path: root/Documentation/acpi
diff options
authorAaron Lu <>2013-06-20 15:08:57 +0800
committerRafael J. Wysocki <>2013-06-21 00:16:05 +0200
commit86393865f0c9f7002aa1fc2b8bdce11f3aefcebe (patch)
tree45deb041cd5782bced7a6876d91d76145c9bcb7f /Documentation/acpi
parent70e66e4df19167653aba95b4dacbcfd3254a4019 (diff)
ACPI / video: update video_extension.txt for backlight control
The ACPI video driver has changed a lot, and it doesn't export interfaces in /proc any more, so the documentation for it should be updated. This update focuses on ACPI video driver's backlight control. Signed-off-by: Aaron Lu <> Signed-off-by: Rafael J. Wysocki <>
Diffstat (limited to 'Documentation/acpi')
1 files changed, 95 insertions, 26 deletions
diff --git a/Documentation/acpi/video_extension.txt b/Documentation/acpi/video_extension.txt
index b2f9b1598ac2..78b32ac02466 100644
--- a/Documentation/acpi/video_extension.txt
+++ b/Documentation/acpi/video_extension.txt
@@ -8,30 +8,99 @@ defining the video POST device, retrieving EDID information or to
setup a video output, etc. Note that this is an ref. implementation
only. It may or may not work for your integrated video device.
-Interfaces exposed to userland through /proc/acpi/video:
-VGA/info : display the supported video bus device capability like Video ROM, CRT/LCD/TV.
-VGA/ROM : Used to get a copy of the display devices' ROM data (up to 4k).
-VGA/POST_info : Used to determine what options are implemented.
-VGA/POST : Used to get/set POST device.
-VGA/DOS : Used to get/set ownership of output switching:
- Please refer ACPI spec B.4.1 _DOS
-VGA/CRT : CRT output
-VGA/LCD : LCD output
-VGA/TVO : TV output
-VGA/*/brightness : Used to get/set brightness of output device
-Notify event through /proc/acpi/event:
+The ACPI video driver does 3 things regarding backlight control:
+1 Export a sysfs interface for user space to control backlight level
+If the ACPI table has a video device, and acpi_backlight=vendor kernel
+command line is not present, the driver will register a backlight device
+and set the required backlight operation structure for it for the sysfs
+interface control. For every registered class device, there will be a
+directory named acpi_videoX under /sys/class/backlight.
+The backlight sysfs interface has a standard definition here:
+And what ACPI video driver does is:
+actual_brightness: on read, control method _BQC will be evaluated to
+get the brightness level the firmware thinks it is at;
+bl_power: not implemented, will set the current brightness instead;
+brightness: on write, control method _BCM will run to set the requested
+brightness level;
+max_brightness: Derived from the _BCL package(see below);
+type: firmware
+Note that ACPI video backlight driver will always use index for
+brightness, actual_brightness and max_brightness. So if we have
+the following _BCL package:
+Method (_BCL, 0, NotSerialized)
+ Return (Package (0x0C)
+ {
+ 0x64,
+ 0x32,
+ 0x0A,
+ 0x14,
+ 0x1E,
+ 0x28,
+ 0x32,
+ 0x3C,
+ 0x46,
+ 0x50,
+ 0x5A,
+ 0x64
+ })
+The first two levels are for when laptop are on AC or on battery and are
+not used by Linux currently. The remaining 10 levels are supported levels
+that we can choose from. The applicable index values are from 0 (that
+corresponds to the 0x0A brightness value) to 9 (that corresponds to the
+0x64 brightness value) inclusive. Each of those index values is regarded
+as a "brightness level" indicator. Thus from the user space perspective
+the range of available brightness levels is from 0 to 9 (max_brightness)
+2 Notify user space about hotkey event
+There are generally two cases for hotkey event reporting:
+i) For some laptops, when user presses the hotkey, a scancode will be
+ generated and sent to user space through the input device created by
+ the keyboard driver as a key type input event, with proper remap, the
+ following key code will appear to user space:
+ etc.
+For this case, ACPI video driver does not need to do anything(actually,
+it doesn't even know this happened).
+ii) For some laptops, the press of the hotkey will not generate the
+ scancode, instead, firmware will notify the video device ACPI node
+ about the event. The event value is defined in the ACPI spec. ACPI
+ video driver will generate an key type input event according to the
+ notify value it received and send the event to user space through the
+ input device it created:
+ event keycode
+ etc.
+so this would lead to the same effect as case i) now.
+Once user space tool receives this event, it can modify the backlight
+level through the sysfs interface.
+3 Change backlight level in the kernel
+This works for machines covered by case ii) in Section 2. Once the driver
+received a notification, it will set the backlight level accordingly. This does
+not affect the sending of event to user space, they are always sent to user
+space regardless of whether or not the video module controls the backlight level
+directly. This behaviour can be controlled through the brightness_switch_enabled
+module parameter as documented in kernel-parameters.txt. It is recommended to
+disable this behaviour once a GUI environment starts up and wants to have full
+control of the backlight level.