Input: clear BTN_RIGHT/MIDDLE on buttonpads

Buttonpads are expected to map the INPUT_PROP_BUTTONPAD property bit
and the BTN_LEFT key bit.

As explained in the specification, where a device has a button type
value of 0 (click-pad) or 1 (pressure-pad) there should not be
discrete buttons:
https://docs.microsoft.com/en-us/windows-hardware/design/component-guidelines/touchpad-windows-precision-touchpad-collection#device-capabilities-feature-report

However, some drivers map the BTN_RIGHT and/or BTN_MIDDLE key bits even
though the device is a buttonpad and therefore does not have those
buttons.

This behavior has forced userspace applications like libinput to
implement different workarounds and quirks to detect buttonpads and
offer to the user the right set of features and configuration options.
For more information:
https://gitlab.freedesktop.org/libinput/libinput/-/merge_requests/726

In order to avoid this issue clear the BTN_RIGHT and BTN_MIDDLE key
bits when the input device is register if the INPUT_PROP_BUTTONPAD
property bit is set.

Notice that this change will not affect udev because it does not check
for buttons. See systemd/src/udev/udev-builtin-input_id.c.

List of known affected hardware:

 - Chuwi AeroBook Plus
 - Chuwi Gemibook
 - Framework Laptop
 - GPD Win Max
 - Huawei MateBook 2020
 - Prestigio Smartbook 141 C2
 - Purism Librem 14v1
 - StarLite Mk II   - AMI firmware
 - StarLite Mk II   - Coreboot firmware
 - StarLite Mk III  - AMI firmware
 - StarLite Mk III  - Coreboot firmware
 - StarLabTop Mk IV - AMI firmware
 - StarLabTop Mk IV - Coreboot firmware
 - StarBook Mk V

Acked-by: Peter Hutterer <peter.hutterer@who-t.net>
Acked-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Link: https://lore.kernel.org/r/20220208174806.17183-1-jose.exposito89@gmail.com
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/input.c

@@ -2285,6 +2285,12 @@ int input_register_device(struct input_dev *dev)
 	/* KEY_RESERVED is not supposed to be transmitted to userspace. */
 	__clear_bit(KEY_RESERVED, dev->keybit);
 
+	/* Buttonpads should not map BTN_RIGHT and/or BTN_MIDDLE. */
+	if (test_bit(INPUT_PROP_BUTTONPAD, dev->propbit)) {
+		__clear_bit(BTN_RIGHT, dev->keybit);
+		__clear_bit(BTN_MIDDLE, dev->keybit);
+	}
+
 	/* Make sure that bitmasks not mentioned in dev->evbit are clean. */
 	input_cleanse_bitmasks(dev);