> 1. After successful boot, I've noticed, that buttons above the
> touchpad[1] stopped working. Tried to debug the thing, but they do
> nothing even under xev or in showkey - seems like they dead. Didn't find
> anything interesting by searching the net.
>

Longshot (since i've had one instance in the last few years where i need to
rebuild xorg after a kernel update) have you tried rebuilding the
xf86-input-* packages against the new kernel?

>
> 2. This one is more annoying. After boot to the new shiny 4.x kernel,
> devices connected via the usb seems to have aggressive power safe mode.
> For example - if I typing for a while everything is fine, if I stop for
> a couple of seconds, and then start to type, for approximately 2 seconds
> there is no response from the device, and after that i can type again.
> Note, that during those 2s no buffering is done, so whatever I'm typing
> in this short period, is lost. Same goes to the mouse movement and
> buttons.
>

I haven't had much luck with USB power saving working in a useful way and
leave it off. Strange its been enabled by default for you.

cat /sys/bus/usb/devices/*/power/control will show you what the current
settings are.

The options are on/auto.

"on" means that the device should be resumed and autosuspend is not allowed.

so that's what you want. It can blanket set with;

echo on | tee /sys/bus/usb/devices/*/power/control

Or you could work out which devices are actually a problem and just "on"
those particular ones.