[PATCH] Documentation: thinkpad-acpi: fix backlight power attribute name

[Anghel Mihai-Alexandru <mih@ianghel.com>]

The sysfs notes called the thinkpad_screen attribute "power". The
generic backlight class names it "bl_power", so the docs now use that
name and describe its values (0 on, 4 off). The intro now points to
the backlight class documentation instead of saying it is poorly
documented.

AI-assisted: Claude Sonnet 5.5 and Claude Haiku 5.5.
Signed-off-by: Anghel Mihai-Alexandru <mih@ianghel.com>
---
 .../admin-guide/laptops/thinkpad-acpi.rst     | 29 ++++++++++++-------
 1 file changed, 18 insertions(+), 11 deletions(-)

diff --git a/Documentation/admin-guide/laptops/thinkpad-acpi.rst b/Documentation/admin-guide/laptops/thinkpad-acpi.rst
index db4588af0278..f6377fad3aa0 100644
--- a/Documentation/admin-guide/laptops/thinkpad-acpi.rst
+++ b/Documentation/admin-guide/laptops/thinkpad-acpi.rst
@@ -1066,8 +1066,9 @@ The available commands are::
 Sysfs notes
 ^^^^^^^^^^^
 
-The interface is implemented through the backlight sysfs class, which is
-poorly documented at this time.
+The interface is implemented through the backlight sysfs class.  See
+Documentation/gpu/backlight.rst and
+Documentation/ABI/stable/sysfs-class-backlight for details.
 
 Locate the thinkpad_screen device under /sys/class/backlight, and inside
 it there will be the following attributes:
@@ -1082,17 +1083,23 @@ it there will be the following attributes:
 	brightness:
 		Writes request the driver to change brightness to the
 		given value.  Reads will tell you what brightness the
-		driver is trying to set the display to when "power" is set
-		to zero and the display has not been dimmed by a kernel
+		driver is trying to set the display to when "bl_power" is
+		zero and the display has not been dimmed by a kernel
 		power management event.
 
-	power:
-		power management mode, where 0 is "display on", and 1 to 3
-		will dim the display backlight to brightness level 0
-		because thinkpad-acpi cannot really turn the backlight
-		off.  Kernel power management events can temporarily
-		increase the current power management level, i.e. they can
-		dim the display.
+	bl_power:
+		Backlight power management mode, provided by the generic
+		backlight class.  0 (BACKLIGHT_POWER_ON) turns the display
+		on, and 4 (BACKLIGHT_POWER_OFF) powers the display down;
+		the driver decides how to blank the output.  1
+		(BACKLIGHT_POWER_REDUCED) is deprecated.  Any other
+		non-zero value is also treated as blank.  Kernel power
+		management events can temporarily blank the display.
+
+		thinkpad-acpi gets its brightness from
+		backlight_get_brightness(), which yields zero while the
+		display is blank, so the backlight level is set to zero
+		in that case.
 
 
 WARNING:
-- 
2.54.0