what's the output from udevadm info /sys/class/input/event5 (adjusted for the event node if need be)

Output for udevadm info /sys/class/input/event5:
P: /devices/platform/i8042/serio1/input/input7/event5
N: input/event5
E: DEVNAME=/dev/input/event5
E: DEVPATH=/devices/platform/i8042/serio1/input/input7/event5
E: ID_BUS=i8042
E: ID_INPUT=1
E: ID_INPUT_MOUSE=1
E: ID_INPUT_POINTINGSTICK=1
E: LIBINPUT_DEVICE_GROUP=11/2/8/800:isa0060/serio1
E: MAJOR=13
E: MINOR=69
E: SUBSYSTEM=input
E: USEC_INITIALIZED=19241595

have a look at this file:
https://github.com/systemd/systemd/blob/master/hwdb/70-pointingstick.hwdb

looks like you need an entry with a CONST_ACCEL of less than 1 to slow the thing down a bit. at the top of the file are instructions on how to install a custom hwdb entry for testing

I'm tried that but it's doesn't work.

My output for/sys/class/dmi/id/modalias
dmi:bvnDellInc.:bvr1.11.3:bd11/14/2016:svnDellInc.:pnLatitudeE5570:pvr:rvnDellInc.:rn0GMHGH:rvrA00:cvnDellInc.:ct9:cvr:

So, I'm created a local rule 
evdev:name:*DualPoint Stick:dmi:bvn*:bvr*:bd*:svnDellInc.:pnLatitudeE5570*:pvr*
 POINTINGSTICK_CONST_ACCEL=0.2

After that I'm update hwdb and execute udevadm trigger /dev/input/event*
But nothing changed. No matter what value I'm set to CONST_ACCEL.

does the property show up? 
https://wayland.freedesktop.org/libinput/doc/latest/faq.html#faq_hwdb_changes

If it shows up in udev, does libinput-debug-events --verbose show the const accel value?

Yes and yes. udevamd test produce this output

ID_INPUT_POINTINGSTICK=1
LIBINPUT_DEVICE_GROUP=11/2/8/800:isa0060/serio1
MAJOR=13
MINOR=69
POINTINGSTICK_CONST_ACCEL=0.5

And libinput-debug-events --verbose 
input device 'AlpsPS/2 ALPS DualPoint Stick', /dev/input/event5 is tagged by udev as: Mouse Pointingstick
Device 'AlpsPS/2 ALPS DualPoint Stick' set to const accel 0.50

But nothing changed in pointstick behavior

strange. You restarted X/Wayland, I take it?

Damn. I've never restart wayland after changes, only hwdb update. Actually, restarting a wayland works. Thank you so much!

I also have a Dell laptop with an ALPS dual pointing stick and had the same issue that the stick was much too fast. However, I am unable to achieve acceptable results with all the different knobs that are available. If I tune down acceleration, I am able to do detailed pointing, but the stick is generally so slow, that it takes ages to cross the screen. On the other hand, if I increase the acceleration, even the slightest movement of the stick makes it jump significantly on the screen so that I am unable to do detailed selections.

Setting POINTINGSTICK_CONST_ACCEL has an effect but as described, not the desired. Is there anything else that can be tuned to get this going?

No, there's no other setting exposed right now, sorry. It's a long-standing issue with libinput but no-one has found the time or motivation to work on it yet.

This fix doesn't work on my Dell E5570, including restarting Xorg.  :-(

Running Linux Mint 18.1 with Cinnamon.  If I stick to an old Kernel, 4.4.0-53 generic, then the mouse is fine.  But if I use any later Kernel, including various versions of 4.8 and 4.10, then the mouse pointer-stick has uncontrollable acceleration.  The touch pad is fine.

Dominik

is the property applied in both cases?

yes, if you mean with both kernels.  Yes.

... But in fact the lower 4.4 kernel doesn't need the hwdb setting.  The pointer-stick/trackpoint acceleration is fine, out of the box.

I think I'm going to close this this as dupe of bug 91369 because it ends up being the same issue. That bug has a test branch in bug 91369#c48, please give that one a try.

*** This bug has been marked as a duplicate of bug 91369 ***