ClickPad support in the synaptics driver

[update 20 Apr: clarify middle finger emulation]
One of the features added to the upcoming xf86-input-synaptics version 1.6 is support for clickpad-style devices. This post outlines what clickpads are, how they are supported and how you go about enabling the new features.

What are clickpads

The name ClickPad comes from the Synaptics product of the same name. It describes a touchpad that does not have physical buttons. Instead, the whole touchpad works as a button. Devices like this have been around for a while, most notably the touchpads found in Apple laptops and the Dell Mini 10 series. The challenges for us were to handle the data streams correctly. Most of this work was done by Chase Douglas.

Clickpads give us the position for each finger, and a button 1 event when the pad is pressed. The design of the hardware however means that whenever you click the pad, you always add one more finger to the mix. Decoupling that finger from the ones that actually matter is the challenge. And integrating it with all the other whacky features that the synaptics driver currently has.

Clickpad Support

Clickpad support requires server 1.12 [0]. It heavily relies on multitouch support.
Central to the new feature is the "ClickPad" option and property. It is enabled if the kernel sets the INPUT_PROP_BUTTONPAD property on the device, otherwise you can enable it in an xorg.conf.d snippet or at runtime with either xinput or synclient:

xinput set-prop "device name" "Synaptics ClickPad" 1
synclient ClickPad=1

This toggles a few driver behaviours to make the clickpad much more usable. Most notably, you can use one finger to press the button and another finger to move the cursor around.

Word of warning here: if you enable clickpad support manually at runtime, you will also have to manually disable traditional middle mouse button emulation (synclient EmulateMidButtonTime=0). For autoconfigured devices or xorg.conf.d configured devices this is done automatically.

The second big feature for clickpads is support for software buttons. Since the device only gives us left button clicks, we expose an option to allow for right and middle button clicks. By default, we ship this xorg.conf.d snippet:

Section "InputClass"
        Identifier "Default clickpad buttons"
        MatchDriver "synaptics"
        Option "SoftButtonAreas" "50% 0 82% 0 0 0 0 0"
EndSection

The order of soft button edges is left, right, top, bottom for the right button, then left, right, top, bottom for the middle button. So the above snippet sets the right half of the bottom 18% to be a right button, with no middle button present. A coordinate of 0 means "stretch to edge of touchpad". [1]

Traditional middle mouse button emulation is disabled by default and we don't handle it for clickpads. Traditional middle mouse button emulation refers to the method of generating a button 2 press when both button 1 and button 3 are pressed simultaneously. Mostly because, well, I'm not sure how you would even trigger middle mouse button emulation for clickpads given that all buttons except the first are already emulated anyway. You can still emulate middle mouse button emulations through clickfingers (see below), tapping, or the soft button areas as described above.

Tapping works as on any other touchpad.

ClickFinger functionality works, albeit slightly different. In the default ClickFinger setting, if you have 2 fingers on the touchpad and you press the left button, you will see a ClickFinger2 action performed (right button by default). On clickpads we guess the number of fingers by proximity (since you need one finger to actually press the button). Fingers closer together than 30% of the touchpad's size [2] will count towards ClickFinger actions, others will be skipped. So in the default use-case, where you have two fingers down on the touchpad in the middle and you use a third to click the button, you will still get a ClickFinger2 action. Likewise, if you press with two fingers down, you will also get a ClickFinger2 action.

All other functions are not affected by clickpad support and behave as before.

• [0] Ubuntu 12.04 will ship a 1.11/1.12 mix, that will work as well
• [1] "edge of touchpads" should be 100% but it isn't due to a years-old mishandling of the coordinate ranges. So 0 is better to use here.
• [2] All touchpads lie about their size, so we just hardcode 30% since anything we infer is going to be wrong anyways