[PATCH 3/5] acpi_video: Add workaround for broken Windows 8 backlight implementations

[Seth Forshee <seth.forshee@canonical.com>]

Windows 8 requires that all backlights report 101 brightness levels.
When Lenovo updated the firmware for some machines for Windows 8 they
met this requirement my making _BCL return a larger set of values for
Windows 8 than for other OSes. However, only the values in the smaller
set actually change the brightness at all. The rest of the values are
silently discarded.

As a workaround, change acpi_video to set all intermediate backlight
levels when setting the brightness. This isn't perfect, but it will mean
that most brightness changes done by common userspace utilities will hit
at least one valid brightness value.

[1] http://msdn.microsoft.com/en-us/library/windows/hardware/jj128256.aspx

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
---
 drivers/acpi/video.c |   51 ++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 41 insertions(+), 10 deletions(-)

diff --git a/drivers/acpi/video.c b/drivers/acpi/video.c
index edfcd74..b83fbbd 100644
--- a/drivers/acpi/video.c
+++ b/drivers/acpi/video.c
@@ -352,25 +352,56 @@ acpi_video_device_lcd_query_levels(struct acpi_video_device *device,
 static int
 acpi_video_device_lcd_set_state(struct acpi_video_device *device, int state)
 {
-	int level = device->brightness->levels[state];
 	union acpi_object arg0 = { ACPI_TYPE_INTEGER };
 	struct acpi_object_list args = { 1, &arg0 };
+	int curr_state, offset;
 	acpi_status status;
+	int result = 0;
 
-	arg0.integer.value = level;
+	curr_state = device->brightness->curr_state;
 
-	status = acpi_evaluate_object(device->dev->handle, "_BCM",
-				      &args, NULL);
-	if (ACPI_FAILURE(status)) {
-		ACPI_ERROR((AE_INFO, "Evaluating _BCM failed"));
-		return -EIO;
+	/*
+	 * Some Lenovo firmware has a broken backlight implementation
+	 * for Windows 8 where _BCL returns 101 backlight levels but
+	 * only 16 or so levels actually change the brightness at all.
+	 * As a workaround for these machines we set every intermediate
+	 * value between the old and new brightness levels whenever the
+	 * system has made the Windows 8 OSI call, hoping that at least
+	 * one of them will cause a change in brightness.
+	 */
+	if (acpi_osi_windows_version() == ACPI_OSI_WIN_8) {
+		if (state == curr_state)
+			offset = 0;
+		else
+			offset = state > curr_state ? 1 : -1;
+	} else {
+		offset = state - curr_state;
 	}
 
-	device->brightness->curr_state = state;
+	do {
+		curr_state += offset;
+		arg0.integer.value = device->brightness->levels[curr_state];
+
+		status = acpi_evaluate_object(device->dev->handle, "_BCM",
+					      &args, NULL);
+		if (ACPI_FAILURE(status)) {
+			ACPI_ERROR((AE_INFO, "Evaluating _BCM failed"));
+
+			/*
+			 * Change curr_state back to that of last
+			 * successful _BCM call
+			 */
+			curr_state -= offset;
+			result = -EIO;
+			break;
+		}
+	} while (curr_state != state);
+
+	device->brightness->curr_state = curr_state;
 	if (device->backlight)
-		device->backlight->props.brightness = state - 2;
+		device->backlight->props.brightness = curr_state - 2;
 
-	return 0;
+	return result;
 }
 
 static int
-- 
1.7.9.5