== USB mouse polling rates under Linux

I'm behind the times, so I only recently discovered that USB mice
have a polling rate, that this polling rate can often be adjusted
under Linux, and that you might want to do so. A good starting
point for this is [[the Arch Linux wiki page on mouse polling
rate https://wiki.archlinux.org/index.php/Mouse_polling_rate]],
but it assumes some basic background that I had to think through.

The advantage of a higher mouse polling rate is not that your mouse
moves any faster. Instead, it is that the mouse reports your motion
changes sooner (what I think gamers are interested in) and that it
reports them more frequently. More frequent reporting usually means
more frequent updating, which in turn can lead to smoother, more
continuous motion. On the other side, too-slow updates can make
mouse motion feel subtly jerky.

(Mouse movement under X is generally already accelerated to some
degree once you move the mouse far enough. See at least _xset_'s
mouse settings and probably the preferences for your desktop
environment if you use one.)

If what you mostly care about is smooth motion, I think that there's
not much benefit to a mouse polling rate that is massively above
the refresh rate of your display. After all, updating the mouse
cursor position 500 times a second is relatively pointless if you
only see it 60 times a second. At the same time, a relatively high
polling rate has the advantage that it gives X and programs a lot
of time to respond to your mouse movement before the next display
update, rather than perhaps only getting the position update at the
last moment.

.pn prewrap on

[[The Arch page https://wiki.archlinux.org/index.php/Mouse_polling_rate]]
is a little bit confusing about the normal default Linux mouse
polling rate. Based on looking at the kernel source, how it appears
to work is that USB mice can tell us their desired polling rate and
by default the kernel just accepts that. The normal USB mouse rate
is 125 Hertz, so most USB mice are going to tell the kernel to use
that; however, I believe some mice can be (or are) set to ask for
a higher polling rate, and if they do the kernel will honour that
by default.

(Your LCD panel likely refreshes at a nominal 60 Hz rate, so I'd
expect that 125 Hz provides plenty of responsiveness headroom;
still, increasing the polling rate is unlikely to hurt.)

As covered in [[the Arch page
https://wiki.archlinux.org/index.php/Mouse_polling_rate]], it's
possible to explicitly set the Linux mouse polling rate and thus
increase it from 125 Hertz. This is done by setting a non-zero value
for the usbhid module's _mousepoll_ module parameter (see [[the
Arch page]] for what values you want). The Arch page describes an
elaborate procedure to change the _mousepoll_ parameter without
rebooting, which is fortunately not necessary these days as kernel
modules expose their module parameters in _/sys/module/<NAME>/parameters_.
To change and test mouse polling rates on the fly, you first
write the new _mousepoll_ value with:

>  echo N >/sys/module/usbhid/parameters/mousepoll

and then *unplug and replug your USB mouse* (or mice). The unplug
and replug bit is necessary because the usbhid module only sets up
the mouse polling interval for a specific USB mouse when the mouse
is plugged in. Changing _mousepoll_ thus only changes the polling
rate for future mice, not currently attached ones.

(A _mousepoll_ value of 0 means 'use whatever the mouse would like',
which is probably mostly going to be '125 Hertz'.  Note that this
implies that setting an explicit conservative _mousepoll_ value may
cause some USB mice to be polled at a lower rate than you'd get if
you just left things alone.)

What you've set Linux's mouse polling rate to is not necessarily
the polling rate that you actually get, for various reasons. You
can see what the actual achieved rate is using [[the useful evhz
program https://github.com/ian-kelling/evhz]], which measures the
rate based on how fast it receives evdev events. This isn't restricted
to just USB mice; it will also reports values for PS/2 mice and
probably any mouse-like thing that Linux's evdev system supports.

The Arch page also talks about displaying the USB device polling
rate, which can be dug out of _/sys/kernel/debug/usb/devices_.
Based on my own testing on Fedora 22, the reported device _Ivl_
does not have much to do with the mouse polling rate. All of my USB
mice report 10ms (which is only 100 Hz), but [[evhz
https://github.com/ian-kelling/evhz]] disagrees and reports 125 Hz
normally for a standard USB mouse and a 500 Hz polling rate if I
set _mousepoll_ to _2_.

Regardless of what you have _mousepoll_ set to, it's possible for
a USB mouse (or at least something claiming to be one) to respond
at a slower rate. In other words, Linux's USB mouse polling rate
is a maximum, not a minimum. Hopefully such hardware is rare,
especially when it comes to real mice.

(I suspect that KVM over IP systems that support virtual USB mice
have relatively low maximum mouse poll rates no matter what Linux
asks for.)