I have Debian 11 (bullseye) on my Toshiba Satellite Pro C665 laptop. It uses libinput by default but I found the cursor difficult to control. With a low acceleration I had precise control but it was tedious traversing the screen. With a high acceleration I could traverse the screen OK but precise control of the cursor was difficult to impossible. So, I began to learn far more than I ever wanted to know about Xorg X Server, libinput, synaptic, evdev and pointers in general. I have only learned a little: not yet enough to achieve reasonable behaviour.
My touchpad works with the libinput and synaptic input drivers but doesn't work with evdev. For reasons unknown, when I configure evdev there are no errors logged but the cursor doesn't move. The buttons are recognized but I can't move the cursor. Other's report this behaviour and the most common 'solution' is to use synaptic or libinput drivers. The events are generated. I have no idea why they are ignored when I try the evdev input driver. Maybe evdev is essentially unsupported software recent years and broke at some point and no one cares.
This thread addresses another fault in libinput but it includes instructions for re-building libinput from source. I have now done this both via 'apt build-dep' etc. as indicated there, and also from xf86-input-libinput which I built and installed with no ill effect.
The person who responded with the instructions for for building libinput said, among other things: "I agree that this isn't an ideal user experience. On the other hand, recompiling libinput in 2020 takes about as much time as taking out a credit card from your wallet and paying for another closed source macOS app." and there was some subsequent back and forth among respondents about how reasonable this workaround is.
I have been aware of X Server since the 1980's. I have been using Linux since the mid 90's, written device drivers from scratch, hacked parts of the kernel and built and debugged many packages through the years. I have been searching for a way to improve my touchpad behaviour for two days now and have learned only a little of what software is involved, how it all relates and how it is all configured. Had I not stumbled on the above linked thread, it might have been many more days before I learned how to build libinput from source and much longer to understand what all the code was doing to resolve that problem. I have looked at the code that implements the acceleration profile but it will be a long while before I understand how to add my own profile and configuration options (i.e. changes to processing of the Xorg configuration files, xinput, xset and who knows what other places might be required to make something that works).
The documentation (in all forms I can find, including man pages, tutorials and forum discussions) is typically terse, often using undefined terms and often inconsistent with the current implementation on my laptop. The software and how to configure it has obviously changed through the years and it is difficult to determine what advice applies to what version of the software. Even current Xorg documentation on the official Xorg website is inconsistent with my experience when using the libinput device driver.
My conclusions thus far:
The evdev device driver doesn't work: there are no errors but the cursor doesn't move when I use the touchpad. No idea why. Much of the documentation I can find about setting acceleration profiles seems to be specific to the evdev input device driver. I would like to try some of these profiles but I can't because the cursor doesn't move at all.
The synaptic device driver works but it is difficult to understand how to configure acceleration. Synaptics Pointer Acceleration makes some convincing arguments that the Synaptic driver is complex to the point of being unpredictable, though it might not be so intractable if one cares only about a single hardware configuration. The author is one of the main contributors to both the Synaptic and libinput device drivers, so well informed. After some effort I was unable to achieve what I wanted with this input driver.
The libinput driver is the default on my and many systems. Many complain about the acceleration. Documentation of it is misleading and there are very limited configuration options. I have been unable to configure it to allow my both fine control of the cursor and easy traversal of larger areas. The source code suggests (to my as-yet superficial understanding) that the implementation is more complex than and inconsistent with the documentation, with obscure heuristics determining the acceleration parameters, rather than configuration parameters.
It is a dismaying prospect to have to spend many more days if I want to understand what all the configuration options (across GUI configuration tools, command line tools and various configuration files for Xorg, udev, xfce4, etc., etc., etc.) are and what they do and which part(s) of the software they affect.
Presumably there is code in the hardware device driver, Xorg input driver and Xorg X Server that could and possibly do contribute to the overall mapping from the hardware to the cursor position. But it is a morass of poorly documented components, leading to confusion and every experiment thus far requires an hour or two of researching errors, installing and configuring tools, often to achieve little to nothing.
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