1 files changed, 57 insertions, 10 deletions

diff --git a/drivers/acpi/video.c b/drivers/acpi/video.c
index 3cdd0471bc63..ed192e5d82b6 100644
--- a/drivers/acpi/video.c
+++ b/drivers/acpi/video.c
@@ -632,6 +632,56 @@ acpi_video_cmp_level(const void *a, const void *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);