Patch "ACPI / video: Default lcd_only to true on Win8-ready and newer machines" has been added to the 4.15-stable tree

This is a note to let you know that I've just added the patch titled

ACPI / video: Default lcd_only to true on Win8-ready and newer machines

to the 4.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
acpi-video-default-lcd_only-to-true-on-win8-ready-and-newer-machines.patch
and it can be found in the queue-4.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Hans de Goede <[email protected]>
Date: Sat, 23 Dec 2017 19:41:47 +0100
Subject: ACPI / video: Default lcd_only to true on Win8-ready and newer machines

From: Hans de Goede <[email protected]>

[ 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

---
 drivers/acpi/acpi_video.c |   14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

@@ -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();
+
 	dmi_check_system(video_dmi_table);
 
 	ret = acpi_bus_register_driver(&acpi_video_bus);

Patches currently in stable-queue which might be from [email protected] are

queue-4.15/serdev-fix-serdev_uevent-failure-on-acpi-enumerated-serdev-controllers.patch queue-4.15/acpi-video-default-lcd_only-to-true-on-win8-ready-and-newer-machines.patch queue-4.15/bluetooth-hci_bcm-mandate-presence-of-shutdown-and-device-wake-gpio.patch queue-4.15/input-goodix-disable-irqs-while-suspended.patch queue-4.15/asoc-intel-cht_bsw_rt5645-analog-mic-support.patch queue-4.15/power-supply-axp288_charger-properly-stop-work-on-probe-error-remove.patch queue-4.15/pinctrl-baytrail-enable-glitch-filter-for-gpios-used-as-interrupts.patch