When we released graphics tablet support in libinput earlier this year, only tablet tools were supported. So while you could use the pen normally, the buttons, rings and strips on the physical tablet itself (the "pad") weren't detected by libinput and did not work. I have now merged the patches for pad support into libinput.
The reason for the delay was simple: we wanted to get it right . Pads have a couple of properties that tools don't have and we always considered pads to be different to pens and initially focused on a more generic interface (the "buttonset" interface) to accommodate for those. After some coding, we have now arrived at a tablet pad-specific interface instead. This post is a high-level overview of the new tablet pad interface and how we intend it do be used.
The basic sign that a pad is present is when a device has the tablet pad capability. Unlike tools, pads don't have proximity events, they are always considered in proximity and it is up to the compositor to handle the focus accordingly. In most cases, this means tying it to the keyboard focus. Usually a pad is available as soon as a tablet is plugged in, but note that the Wacom ExpressKey Remote (EKR) is a separate, wireless device and may be connected after the physical pad. It is up to the compositor to link the EKR with the correct tablet (if there is more than one).
Pads have three sources of events: buttons, rings and strips. Rings and strips are touch-sensitive surfaces and provide absolute values - rings in degrees, strips in normalized [0.0, 1.0] coordinates. Similar to pointer axis sources we provide a source notification. If that source is "finger", then we send a terminating out-of-range event so that the caller can trigger things like kinetic scrolling.
Buttons on a pad are ... different. libinput usually re-uses the Linux kernel's include/input.h event codes for buttons and keys. But for the pad we decided to use plain sequential button numbering, starting at index 0. So rather than a semantic code like BTN_LEFT, you'd simply get a button 0 event. The reasoning behind this is a caveat in the kernel evdev API: event codes have semantic meaning (e.g. BTN_LEFT) but buttons on a tablet pad don't those meanings. There are some generic event ranges (e.g. BTN_0 through to BTN_9) and the Wacom tablets use those but once you have more than 10 buttons you leak into other ranges. The ranges are simply too narrow so we end up with seemingly different buttons even though all buttons are effectively the same. libinput's pad support undoes that split and combines the buttons into a simple sequential range and leaves any semantic mapping of buttons to the caller. Together with libwacom which describes the location of the buttons a caller can get a relatively good idea of how the layout looks like.
Mode switching is a commonly expected feature on tablet. One button is designated as mode switch button and toggles all other buttons between the available modes. On the Intuos Pro series tablets, that button is usually the button inside the ring. Button mapping and thus mode switching is however a feature we leave up to the caller, if you're working on a compositor you will have to implemented mode switching there.
Other than that, pad support is relatively simple and straightforward and should not cause any big troubles.
 or at least less wrong than in the past
 They're actually linux/input-event-codes.h in recent kernels