git: b950b48ad7 - main - [handbook] Add reference to acpi_video(4)

[Fernando Apesteguía]

The branch main has been updated by fernape:

URL: https://cgit.FreeBSD.org/doc/commit/?id=b950b48ad7ac9e148e81ecffa9100494806b29a5

commit b950b48ad7ac9e148e81ecffa9100494806b29a5
Author:     Fernando Apesteguía <fernape at FreeBSD.org>
AuthorDate: 2021-04-03 17:56:40 +0000
Commit:     Fernando Apesteguía <fernape at FreeBSD.org>
CommitDate: 2021-04-05 13:24:10 +0000

    [handbook] Add reference to acpi_video(4)
    
    Section 12.13.1 explains how to configure ACPI. Add a small paragraph explaining
    how to use the acpi_video(4) driver so some laptop brightness keys can work.
    
    Approved by: gbe (mentor)
    Differential Revision: https://reviews.freebsd.org/D29567
---
 .../content/en/books/handbook/config/_index.adoc          | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/documentation/content/en/books/handbook/config/_index.adoc b/documentation/content/en/books/handbook/config/_index.adoc
index 9eaca96dd2..364df797f9 100644
--- a/documentation/content/en/books/handbook/config/_index.adoc
+++ b/documentation/content/en/books/handbook/config/_index.adoc
@@ -1347,6 +1347,21 @@ ACPI and APM cannot coexist and should be used separately. The last one to load
 
 ACPI can be used to put the system into a sleep mode with `acpiconf`, the `-s` flag, and a number from `1` to `5`. Most users only need `1` (quick suspend to RAM) or `3` (suspend to RAM). Option `5` performs a soft-off which is the same as running `halt -p`.
 
+The man:acpi_video[4] driver uses
+link:https://uefi.org/specs/ACPI/6.4/Apx_B_Video_Extensions/Apx_B_Video_Extensions.html[ACPI
+Video Extensions] to control display switching and backlight brightness. It must
+be loaded after any of the DRM kernel modules. After loading the driver,
+the kbd:[Fn] brightness keys will change the brightness of the screen. It is
+possible to check the ACPI events by inspecting [.filename]#/var/run/devd.pipe#:
+
+[source,shell]
+...
+# cat /var/run/devd.pipe
+!system=ACPI subsystem=Video type=brightness notify=62
+!system=ACPI subsystem=Video type=brightness notify=63
+!system=ACPI subsystem=Video type=brightness notify=64
+...
+
 Other options are available using `sysctl`. Refer to man:acpi[4] and man:acpiconf[8] for more information.
 
 [[ACPI-comprob]]