ACPI: thinkpad-acpi: prefer standard ACPI backlight level control

Newer Lenovo BIOSes support the standard ACPI backlight brightness
interface (_BCM, _BQC, _BCL).  It should be used instead of the native
thinkpad backlight brightness control interface when possible.

This patch disables the native brightness support in the driver by default
when we detect that the standard ACPI interface is available.  The local
admin can still enable it using the module parameter "brightness_enable".

Note that we need to detect the standard ACPI backlight interface only in
boxes for which we would load the native backlight interface in the first
place, and that no ThinkPad BIOS has _BCL but misses the other methods, so
the detection routines can be really simple.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>

1 files changed, 6 insertions, 2 deletions

diff --git a/Documentation/thinkpad-acpi.txt b/Documentation/thinkpad-acpi.txt
index f877c13c5ef8..bb1c7a60b9bb 100644
--- a/Documentation/thinkpad-acpi.txt
+++ b/Documentation/thinkpad-acpi.txt
@@ -942,11 +942,15 @@ and CMOS.  The driver tries to autodetect which interface to use.
 
 When display backlight brightness controls are available through the
 standard ACPI interface, it is best to use it instead of this direct
-ThinkPad-specific interface.
+ThinkPad-specific interface.  The driver will disable its native
+backlight brightness control interface if it detects that the standard
+ACPI interface is available in the ThinkPad.
 
 The brightness_enable module parameter can be used to control whether
 the LCD brightness control feature will be enabled when available.
-brightness_enable=0 forces it to be disabled.
+brightness_enable=0 forces it to be disabled.  brightness_enable=1
+forces it to be enabled when available, even if the standard ACPI
+interface is also available.
 
 Procfs notes: