@gmail.com>
Reported-by: Cheppes <cheppes@mailinator.com>
Signed-off-by: Aaron Lu <aaron.lu@intel.com>
Acked-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

drivers/acpi/video.c

@@ -631,6 +631,56 @@ acpi_video_cmp_level(const void *a, const void *b)
 	return *(int *)a - *(int *)b;
 }
 
+/*
+ * Decides if _BQC/_BCQ for this system is usable
+ *
+ * We do this by changing the level first and then read out the current
+ * brightness level, if the value does not match, find out if it is using
+ * index. If not, clear the _BQC/_BCQ capability.
+ */
+static int acpi_video_bqc_quirk(struct acpi_video_device *device,
+				int max_level, int current_level)
+{
+	struct acpi_video_device_brightness *br = device->brightness;
+	int result;
+	unsigned long long level;
+	int test_level;
+
+	/* don't mess with existing known broken systems */
+	if (bqc_offset_aml_bug_workaround)
+		return 0;
+
+	/*
+	 * Some systems always report current brightness level as maximum
+	 * through _BQC, we need to test another value for them.
+	 */
+	test_level = current_level == max_level ? br->levels[2] : max_level;
+
+	result = acpi_video_device_lcd_set_level(device, test_level);
+	if (result)
+		return result;
+
+	result = acpi_video_device_lcd_get_level_current(device, &level, true);
+	if (result)
+		return result;
+
+	if (level != test_level) {
+		/* buggy _BQC found, need to find out if it uses index */
+		if (level < br->count) {
+			if (br->flags._BCL_reversed)
+				level = br->count - 3 - level;
+			if (br->levels[level + 2] == test_level)
+				br->flags._BQC_use_index = 1;
+		}
+
+		if (!br->flags._BQC_use_index)
+			device->cap._BQC = device->cap._BCQ = 0;
+	}
+
+	return 0;
+}
+
+
 /*
  *  Arg:	
  *  	device	: video output device (LCD, CRT, ..)
@@ -742,18 +792,15 @@ acpi_video_init_brightness(struct acpi_video_device *device)
 	if (result)
 		goto out_free_levels;
 
-	/*
-	 * Set the level to maximum and check if _BQC uses indexed value
-	 */
-	result = acpi_video_device_lcd_set_level(device, max_level);
-	if (result)
-		goto out_free_levels;
-
-	result = acpi_video_device_lcd_get_level_current(device, &level, true);
+	result = acpi_video_bqc_quirk(device, max_level, level_old);
 	if (result)
 		goto out_free_levels;
-
-	br->flags._BQC_use_index = (level == max_level ? 0 : 1);
+	/*
+	 * cap._BQC may get cleared due to _BQC is found to be broken
+	 * in acpi_video_bqc_quirk, so check again here.
+	 */
+	if (!device->cap._BQC)
+		goto set_level;
 
 	if (use_bios_initial_backlight) {
 		level = acpi_video_bqc_value_to_level(device, level_old);