Is there any performance difference between udev and evdev in xorg?

[Niclas Zeising]

On 2020-09-16 09:37, Alexey Dokuchaev wrote:
> On Wed, Sep 16, 2020 at 08:41:28AM +0200, Niclas Zeising wrote:
>> On 2020-09-16 06:01, Alexey Dokuchaev wrote:
>>> On Tue, Sep 15, 2020 at 10:55:31PM -0400, Aryeh Friedman wrote:
>>>> What if any is the performance difference between udev and evdev when
>>>> configuring xorg?  Also do I need to use one or the other consistently
>>>> or can I intermix them?
>>>
>>> If you don't need them (e.g. because this is desktop system without
>>> fancy input devices), you'd better off with disabling both of them
>>> altogether and use good old traditional way, that is, simply install
>>> xf86-input-{keyboard,mouse} and let X.org handle those peripherals.
>>>
>>> Yes, you would still be able to plug and unplug your USB mice and
>>> they will be detected and working as expected.
>>>
>>> TL;DR: DEVD/UDEV support is overrated and usually not needed at all.
>>
>> This is bad advice.
> 
> OK, let's see why is it bad. :-)
> 
>> The DEVD support in xorg-server might go away, since it is a FreeBSD
>> only solution and the udev/evdev is similar to what is used on Linux.
> 
> Does this imply that DEVD support in X.org is technically inferior to
> udev/evdev, or it might get deprecated just because they prefer Linux
> way, regardless of the actual design and implementation quality?  Kind
> of tangentially related question, but this might help to foresee what
> to expect from future X.org development.

The DEVD support in xorg is only used by FreeBSD, which is a fairly 
small installation base.  To my knowledge only FreeBSD and derivates 
(and maybe dfly) has devd, the other BSDs does something else to handle 
devices.  The devd support hasn't been upstreamed, since we in FreeBSD 
grew udev/evdev support instead.

I'm not sure who "they" are in this context, but upstream is using 
udev/evdev by default, at least on Linux (which today is the big 
consumer).  By using that stack we get the benefit of that big 
installation base when it comes to features, bug fixes and so on.

Having a uniform way, similar to Linux, to handle this also means that 
we get the benefit that we don't need to add and maintain FreeBSD compat 
stuff to other software dealing with input devices, such as Wayland, and 
possibly other ports.  It also means that we can use the configuration 
utilities in desktop environments, etc.

It is also the only thing available for Wayland, which is relevant since 
a lot of upstream effort is going in that direction.

> 
>> If you are using Wayland, it is also the only way to use input devices.
> 
> Wayland is overrated and unneeded as well.  Plus, we're discussing X11
> here and X.org server in particular, how's that even relevant?

Upstream doesn't seem to agree here.  A lot of upstream effort is aimed 
at Wayland currently.

> 
>> If you are using the default configuration of xorg on FreeBSD 12.1 or
>> later, using udev is the default.  This means using xf86-input-libinput
>> as the input device driver in X, and not xf86-input-{keyboard,mouse}.
>> This gives much better support for things like synaptics touchpads and
>> similar devices.
> 
> Like I've said initially, these might come handy, but for "desktop system
> without fancy input devices", what's the point of bringing another layer
> of abstraction (xf86-input-libinput) rather then let X.org talk to device
> drivers drivers directly and not having to deal with evdev/libinput bugs,
> tinker with sysctls (kern.evdev.rcpt_mask), etc.?

I'm not sure there's much more abstraction when using udev/evdev rather 
than when using the old method.  Most things are in libinput, and the 
evdev drivers are in the FreeBSD kernel, which libinput and 
xf86-input-libinput interfaces with.

Without this you still need xf86-input-{mouse,keyboard} to talk directly 
to the kernel driver, which is the same thing.  You also might need 
special xf86-input-* which can be handled by libinput if you have 
special hardware.

The kern.evdev.rcpt_mask problem is a transition problem.  To change the 
default there, we first had to change the default in ports, and to 
change the defaults in ports, we first needed to add the relevant 
drivers to GENERIC.
In FreeBSD 13, as well as the upcoming 12.2, kern.evdev.rcpt_mask has a 
sensible default to work with the new input stack.

> 
>> You can configure such devices either by adding X configuration snippets
>> to /usr/local/etc/X11/xorg.conf.d/ or by using xinput on the command line
> 
> Right, and with the old way, device configuration snippets are not needed.
> Just that simple.  So, the advice does not look that bad after all. :-)
> 
> TL;DR: if there's a simpler solution/approach which is sufficient for one's
> needs, e.g. for simple three-button mouse and pc104 keyboard, just dump the
> extra xf86-input-libinput bloat and stick to old, well-tested, solid code
> which just works(tm).

You don't need configuration snippets for the defaults when using 
udev/evdev either, but it's one way to change for instance keyboard 
layouts, or mice handling.
I personally prefer to use setxkbmap and xinput for these (to for 
instance get my preferred layout and a mouse pointer speed I'm 
comfortable with).

Regards
-- 
Niclas Zeising