Why libinput doesn't support edge scrolling

libinput supports edge scrolling since version 0.7.0. Whoops, how does the post title go with this statement? Well, libinput supports edge scrolling, but only on some devices and chances are your touchpad won't be one of them. Bug 89381 is the reference bug here.

First, what is edge scrolling? As the libinput documentation illustrates, it is scrolling triggered by finger movement within specific regions of the touchpad - the left and bottom edges for vertical and horizontal scrolling, respectively. This is in contrast to two-finger scrolling, triggered by a two-finger movement, anywhere on the touchpad. synaptics had edge scrolling since at least 2002, the earliest commit in the repo. Back then we didn't have multitouch-capable touchpads, these days they're the default and you'd be struggling to find one that doesn't support at least two fingers. But back then edge-scrolling was the default, and touchpads even had the markings for those scroll edges painted on.

libinput adds a whole bunch of features to the touchpad driver, but those features make it hard to support edge scrolling. First, libinput has quite smart software button support. Those buttons are usually on the lowest ~10mm of the touchpad. Depending on finger movement and position libinput will send a right button click, movement will be ignored, etc. You can leave one finger in the button area while using another finger on the touchpad to move the pointer. You can press both left and right areas for a middle click. And so on. On many touchpads the vertical travel/physical resistance is enough to trigger a movement every time you click the button, just by your finger's logical center moving.

libinput also has multi-direction scroll support. Traditionally we only sent one scroll event for vertical/horizontal at a time, even going as far as locking the scroll direction. libinput changes this and only requires a initial threshold to start scrolling, after that the caller will get both horizontal and vertical scroll information. The reason is simple: it's context-dependent when horizontal scrolling should be used, so a global toggle to disable doesn't make sense. And libinput's scroll coordinates are much more fine-grained too, which is particularly useful for natural scrolling where you'd expect the content to move with your fingers.

Finally, libinput has smart palm detection. The large majority of palm touches are along the left and right edges of the touchpad and they're usually indistinguishable from finger presses (same pressure values for example). Without palm detection some laptops are unusable (e.g. the T440 series).

These features interfere heavily with edge scrolling. Software button areas are in the same region as the horizontal scroll area, palm presses are in the same region as the vertical edge scroll area. The lower vertical edge scroll zone overlaps with software buttons - and that's where you would put your finger if you'd want to quickly scroll up in a document (or down, for natural scrolling). To support edge scrolling on those touchpads, we'd need heuristics and timeouts to guess when something is a palm, a software button click, a scroll movement, the start of a scroll movement, etc. The heuristics are unreliable, the timeouts reduce responsiveness in the UI. So our decision was to only provide edge scrolling on touchpads where it is required, i.e. those that cannot support two-finger scrolling, those with physical buttons. All other touchpads provide only two-finger scrolling. And we are focusing on making 2 finger scrolling good enough that you don't need/want to use edge scrolling (pls file bugs for anything broken)

Now, before you get too agitated: if edge scrolling is that important to you, invest the time you would otherwise spend sharpening pitchforks, lighting torches and painting picket signs into developing a model that allows us to do reliable edge scrolling in light of all the above, without breaking software buttons, maintaining palm detection. We'd be happy to consider it.

libinput scroll sources

This feature got merged for libinput 0.8 but I noticed I hadn't blogged about it. So belatedly, here is a short description of scroll sources in libinput.

Scrolling is a fairly simple concept. You move the mouse wheel and the content moves down. Beyond that the details get quite nitty, possibly even gritty. On touchpads, scrolling is emulated through a custom finger movement (e.g. two-finger scrolling). A mouse wheel moves in discrete steps of (usually) 15 degrees, a touchpad's finger movement is continuous (within the device physical resolution). Another scroll method is implemented for the pointing stick: holding the middle button down while moving the stick will generate scroll events. Like touchpad scroll events, these events are continuous. I'll ignore natural scrolling in this post because it just inverts the scroll direction. Kinetic scrolling ("fling scrolling") is a comparatively recent feature: when you lift the finger, the final finger speed determines how long the software will keep emulating scroll events. In synaptics, this is done in the driver and causes all sorts of issues - the driver may keep sending scroll events even while you start typing.

In libinput, there is no kinetic scrolling at all, what we have instead are scroll sources. Currently three sources are defined, wheel, finger and continuous. Wheel is obvious, it provides the physical value in degrees (see this post) and in discrete steps. The "finger" source is more interesting, it is the hint provided by libinput that the scroll event is caused by a finger movement on the device. This means that a) there are no discrete steps and b) libinput guarantees a terminating scroll event when the finger is lifted off the device. This enables the caller to implement kinetic scrolling: simply wait for the terminating event and then calculate the most recent speed. More importantly, because the kinetic scrolling implementation is pushed to the caller (who will push it to the client when the Wayland protocol for this is ready), kinetic scrolling can be implemented on a per-widget basis.

Finally, the third source is "continuous". The only big difference to "finger" is that we can't guarantee that the terminating event is sent, simply because we don't know if it will happen. It depends on the implementation. For the caller this means: if you see a terminating scroll event you can use it as kinetic scroll information, otherwise just treat it normally.

For both the finger and the continuous sources the scroll distance provided by libinput is equivalent to "pixels", i.e. the value that the relative motion of the device would otherwise send. This means the caller can interpret this depending on current context too. Long-term, this should make scrolling a much more precise and pleasant experience than the old X approach of "You've scrolled down by one click".

The API documentation for all this is here: http://wayland.freedesktop.org/libinput/doc/latest/group__event__pointer.html, search for anything with "pointer_axis" in it.

libinput device groups

I just pushed a patchset into libinput to introduce the concept of device groups. This post will explain what they are in this context and why they are needed.

libinput exposes kernel devices as an opaque struct libinput_device. It only recognises evdev devices at this point, this may change in the future if we see a need for it. libinput also exposes a few bits of information about the device such as the name, PID/VID and a handle to the struct udev_device that matches this device. The latter enables callers to get more information from the device. libinput also provides a bunch of configuration settings for each device. Pointer devices get acceleration settings, absolute devices have calibration, etc. For most devices this works just fine.

Some devices like Wacom tablets are represented as multiple event nodes. On a 3.19 kernel you'd get three event nodes for an Intuos 5 touch - the pad (i.e. the tablet itself), a touch node and one node for all the tools (stylus, eraser, etc. multiplexed). libinput exposes each of these nodes as separate device, but that is problematic when applying certain configuration settings. For example, applying a left-handed configuration to the tablet means it's rotated by 180 degrees so we need to rotate the coordinates accordingly. Of course, such a rotation would have to apply to both the touch and the stylus devices but now the caller is left with having to figure out which other devices to set.

The original idea was to present such devices as a single, merged struct libinput_device with multiple capabilities, i.e. a single physical device that can do touch, tablet and pad buttons. A configuration setting like left-handed-ness would then apply to all devices transparently. The API is clean, usage is simple, everybody is happy. Except when they aren't - this doesn't actually work particularly well. First, having such merged devices means we require devices to change at runtime, adding/removing capabilities on-the-fly which puts a burden on the callers to handle this correctly. Second, not all configuration options apply to all subdevices. If the Intuos is used as a touchpad you may want natural scrolling enabled on the touchpad but the wheel on the Wacom mouse should probably still work normally. Third, the subdevices may have different PID/VIDs and certainly have different udev devices. So now libinput needs a way to get to those. In short, a merged device looks nice in theory but the implementation of it would make the libinput API cumbersome to use for little benefit.

The solution to this are device groups: each device in libinput is now part of a struct libinput_device_group. This is just an opaque object that doesn't do anything but sit there but it's enough to identify how devices are grouped together. If two devices return the same device group, they logically belong together. The caller can then decide what to do with it, e.g. loop through all devices of a group to apply a certain configuration setting to all devices. The basic approach is thus:

new_device = libinput_event_get_device(event);
new_group = libinput_device_get_device_group(new_device);
libinput_device_group_ref(new_group);

for each (device, group) in previously_stored_devices {
   if (group == new_group)
      printf("This device shares a group with %s", device);
}

The device groups' lifetime is as you'd expect: it is created for the first device in the group and ceases once the last device in a group is removed. It's not deleted until the last reference was deleted but it won't get recycled. In other words, if you keep unplugging and re-plugging that Intuos tablet, the device group will be new after every plug.

Note that we're intentionally not providing ways to get the devices from a device group, or counting the devices within a group, etc. This avoids race conditions (the view libinput has of the devices isn't the same as the caller has while going through the event queue) but it also makes the API simpler. libinput's callers are mainly compositors which use toolkits with advanced datastructures (glib, Qt, etc.). Using a pointer as key into a hashmap is simpler and less buggy than using whatever hand-crafted hashmap/list implementation we can provide through the libinput API.

Lenovos X1 Carbon 3rd touchpad woes

Lenovo released a new set of laptops for 2015 with a new (old) feature: the trackpoint device has the physical buttons back. Last year's experiment apparently didn't work out so well.

What do we do in Linux with the last generation's touchpads? The kernel marks them with INPUT_PROP_TOPBUTTONPAD based on the PNPID [1]. In the X.Org synaptics driver and libinput we take that property and emulate top software buttons based on it. That took us a while to get sorted and feed into the myriad of Linux distributions out there but at some point last year the delicate balance of nature was restored and the touchpad-related rage dropped to the usual background noise.

Slow-forward to 2015 and Lenovo introduces the new series. In the absence of unnecessary creativity they are called the X1 Carbon 3rd, T450, T550, X250, W550, L450, etc. Lenovo did away with the un(der)-appreciated top software buttons and re-introduced physical buttons for the trackpoint. Now, that's bound to make everyone happy again. However, as we learned from Agent Smith, happiness is not the default state of humans so Lenovo made sure the harvest is safe.

What we expected to happen was that the trackpoint device has BTN_LEFT, BTN_MIDDLE, BTN_RIGHT and the touchpad has BTN_LEFT and is marked with INPUT_PROP_BUTTONPAD (i.e. it is a Clickpad). That is the case on the x220 generation and the T440 generation. Though the latter doesn't actually have trackpoint buttons and we emulated them in software.

On the X1 Carbon 3rd, the trackpoint has BTN_LEFT, BTN_MIDDLE, BTN_RIGHT but they never send events. The touchpad has BTN_LEFT and BTN_0, BTN_1 and BTN_2 [2]. Clicking the left button on the trackpoint generates BTN_0 on the touchpad device, clicking the right button generates BTN_1 on the touchpad device. So in short, Lenovo has decided to wire the newly re-introduced trackpoint buttons to the touchpad, not the trackpoint. [3] The middle button is currently dead, which is a kernel bug. Meanwhile we think of it as security feature - never accidentally paste your password into your IRC session again!

What does this mean for us? Neither synaptics nor evdev nor libinput currently support this so we've been busy aipodae and writing patches like crazy. The patch goes into the kernel and udev.... The two patches needed go into the kernel and udev, and libinput. No, the three patches needed go into the kernel, udev and libinput, and synaptics. The four patches, no, wait. Amongst the projects needing patches are the kernel, udev, libinput and synaptics. I'll try again:

• The kernel needs this patchset here. It removes INPUT_PROP_TOPBUTTONPAD from the device, makes the middle button work and fixes the button behaviour.
• The udev hwdb marks touchpads that need special handling.
• libinput detects devices marked thusly and re-routes button events through the trackpoint device.
• synaptics simply mangles the buttons [4] and sends them as left/right/middle buttons accordingly.

With those put together, things pretty much work as they're supposed to. libinput handles middle button scrolling as well this way but synaptics won't, much for the same reason it didn't work in the previous generation: synaptics can't talk to evdev and vice versa. And given that synaptics is on life support or in pallative care, depending how you look at it, I recommend not holding your breath for a fix. Otherwise you may join it quickly.

Note that all the patches are fresh off the presses and there may be a few bits changing before they are done. If you absolutely can't live without the trackpoint's buttons you can work around it by disabling the synaptics kernel driver until the patches have trickled down to your distribution.

The tracking bug for all this is Bug 88609. Feel free to CC yourself on it. Bring popcorn.

Final note: I haven't seen logs from the T450, T550, ... devices yet yet so this is so far only confirmed on the X1 Carbon so far. Given the hardware is essentially identical I expect it to be true for the rest of the series though.

[1] We also apply quirks for the 2013 generation because the firmware was buggy - a problem Synaptics Inc. has since fixed (but currently gives us slight headaches).
[2] It is also marked with INPUT_PROP_TOPBUTTONPAD which is a bug. It uses a new PNPID but one that was in the range we previously believed was for pads without trackpoint buttons. That's an an easy thing to fix.
[3] The reason for that seems to be HW design: this way they can keep the same case/keyboard and just swap the touchpad bits.
[4] synaptics is old enough to support dedicated scroll buttons. Buttons that used to send BTN_0 and BTN_1 and are thus interpreted as scroll up/down event.

Lenovo T440, T540 latest generation touchpad issues

It seems we can't ever get rid of the issues with this series. Daniel Martin filed a kernel bug for the latest series of these devices (Oct 2014) and it looks like they all need manual fixing again.

When the *40 series first came out, the PS/2 firmware was buggy and advertised bogus coordinate ranges for the x/y axes. Since we use those coordinate ranges to set up the size and position of software buttons (very much needed since that series did away with the physical trackpoint buttons) we added kernel patches for each of those laptops. The kernel would match on the PNPID (e.g. LEN0036 on a T440) and fix the min/max range for both axes based actual measurements. Since this requires someone to have a laptop, measure it, file a bug or send a patch, wait for it to get into the kernel, wait for it to get into distros it took quite a while to get all models supported.

Lenovo has updated the series in Oct 2014 and it's starting to get in the hands of users. And as it turns out, the touchpads now have different coordinate ranges but the same PNPID. And the values reported by the firmware are still bogus, so we need the same quirk, again, for each touchpad. Update 22/01/15: looks like the ranges are correct enough, so we don't need to update all ranges separately.

So in short: if you have one of the latest series *40 touchpads, your touchpad software buttons will be off. CC yourself on the kernel bug and if you have a model that's not listed there yet, add the required data. Eventually that will end up in the kernel and then everything is hunky-dory again. Until then, have a drink on behalf of the Synaptics/Lenovo QA departments.

Now the obvious question: why does this work with Windows? They don't use the PS/2 protocol but the SMBus/RMI4 interface and thus PS/2 firmware correctness is apparently not top priority for the QA departments. But the SMBus protocol requires the Host Notify feature, which caused Synaptics to reimplement the i2c driver for Windows. And that's what is shipped/preinstalled as driver. We don't support Host Notify on Linux, so there goes that idea. But there's strong suspicion that's not the only piece of the puzzle that's missing anyway...

Update 22/01/15: The min/max ranges advertised seem to be correct in the newer versions which would indicate that Synaptics has fixed the firmware. That's great (except for re-using the PNPID). Now we need to just detect that and drop the quirks for the newer touchpads. Hans has a good suggestion for how to do this, so with a bit of luck this will end up being only one kernel patch instead of one per device.

xf86-input-libinput compatibility with evdev and synaptics

A Fedora 22 feature is to use the libinput X.Org driver as default driver for all non-tablet devices. This replaces the current defaults of synaptics for touchpads and evdev for anything else (tablets usually use the wacom driver, if installed). As expected, changing a default has some repercussions both for users and for developers. These are outlined below, based on the libinput 0.8 release. Future versions may add features, so check with your latest local version. Generally, the behaviour should roughly stay the same, big changes such as devices not being detected or not working is most likely a bug. Some behaviours are new, e.g. always-on palm detection, top software buttons on specific touchpads, etc. If in doubt, check the libinput documentation for hints on whether something is supposed to work in a particular manner.

Changes visible to Users

Any custom xorg.conf snippets will cease to work, if they are properly stacked. Options set by snippets are almost always exclusive to one particular driver. When the default driver changes, the snippet may not apply to the device anymore. Whether they stop working depends whether the Driver line is present. Consider this example snippet:

Section "InputClass"
  Identifier "enable tapping"
  MatchProduct "my touchpad"
  Driver "synaptics"
  Option "TapButton1" "1"
EndSection

This snippet does two things: it assigns the synaptics driver to the "my mouse" device and sets the option TapButton1. The assignment will override the default libinput assignment, i.e. this device won't change behaviour, you just don't get to use any new features. If the Driver line is not present then this snippet won't do anything, the libinput driver does not provide a TapButton1 option. It is safe to leave the snippet in place, options that are not supported by a driver are simply ignored.

The xf86-input-libinput man page has a list of options that can be set. For example, the above snippet would have an equivalent as

Section "InputClass"
  Identifier "enable tapping"
  MatchDriver "libinput"
  MatchProduct "my touchpad"
  Option "Tapping" "on"
EndSection

Note that this matches on a driver rather than assign the driver. Since options are driver-specific this is the correct approach.

The other visible change is a difference in default pointer speed. We have fine-tuning pointer acceleration on our TODO lists, but we're not quite there yet and any help would be appreciated. In the meantime you may see changes in pointer acceleration.

Finally, you may see certain features have gone the way of the dodo. Specifically the touchpad code exposes a lot less knobs to tweak. Most of that is intentional, some of it may warrant discussion. If there is a particular feature you are missing or doesn't work as you want to, please file a bug.

Changes visible to developers

These changes affect desktop environments, specifically the part that configures input devices. The changes affect three categories: pointer acceleration, button mapping, touchpad disabling and device properties. The property "libinput Send Events Modes Available" exists on all devices, it can be used to determine if a device is handled by the libinput driver.

Pointer acceleration

The X server exposes a variety of knobs for its pointer acceleration code. The oldest knob (and specified in the core protocol) is the XChangePointerControl request. In some environments this is exposed as a single slider, in others it's split into multiple settings (Acceleration and Threshold, for example).

libinput does away with this and only exposes a single 1-value float property "libinput Accel Speed" with a range of -1 (slowest) to 1 (fastest). The XChangePointerControl request has no effect on a libinput device. It is up to you how to map the current speed mappings into the [-1, 1] range.

Button mapping

The X server provides button mapping through the XSetPointerMapping request. This is most commonly used to apply a left-handed configuration and to enable natural scrolling. The call will continue to work with the libinput driver, but better methods are available.

The property "libinput Left Handed Enabled" takes a single boolean 8-bit value to enable and disable left-handed mode. Unlike the X request this will automatically take care of the tapping configuration (and other things in the future). If the property is not available on a device, that device has no left-handed mode.

The property "libinput Natural Scrolling Enabled" takes a single boolean 8-bit value to enable and disable natural scrolling. This applies to smooth scrolling and legacy button scrolling (which the libinput driver doesn't do anyway). If the property is not available on a device, that device has no natural scrolling mode.

Touchpad disabling

In the synaptics driver, disabling the touchpad is usually done with the "Synaptics Off" property. This is used by syndaemon to turn the touchpad off while typing. libinput does this by default, so it is safe to simply ignore this at all. Don't bother starting syndaemon, it won't control the libinput driver.

Device properties

Any code that handles a driver-specific property (prefixed by "evdev" or "synaptics") will stop working. These properties are not exposed by the libinput driver (we tried, it was not viable). KDE's kcm_touchpad module is a particularly bad offender here, it exposes almost every toggle the driver ever had. Make sure the code behaves well if the properties are not present and you're basically good to go.

If you decide to handle libinput-specific properties, the general rule is: if a single-value property is not present, that configuration does not apply to this device. Bitmask-style properties are split into an "libinput foo Available" and "libinput foo Enabled". The former lists the available settings, the latter enables a specific setting. Have a look at the xf86-input-libinput source for details on each property.

Providing the physical movement of wheel events in libinput

libinput 0.8 was released yesterday. One feature I'd like to talk about here: the change to provide mouse wheel events as physical distances.

Mouse wheels are clicks. In the evdev protocol they're sent via the REL_WHEEL and REL_HWHEEL axes, with a value of 1 per click. Spinning the wheel fast enough will give you a higher value per event but it's still just a multiple of the physical clicks. This is a conundrum for libinput.

libinput exports scroll events as "axis" events, the value returned by libinput_event_pointer_get_axis_value() for touchpads and button scrolling is in "pixels". libinput doesn't have a concept of pixels of course but the compositor will likely take the relative motion and apply it to the cursor. Scroll events are in the same coordinate space, i.e. the scrolling for two-finger scrolling has the same feel as moving the pointer. This continuous coordinate space is at odds with the discrete values coming from a wheel. We added axis sources to the API so you can now tell whether an event was generated by the wheel or some other scroll methods. But still, the discrete values from a wheel are at odds with the other sources.

For libinput 0.8, we changed the default reporting mode of the wheel. For the click count, a new call libinput_event_pointer_get_axis_value_discrete() provides that number (positive and negative for both direction). The libinput_event_pointer_get_axis_value() on a wheel now returns the movement of the wheel in degrees. That gives us a continuous coordinate space that also opens up new possibilities: if you know the rotation of a mouse wheel in degrees, you know things like "has the wheel been turned a full rotation". I don't quite know how, but I'm sure there are interesting interfaces you can make from that :)

Of course, the physical properties don't change, the degrees are always a multiple of the click count, and on most mice one click count is a 15 degree movement. The Logitech M325 is a notable exception here with a 20 degree angle. This isn't advertised by the hardware of course so we rely on the udev hwdb to set it where it differs from the default. The patch for this has been pushed to systemd and will soon arrive at a distribution near you.

And to answer a question I'm sure will come up: those mice that support a free spinning scrollwheel don't change the reporting mode. The G500s for example merely moves a physical bit that provides friction and the click feel/noise. The device still reports in 15 degree angle counts. I strongly suspect that other mice with this feature are the same (if not, we can now work this continuous motion into libinput and handle it propertly).