On my computer, the minimum brightness enforced by clamping in
backlight is too bright.
Let udev property ID_BACKLIGHT_CLAMP control whether the brightness
is clamped or not.
---
man/systemd-backli...@.service.xml | 11 ++++++++++-
src/backlight/backlight.c | 5 ++++-
2 files changed, 14 insertions(+), 2 deletions(-)
diff --git a/man/systemd-backli...@.service.xml
b/man/systemd-backli...@.service.xml
index 453afbf..4105685 100644
--- a/man/systemd-backli...@.service.xml
+++ b/man/systemd-backli...@.service.xml
@@ -58,7 +58,16 @@
is a service that restores the display backlight
brightness at early boot and saves it at shutdown. On
disk, the backlight brightness is stored in
- <filename>/var/lib/systemd/backlight/</filename>.</para>
+ <filename>/var/lib/systemd/backlight/</filename>. During
+ loading, if udev property ID_BACKLIGHT_CLAMP is not
+ present or is set to <literal>1</literal>, the
+ brightness is clamped to at least value
+ <literal>1</literal> or 5% of maximum brightness. This
+ is to avoid an unpowered display due to poor API
+ design in kernel (unfixed as of 2015-01-28) that does
+ not allow user space to know the difference between
+ lowest brightness and powering off the
+ backlight.</para>
</refsect1>
<refsect1>
diff --git a/src/backlight/backlight.c b/src/backlight/backlight.c
index 1271a66..df53b75 100644
--- a/src/backlight/backlight.c
+++ b/src/backlight/backlight.c
@@ -373,6 +373,7 @@ int main(int argc, char *argv[]) {
if (streq(argv[1], "load")) {
_cleanup_free_ char *value = NULL;
+ const char *clamp;
if (!shall_restore_state())
return EXIT_SUCCESS;
@@ -390,7 +391,9 @@ int main(int argc, char *argv[]) {
return EXIT_FAILURE;
}
- clamp_brightness(device, &value, max_brightness);
+ clamp = udev_device_get_property_value(device,
"ID_BACKLIGHT_CLAMP");
+ if (clamp == NULL || streq(clamp, "1"))
+ clamp_brightness(device, &value, max_brightness);
r = udev_device_set_sysattr_value(device, "brightness", value);
if (r < 0) {
--
2.1.4
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