Wednesday, June 13, 2018

libinput and its device quirks files

This post does not describe a configuration system. If that's all you care about, read this post here and go be angry at someone else. Anyway, with that out of the way let's get started.

For a long time, libinput has supported model quirks (first added in Apr 2015). These model quirks are bitflags applied to some devices so we can enable special behaviours in the code. Model flags can be very specific ("this is a Lenovo x230 Touchpad") or generic ("This is a trackball") and it just depends on what the specific behaviour is that we need. The x230 touchpad for example has a custom pointer acceleration but trackballs are marked so they get some config options mice don't have/need.

In addition to model tags we also have custom attributes. These are free-form and provide information that we cannot get from the kernel. These too can be specific ("this model needs a pressure threshold of N") or generic ("bluetooth keyboards are an external keyboards").

Overall, it's a good system. Most users never have to care that we even have this. The whole point is that any device-specific quirks need to be merged only once for each model, then everyone with the same device gets to benefit on the next update.

Originally quirks were hardcoded but this required rebuilding libinput for any changes. So we moved this to utilise the udev hwdb. For the trivial work of fetching udev properties we got a lot of flexibility in how we can match against devices. For example, an entry may look like this:

libinput:name:*AlpsPS/2 ALPS GlidePoint:dmi:*svnDellInc.:pnLatitudeE6220:*
 LIBINPUT_ATTR_PRESSURE_RANGE=100:90
The above uses a name match and the dmi modalias match to apply a property for the touchpad on the Dell Latitude E6330. The exact match format is defined by a bunch of udev rules that ship as part of libinput.

Using the udev hwdb maked the quirk storage a plaintext file that can be updated independently of libinput, including local overrides for testing things before merging them upstream. Having said that, it's definitely not public API and can change even between stable branch updates as properties are renamed or rescoped to fit the behaviour more accurately. For example, a model-specific tag may be renamed to a behaviour-specific tag as we find more devices affected by the same issue.

The main issue with the quirks now is that we keep accumulating more and more of them and I'm starting to hit limits with the udev hwdb match behaviour. The hwdb is great for single matches but not so great for cascading matches where one match may overwrite another match. The hwdb match system is largely implementation-defined so it's not always predictable which match rule wins out in the end.

Second, debugging the udev hwdb is not at all trivial. It's a bit like git - once you're used to it it's just fine but until then the air turns yellow with all the swearing being excreted by the unsuspecting user.

So long story short, libinput 1.12 will replace the hwdb model quirks database with a set of .ini files. The model quirks will be installed in /usr/share/libinput/ or whatever prefix your distribution prefers instead. It's a bunch of files with fairly simplistic instructions, each [section] has a set of MatchFoo=Bar directives and the ModelFoo=bar or AttrFoo=bar tags. See this file for an example. If all MatchFoo directives apply to a device, the Model and Attr tags are applied. Matching works in inter- and intra-file sequential order so the last section in a file overrides the first section of that file and the highest-sorting file overrides the lowest-sorting file. Otherwise the tags are accumulated, so if two files match on the same device with different tags, both tags are applied. So far, so unexciting.

Sometimes it's necessary to install a temporary local quirk until upstream libinput is updated or the distribution updates its package. For this, the /etc/libinput/local-overrides.quirks file is read in as well (if it exists). Note though that the config files are considered internal API, so any local overrides may stop working on the next libinput update. Should've upstreamed that quirk, eh?

These files give us the same functionality as the hwdb - we can drop in extra files without recompiling. They're more human-readable than a hwdb match and it's a lot easier to add extra match conditions to it. And we can extend the file format at will. But the biggest advantage is that we can quite easily write debugging tools to figure out why something works or doesn't work. The libinput list-quirks tool shows what tags apply to a device and using the --verbose flag shows you all the files and sections and how they apply or don't apply to your device.

As usual, the libinput documentation has details.

Thursday, June 7, 2018

Observations on trackpoint input data

This time we talk trackpoints. Or pointing sticks, or whatever else you want to call that thing between the GHB keys. If you don't have one and you've never seen one, prepare to be amazed. [1]

Trackpoints are tiny joysticks that react to pressure [2], convert that pressure into relative x/y events and pass that on to whoever is interested in it. The harder you push, the higher the deltas. This is where the simple and obvious stops and it gets difficult. But then again, if it was that easy I wouldn't write this post, you wouldn't have anything to read, so somehow everyone wins. Whoop-dee-doo.

All the data and measurements below refer to my trackpoint, a Lenovo T440s. It may not apply to any other trackpoints, including those on on different laptop models or even on the same laptop model with different firmware versions. I've written the below with a lot of cringing and handwringing. I want to write data that is irrefutable, but the universe is against me and what the universe wants, the universe gets. Approximately every second sentence below has a footnote of "actual results may vary". Feel free to re-create the data on your device though.

Measuring trackpoint range is highly subjective, so you'll have to trust me when I describe how specific speeds/pressure ranges feel. There are three ranges of pressure on my trackpoint (sort-of):

  • Pressure range one: When resting the finger on the trackpoint I don't really need to apply noticable pressure to make the trackpoint send events. Just moving the finger on the trackpoint makes it send events, albeit sporadically.
  • Pressure range two: Going beyond range one requires applying real pressure and feels to me like we're getting into RSI territory. Not a problem for short periods, but definitely not something I'd want all the time. It's the pressure I'd use to cross the screen.
  • Pressure range three: I have to push hard. I definitely wouldn't want to do this during everyday interaction and it just feels wrong anyway. This pressure range is for testing maximum deltas, not one you would want to use otherwise.
The first/second range are easier delineated than the second/third range because going from almost no pressure to some real pressure is easy. Going from some pressure to too much pressure is more blurry, there is some overlap between second and third range. Either way, keep these ranges in mind though as I'll be using them in the explanations below.

Ok, so with the physical conditions explained, let's look at what we have to worry about in software:

  • It is impossible to provide a constant input to a trackpoint if you're a puny human. Without a robotic setup you just cannot apply constant pressure so any measurements have some error. You also get to enjoy a feedback loop - pressure influences pointer motion but that pointer motion influences how much pressure you inadvertently apply. This makes any comparison filled with errors. I don't know if I'm applying the same pressure on the two devices I'm testing, I don't know if a user I'm asking to test something uses constant/the same/the right pressure.
  • Not all trackpoints are created equal. Some trackpoints (mostly in Lenovos), have configurable sensibility - 256 levels of it. [3] So one trackpoint measured does not equal another trackpoint unless you keep track of the firmware-set sensibility. Those trackpoints also have other toggles. More importantly and AFAIK, this type of trackpoint also has a built-in acceleration curve. [4] Other trackpoints (ALPS) just have a fixed sensibility, I have no idea whether those have a built-in acceleration curve or merely have a linear-ish pressure->delta mappings.

    Due to some design choices we did years ago, systemd increases the sensitivity on some devices (the POINTINGSTICK_SENSITIVITY property). So even on a vanilla install, you can't actually rely on the trackpoint being set to the manufacturer default. This was in an attempt to make trackpoints behave more consistently, systemd had the hwdb and it seemed like the right place to put device-specific quirks. In hindsight, it was the wrong design choice.
  • Deltas are ... unreliable. At high sensitivity and high pressures you might get a sequence of [7, 7, 14, 8, 3, 7]. At lower pressure you get the deltas at seemingly random intervals. This could be because it's hard to keep exact constant pressure, it could be a hardware issue.
  • evdev has been the default driver for almost a decade and before that it was the mouse driver for a long time. So the kernel will "Divide 4 since trackpoint's speed is too fast" [sic] for some trackpoints. Or by 8. Or not at all. In other words, the kernel adjusts for what the default user space is and userspace is based on what the kernel provides. On the newest ALPS trackpoints the kernel has stopped doing any in-kernel scaling (good!) but that means that the deltas are out by a factor of 8 now.
  • Trackpoints don't always have the same pressure ranges for x/y. AFAICT the y range is usually a bit less than the x range on many or most trackpoints. A bit weird because the finger position would suggest that strong vertical pressure is easier to apply than sideways pressure.
  • (Some? All?) Trackpoints have built-in calibration procedures to find and set their own center-point. Without that you'll get the trackpoint eventually being ever so slightly off center over time, causing a mouse pointer that just wanders off the screen, possibly into the woods, without the obligatory red cape and basket full of whatever grandma eats when she's sick.

    So the calibration is required but can be triggered accidentally by the user: If you push with the same pressure into the same direction for 2-5 seconds (depending on $THINGS) you trigger the calibration procedure and the current position becomes the new center point. When you release, the cursor wanders off for a few seconds until the calibration sets things straight again. If you ever see the cursor buzz off in a fixed direction or walking backwards for a centimetre or two you've triggered that calibration. The only way to avoid this is to make sure the pointer acceleration mechanism allows you to reach any target within 2 seconds and/or never forces you to apply constant pressure for more than 2 seconds. Now there's a challenge...

Ok. If you've been paying attention instead of hoping for a TLDR that's more elusive than Godot, we're now aware of the various drawbacks of collecting data from a trackpoint. Let's go and look at data. Sensitivity is set to the kernel default of 128 in sysfs, the default reporting rate is 100Hz. All observations are YMMV and whatnot, especially the latter.

Trackpoint deltas are in integers but the dynamic range of delta values is tiny. You mostly get 1 or 2 and it requires quite a fair bit of pressure to get up to 5 or more. At low pressure you get deltas of 1, but less frequently. Visualised, the relationship between deltas and the interval between deltas is like this:

At low pressure, we get deltas of 1 but high intervals. As the pressure increases, the interval between events shrinks until at some point the interval between events matches the reporting rate (100Hz/10ms). Increasing the pressure further now increases the deltas while the intervals remain at the reporting rate. For example, here's an event sequence at low pressure:
E: 63796.187226 0000 0000 0000 # ------------ SYN_REPORT (0) ---------- +20ms
E: 63796.227912 0002 0001 0001 # EV_REL / REL_Y                1
E: 63796.227912 0000 0000 0000 # ------------ SYN_REPORT (0) ---------- +40ms
E: 63796.277549 0002 0000 -001 # EV_REL / REL_X                -1
E: 63796.277549 0000 0000 0000 # ------------ SYN_REPORT (0) ---------- +50ms
E: 63796.436793 0002 0000 -001 # EV_REL / REL_X                -1
E: 63796.436793 0000 0000 0000 # ------------ SYN_REPORT (0) ---------- +159ms
E: 63796.546114 0002 0001 0001 # EV_REL / REL_Y                1
E: 63796.546114 0000 0000 0000 # ------------ SYN_REPORT (0) ---------- +110ms
E: 63796.606765 0002 0000 -001 # EV_REL / REL_X                -1
E: 63796.606765 0000 0000 0000 # ------------ SYN_REPORT (0) ---------- +60ms
E: 63796.786510 0002 0000 -001 # EV_REL / REL_X                -1
E: 63796.786510 0000 0000 0000 # ------------ SYN_REPORT (0) ---------- +180ms
E: 63796.885943 0002 0001 0001 # EV_REL / REL_Y                1
E: 63796.885943 0000 0000 0000 # ------------ SYN_REPORT (0) ---------- +99ms
E: 63796.956703 0002 0000 -001 # EV_REL / REL_X                -1
E: 63796.956703 0000 0000 0000 # ------------ SYN_REPORT (0) ---------- +71ms
This was me pressing lightly but with perceived constant pressure and the time stamps between events go from 20m to 180ms. Remember what I said above about unreliable deltas? Yeah, that.

Here's an event sequence from a trackpoint at a pressure that triggers almost constant reporting:

E: 72743.926045 0002 0000 -001 # EV_REL / REL_X                -1
E: 72743.926045 0002 0001 -001 # EV_REL / REL_Y                -1
E: 72743.926045 0000 0000 0000 # ------------ SYN_REPORT (0) ---------- +10ms
E: 72743.939414 0002 0000 -001 # EV_REL / REL_X                -1
E: 72743.939414 0002 0001 -001 # EV_REL / REL_Y                -1
E: 72743.939414 0000 0000 0000 # ------------ SYN_REPORT (0) ---------- +13ms
E: 72743.949159 0002 0000 -002 # EV_REL / REL_X                -2
E: 72743.949159 0002 0001 -002 # EV_REL / REL_Y                -2
E: 72743.949159 0000 0000 0000 # ------------ SYN_REPORT (0) ---------- +10ms
E: 72743.956340 0002 0000 -001 # EV_REL / REL_X                -1
E: 72743.956340 0002 0001 -001 # EV_REL / REL_Y                -1
E: 72743.956340 0000 0000 0000 # ------------ SYN_REPORT (0) ---------- +7ms
E: 72743.978602 0002 0000 -001 # EV_REL / REL_X                -1
E: 72743.978602 0002 0001 -001 # EV_REL / REL_Y                -1
E: 72743.978602 0000 0000 0000 # ------------ SYN_REPORT (0) ---------- +22ms
E: 72743.989368 0002 0000 -001 # EV_REL / REL_X                -1
E: 72743.989368 0002 0001 -001 # EV_REL / REL_Y                -1
E: 72743.989368 0000 0000 0000 # ------------ SYN_REPORT (0) ---------- +11ms
E: 72743.999342 0002 0000 -001 # EV_REL / REL_X                -1
E: 72743.999342 0002 0001 -001 # EV_REL / REL_Y                -1
E: 72743.999342 0000 0000 0000 # ------------ SYN_REPORT (0) ---------- +10ms
E: 72744.009154 0002 0000 -001 # EV_REL / REL_X                -1
E: 72744.009154 0002 0001 -001 # EV_REL / REL_Y                -1
E: 72744.009154 0000 0000 0000 # ------------ SYN_REPORT (0) ---------- +10ms
E: 72744.018965 0002 0000 -002 # EV_REL / REL_X                -2
E: 72744.018965 0002 0001 -003 # EV_REL / REL_Y                -3
E: 72744.018965 0000 0000 0000 # ------------ SYN_REPORT (0) ---------- +9ms
Note how there is an events in there with 22ms? Maintaining constant pressure is hard. You can re-create the above recordings by running evemu-record.

Pressing hard I get deltas up to maybe 5. That's staying within the second pressure range outlined above, I can force higher deltas but what's the point. So the dynamic range for deltas alone is terrible - we have a grand total of 5 values across the comfortable range.

Changing the sensitivity setting higher than the default will send higher deltas, including deltas greater than 1 before reaching the report rate. Setting it to lower than the default (does anyone do that?) sends smaller deltas. But doing so means changing the hardware properties, similar to how some gaming mice can switch dpi on the fly.

I leave you with a fun thought exercise in correlation vs. causation: your trackpoint uses PS/2, your touchpad probably uses PS/2. Your trackpoint has a reporting rate of 100Hz but when you touch the touchpad half the bandwidth is used by the touchpad. So your trackpoint sends half the events when you have the palm resting on the touchpad. From my observations, the deltas don't double in size. In other words, your trackpoint just slows down to roughly half the speed. I can reduce the reporting rate to approximately a third by putting two or more fingers onto the touchpad. Trackpoints haven't changed that much over the years but touchpads have. So the takeway is: 10 years ago touchpads were smaller and trackpoints were faster. Simply because you could use them without touching the touchpad. Mind blown (if true, measuring these things is hard...)

Well, that was fun, wasn't it. I'm glad you stayed that long, because I did and it'd feel lonely otherwise. In the next post I'll outline the pointer acceleration curves for trackpoints and what we're going to to about that. Besides despairing, that is.

[1] I doubt you will be, but it always pays to be prepared.
[2] In this post I'm using "pressure" here as side-ways pressure, not downwards pressure. Some trackpoints can handle downwards pressure and modify the acceleration based on it (or expect userland to do so).
[3] Not that this number is always correct, the Lenovo CompactKeyboard USB with Trackpoint has a default sensibility of 5 - any laptop trackpoint would be unusable at that low value (their default is 128).
[4] I honestly don't know this for sure but ages ago I found a hw spec document that actually detailed the process. Search for ""TrackPoint System Version 4.0 Engineering Specification", page 43 "2.6.2 DIGITAL TRANSFER FUNCTION"

Wednesday, June 6, 2018

libinput is now on gitlab.freedesktop.org

Thanks to Daniel Stone's efforts, libinput is now on gitlab. For a longer explanation on the move from the old freedesktop infrastructure (cgit, bugzilla, etc.) to the gitlab instance hosted by freedesktop.org, see this email.

All open bugs have been migrated from bugzilla to gitlab too, the documentation has been updated acccordingly, and we're ready to go. The new base URL for libinput in gitlab is: https://gitlab.freedesktop.org/libinput/.

Tuesday, May 29, 2018

libinput-record and libinput-replay - tools to record and replay kernel devices

libinput 1.11 is just around the corner and one of the new features added are the libinput-record and libinput-replay tools. These are largely independent of libinput itself (libinput-replay is a python script) and replace the evemu-record and evemu-replay tools. The functionality is roughly the same with a few handy new features. Note that this is a debugging tool, if you're "just" a user, you may never have to use either tool. But for any bug report expect me to ask for a libinput-record output, same as I currently ask everyone for an evemu recording.

So what does libinput-record do? Simple - it opens an fd to a kernel device node and reads events from it. These events are converted to YAML and printed to stdout (or the provided output file). The output is a combination of machine-readable information and human-readable comments. Included in the output are the various capabilities of the device but also some limited system information like the kernel version and the dmi modalias. The YAML file can be passed to libinput-replay, allowing me to re-create the event device on my test machines and hopefully reproduce the bug. That's about it. evemu did exactly the same thing and it has done wonders for how efficiently we could reproduce and fix bugs.

Alas, evemu isn't perfect. It's becoming 8 years old now and its API is a bit crufty. Originally two separate tools generated two separate files (machine-readable only), two different tools for creating the device and playing events. Over the years it got more useful. Now we only have one tool each to record or replay events and the file includes human-readable comments. But we're hitting limits, its file format is very inflexible and the API is the same. So we'd have to add a new file format and the required parsing, break the API, deal with angry users, etc. Not worth it.

Thus libinput-record is the replacement for evemu. The main features that libinput-record adds are a more standardised file format that can be expanded and parsed easily, the ability to record and replay multiple devices at once and the interleaving of evdev events with libinput events to check what's happening. And it's more secure by default, all alphanumeric keys are (by default) printed as KEY_A so there's no risk of a password leaking into a file attached to Bugzilla. evemu required python bindings, for libinput-record's output format we don't need those since you can just access YAML as array in Python. And finally - it's part of libinput which means it's going to be easier to install (because distributions won't just ignore libinput) and it's going to be more up-to-date (because if you update libinput, you get the new libinput-record).

It's new code so it will take a while to iron out any leftover bugs but after that it'll be the glorious future ;)

Thursday, May 10, 2018

X server pointer acceleration analysis - part 4

This post is part of a four part series: Part 1, Part 2, Part 3, Part 4.

In the first three parts, I covered the X server and synaptics pointer acceleration curves and how libinput compares to the X server pointer acceleration curve. In this post, I will compare libinput to the synaptics acceleration curve.

Comparison of synaptics and libinput

libinput has multiple different pointer acceleration curves, depending on the device. In this post, I will only consider the one used for touchpads. So let's compare the synaptics curve with the libinput curve at the default configurations:

But this one doesn't tell the whole story, because the touchpad accel for libinput actually changes once we get faster. So here are the same two curves, but this time with the range up to 1000mm/s. These two graphs show that libinput is both very different and similar. Both curves have an acceleration factor less than 1 for the majority of speeds, they both decelerate the touchpad more than accelerating it. synaptics has two factors it sticks to and a short curve, libinput has a short deceleration curve and its plateau is the same or lower than synaptics for the most part. Once the threshold is hit at around 250 mm/s, libinput's acceleration keeps increasing until it hits a maximum much later.

So, anything under ~20mm/s, libinput should be the same as synaptics (ignoring the <7mm/s deceleration). For anything less than 250mm/s, libinput should be slower. I say "should be" because that is not actually the case, synaptics is slower so I suspect the server scaling slows down synaptics even further. Hacking around in the libinput code, I found that moving libinput's baseline to 0.2 matches the synaptics cursor's speed. However, AFAIK that scaling depends on the screen size, so your mileage may vary.

Comparing configuration settings

Let's overlay the libinput speed toggles. In Part 2 we've seen the synaptics toggles and they're open-ended, so it's a bit hard to pick a specific set to go with to compare. I'll be using the same combined configuration options from the diagram there.

And we need the diagram from 0-1000mm/s as well. There isn't much I can talk about here in direct comparison, the curves are quite different and the synaptics curves vary greatly with the configuration options (even though the shape remains the same).

Analysis

It's fairly obvious that the acceleration profiles are very different once depart from the default settings. Most notable, only libinput's slowest speed setting matches the 0.2 speed that is the synaptics default setting. In other words, if your touchpad is too fast compared to synaptics, it may not be possible to slow it down sufficiently. Likewise, even at the fastest speed, the baseline is well below the synaptics baseline for e.g. 0.6 [1], so if your touchpad is too slow, you may not be able to speed it up sufficiently (at least for low speeds). That problem won't exist for the maximum acceleration factor, the main question here is simply whether they are too high. Answer: I don't know.

So the base speed of the touchpad in libinput needs a higher range, that's IMO a definitive bug that I need to work on. The rest... I don't know. Let's see how we go.

[1] A configuration I found suggested in some forum when googling for MinSpeed, so let's assume there's at least one person out there using it.

X server pointer acceleration analysis - part 3

This post is part of a four part series: Part 1, Part 2, Part 3, Part 4.

In Part 1 and Part 2 I showed the X server acceleration code as used by the evdev and synaptics drivers. In this part, I'll show how it compares against libinput.

Comparison to libinput

libinput has multiple different pointer acceleration curves, depending on the device. In this post, I will only consider the default one used for mice. A discussion of the touchpad acceleration curve comes later. So, back to the graph of the simple profile. Let's overlay this with the libinput pointer acceleration curve:

Turns out the pointer acceleration curve, mostly modeled after the xserver behaviour roughly matches the xserver behaviour. Note that libinput normalizes to 1000dpi (provided MOUSE_DPI is set correctly) and thus the curves only match this way for 1000dpi devices.

libinput's deceleration is slightly different but I doubt it is really noticeable. The plateau of no acceleration is virtually identical, i.e. at slow speeds libinput moves like the xserver's pointer does. Likewise for speeds above ~33mm/s, libinput and the server accelerate by the same amount. The actual curve is slightly different. It is a linear curve (I doubt that's noticeable) and it doesn't have that jump in it. The xserver acceleration maxes out at roughly 20mm/s. The only difference in acceleration is for the range of 10mm/s to 33mm/s.

30mm/s is still a relatively slow movement (just move your mouse by 30mm within a second, it doesn't feel fast). This means that for all but slow movements, the current server and libinput acceleration provides but a flat acceleration at whatever the maximum acceleration is set to.

Comparison of configuration options

The biggest difference libinput has to the X server is that it exposes a single knob of normalised continuous configuration (-1.0 == slowest, 1.0 == fastest). It relies on settings like MOUSE_DPI to provide enough information to map a device into that normalised range.

Let's look at the libinput speed settings and their effect on the acceleration profile (libinput 1.10.x).

libinput's speed setting is a combination of changing thresholds and accel at the same time. The faster you go, the sooner acceleration applies and the higher the maximum acceleration is. For very slow speeds, libinput provides deceleration. Noticeable here though is that the baseline speed is the same until we get to speed settings of less than -0.5 (where we have an effectively flat profile anyway). So up to the (speed-dependent) threshold, the mouse speed is always the same.

Let's look at the comparison of libinput's speed setting to the accel setting in the simple profile:

Clearly obvious: libinput's range is a lot smaller than what the accel setting allows (that one is effectively unbounded). This obviously applies to the deceleration as well: I'm not posting the threshold comparison, as Part 1 shows it does not effect the maximum acceleration factor anyway.

Analysis

So, where does this leave us? I honestly don't know. The curves are different but the only paper I could find on comparing acceleration curves is Casiez and Roussel' 2011 UIST paper. It provides a comparison of the X server acceleration with the Windows and OS X acceleration curves [1]. It shows quite a difference between the three systems but the authors note that no specific acceleration curve is definitely superior. However, the most interesting bit here is that both the Windows and the OS X curve seem to be constant acceleration (with very minor changes) rather than changing the curve shape.

Either way, there is one possible solution for libinput to implement: to change the base plateau with the speed. Otherwise libinput's acceleration curve is well defined for the configurable range. And a maximum acceleration factor of 3.5 is plenty for a properly configured mouse (generally anything above 3 is tricky to control). AFAICT, the main issues with pointer acceleration come from mice that either don't have MOUSE_DPI set or trackpoints which are, unfortunately, a completely different problem.

I'll probably also give the windows/OS X approaches a try (i.e. same curve, different constant deceleration) and see how that goes. If it works well, that may be a a solution because it's easier to scale into a large range. Otherwise, *shrug*, someone will have to come with a better solution.

[1] I've never been able to reproduce the same gain (== factor) but at least the shape and x axis seems to match.

X server pointer acceleration analysis - part 2

This post is part of a four part series: Part 1, Part 2, Part 3, Part 4.

In Part 1 I showed the X server acceleration code as used by the evdev driver (which leaves all acceleration up to the server). In this part, I'll show the acceleration code as used by the synaptics touchpad driver. This driver installs a device-specific acceleration profile but beyond that the acceleration is... difficult. The profile itself is not necessarily indicative of the real movement, the coordinates are scaled between device-relative, device-absolute, screen-relative, etc. so often that it's hard to keep track of what the real delta is. So let's look at the profile only.

Diagram generation

Diagrams were generated by gnuplot, parsing .dat files generated by the ptrveloc tool in the git repo. Helper scripts to regenerate all data are in the repo too. Default values unless otherwise specified:

  • MinSpeed: 0.4
  • MaxSpeed: 0.7
  • AccelFactor: 0.04
  • dpi: 1000 (used for converting units to mm)
All diagrams are limited to 100 mm/s and a factor of 5 so they are directly comparable. From earlier testing I found movements above over 300 mm/s are rare, once you hit 500 mm/s the acceleration doesn't really matter that much anymore, you're going to hit the screen edge anyway.

The choice of 1000 dpi is a difficult one. It makes the diagrams directly comparable to those in Part 1 but touchpads have a great variety in their resolution. For example, an ALPS DualPoint touchpad may have resolutions of 25-32 units/mm. A Lenovo T440s has a resolution of 42 units/mm over PS/2 but 20 units/mm over the newer SMBus/RMI4 protocol. This is the same touchpad. Overall it doesn't actually matter that much though, see below.

The acceleration profile

This driver has a custom acceleration profile, configured by the MinSpeed, MaxSpeed and AccelFactor options. The former two put a cap on the factor but MinSpeed also adjusts (overwrites) ConstantDeceleration. The AccelFactor defaults to a device-specific size based on the device diagonal.

Let's look at the defaults of 0.4/0.7 for min/max and 0.04 (default on my touchpad) for the accel factor:

The simple profile from part 1 is shown in this graph for comparison. The synaptics profile is printed as two curves, one for the profile output value and one for the real value used on the delta. Unlike the simple profile you cannot configure ConstantDeceleration separately, it depends on MinSpeed. Thus the real acceleration factor is always less than 1, so the synaptics driver doesn't accelerate as such, it controls how much the deltas are decelerated.

The actual acceleration curve is just a plain old linear interpolation between the min and max acceleration values. If you look at the curves closer you'll find that there is no acceleration up to 20mm/s and flat acceleration from 25mm/s onwards. Only in this small speed range does the driver adjust its acceleration based on input speed. Whether this is in intentional or just happened, I don't know.

The accel factor depends on the touchpad x/y axis. On my T440s using PS/2, the factor defaults to 0.04. If I get it to use SMBus/RMI4 instead of PS/2, that same device has an accel factor of 0.09. An ALPS touchpad may have a factor of 0.13, based on the min/max values for the x/y axes. These devices all have different resolutions though, so here are the comparison graphs taking the axis range and the resolution into account:

The diagonal affects the accel factor, so these three touchpads (two curves are the same physical touchpad, just using a different bus) get slightly different acceleration curves. They're more similar than I expected though and for the rest of this post we can get away we just looking at the 0.04 default value from my touchpad.

Note that due to how synaptics is handled in the server, this isn't the whole story, there is more coordinate scaling etc. happening after the acceleration code. The synaptics acceleration profile also does not acccommodate for uneven x/y resolutions, this is handled in the server afterwards. On touchpads with uneven resolutions the velocity thus depends on the vector, moving along the x axis provides differently sized deltas than moving along the y axis. However, anything applied later isn't speed dependent but merely a constant scale, so these curves are still a good representation of what happens.

The effect of configurations

What does the acceleration factor do? It changes when acceleration kicks in and how steep the acceleration is.

And how do the min/max values play together? Let's adjust MinSpeed but leave MaxSpeed at 0.7.

MinSpeed lifts the baseline (i.e. the minimum acceleration factor), somewhat expected from a parameter named this way. But it looks again like we have a bug here. When MinSpeed and MaxSpeed are close together, our acceleration actually decreases once we're past the threshold. So counterintuitively, a higher MinSpeed can result in a slower cursor once you move faster.

MaxSpeed is not too different here:

The same bug is present, if the MaxSpeed is smaller or close to MinSpeed, our acceleration actually goes down. A quick check of the sources didn't indicate anything enforcing MinSpeed < MaxSpeed either. But otherwise MaxSpeed lifts the maximum acceleration factor.

These graphs look at the options in separation, in reality users would likely configure both MinSpeed and MaxSpeed at the same time. Since both have an immediate effect on pointer movement, trial and error configuration is simple and straightforward. Below is a graph of all three adjusted semi-randomly:

No suprises in there, the baseline (and thus slowest speed) changes, the maximum acceleration changes and how long it takes to get there changes. The curves vary quite a bit though, so without knowing the configuration options, it's impossible to predict how a specific touchpad behaves.

Epilogue

The graphs above show the effect of configuration options in the synaptics driver. I purposely didn't put any specific analysis in and/or compare it to libinput. That comes in a future post.