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author | Rafael J. Wysocki <rafael.j.wysocki@intel.com> | 2013-06-28 13:00:45 +0200 |
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committer | Rafael J. Wysocki <rafael.j.wysocki@intel.com> | 2013-06-28 13:00:45 +0200 |
commit | e8b6cb3947430d62538d88f615c007a51aeb23fe (patch) | |
tree | 9a5c4f74b59584a8763dc48ba296b72853d385ac /Documentation/acpi/video_extension.txt | |
parent | bdc8f09685a25a12f2f5282f56672ba663ecb88c (diff) | |
parent | 73e797f79e5edb3d000d1911a2d859f15c446c1a (diff) | |
download | linux-stable-e8b6cb3947430d62538d88f615c007a51aeb23fe.tar.gz linux-stable-e8b6cb3947430d62538d88f615c007a51aeb23fe.tar.bz2 linux-stable-e8b6cb3947430d62538d88f615c007a51aeb23fe.zip |
Merge branch 'acpi-doc'
* acpi-doc:
Documentation / CPU hotplug: Rephrase the outdated description for MADT entries
ACPI / video: update video_extension.txt for backlight control
ACPI / video: move video_extension.txt to Documentation/acpi
ACPI / video: add description for brightness_switch_enabled
ACPI: Add ACPI namespace documentation
ACPI: Add sysfs ABI documentation
ACPI: Update MAINTAINERS file to include Documentation/acpi
Diffstat (limited to 'Documentation/acpi/video_extension.txt')
-rw-r--r-- | Documentation/acpi/video_extension.txt | 106 |
1 files changed, 106 insertions, 0 deletions
diff --git a/Documentation/acpi/video_extension.txt b/Documentation/acpi/video_extension.txt new file mode 100644 index 000000000000..78b32ac02466 --- /dev/null +++ b/Documentation/acpi/video_extension.txt @@ -0,0 +1,106 @@ +ACPI video extensions +~~~~~~~~~~~~~~~~~~~~~ + +This driver implement the ACPI Extensions For Display Adapters for +integrated graphics devices on motherboard, as specified in ACPI 2.0 +Specification, Appendix B, allowing to perform some basic control like +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. + +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: +Documentation/ABI/stable/sysfs-class-backlight. + +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) +inclusive. + +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: + + EV_KEY, KEY_BRIGHTNESSUP + EV_KEY, KEY_BRIGHTNESSDOWN + 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 + 0x86 KEY_BRIGHTNESSUP + 0x87 KEY_BRIGHTNESSDOWN + 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. |