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Subject[PATCH AUTOSEL for 4.15 018/124] ACPI / video: Default lcd_only to true on Win8-ready and newer machines
From: Hans de Goede <>

[ Upstream commit 5928c281524fe451114e04f1dfa11246a37e859f ]

We're seeing a lot of bogus backlight interfaces on newer machines without
a LCD such as desktops, servers and HDMI sticks. This causes userspace to
show a non-functional brightness slider in e.g. the GNOME3 system menu,
which is undesirable. And, in general, we should simply just not register
a non functional backlight interface.

Checking the LCD flag causes the bogus acpi_video backlight interfaces to
go away (on the machines this was tested on).

This change sets the lcd_only option by default on any machines which
are Win8-ready, to fix this.

This is not entirely without a risk of regressions, but video_detect.c
already prefers native-backlight interfaces over the acpi_video one
on Win8-ready machines, calling acpi_video_unregister_backlight() as soon
as a native interface shows up. This is done because the ACPI backlight
interface often is broken on Win8-ready machines, because win8 does not
seem to actually use it.

So in practice we already end up not registering the ACPI backlight
interface on (most) Win8-ready machines with a LCD panel, thus this
change does not change anything for (most) machines with a LCD panel
and on machines without a LCD panel we actually don't want to register
any backlight interfaces.

This has been tested on the following machines and fixes a bogus backlight
interface showing up there:
- Desktop with an Asrock B150M Pro4S/D3 m.b. using i5-6500 builtin gfx
- Intel Compute Stick STK1AW32SC
- Meegopad T08 HDMI stick

Bogus backlight interfaces have also been reported on:
- Desktop with Asus H87I-Plus m.b.
- Desktop with ASRock B75M-ITX m.b.
- Desktop with Gigabyte Z87-D3HP m.b.
- Dell PowerEdge T20 desktop

Signed-off-by: Hans de Goede <>
Signed-off-by: Rafael J. Wysocki <>
Signed-off-by: Sasha Levin <>
drivers/acpi/acpi_video.c | 14 ++++++++++++--
1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/drivers/acpi/acpi_video.c b/drivers/acpi/acpi_video.c
index 0972ec0e2eb8..f53ccc680238 100644
--- a/drivers/acpi/acpi_video.c
+++ b/drivers/acpi/acpi_video.c
@@ -80,8 +80,8 @@ MODULE_PARM_DESC(report_key_events,
static bool device_id_scheme = false;
module_param(device_id_scheme, bool, 0444);

-static bool only_lcd = false;
-module_param(only_lcd, bool, 0444);
+static int only_lcd = -1;
+module_param(only_lcd, int, 0444);

static int register_count;
static DEFINE_MUTEX(register_count_mutex);
@@ -2136,6 +2136,16 @@ int acpi_video_register(void)
goto leave;

+ /*
+ * We're seeing a lot of bogus backlight interfaces on newer machines
+ * without a LCD such as desktops, servers and HDMI sticks. Checking
+ * the lcd flag fixes this, so enable this on any machines which are
+ * win8 ready (where we also prefer the native backlight driver, so
+ * normally the acpi_video code should not register there anyways).
+ */
+ if (only_lcd == -1)
+ only_lcd = acpi_osi_is_win8();

ret = acpi_bus_register_driver(&acpi_video_bus);
 \ /
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