I've begun testing 4.0 kernel on QNAP TS-212P. The following results likely apply also to other devices matching kirkwood-ts219-6282.dtb like TS-210 and variants of TS-219. So, let's begin with...
The Good:
* mv_cesa works correctly (partially broken in 3.16 - #783718)
* kirkwood-cpufreq works correctly (it didn't seem to do anything in 3.16)
* kirkwood-thermal works correctly (/sys/class/thermal/thermal_zone0 is correctly populated, nothing in 3.16)
The Bad:
* qcontrol service does not start. This appears to be due to the fact that qcontrol expects a symlink "platform-gpio-keys-event" in /dev/input/by-path, while with the 4.0 kernel it's called "platform-gpio_keys-event" instead. As a workaround, adding the expected symlink causes qcontrol to work correctly (warning: without qcontrol the device is on a 5 minute watchdog, you can also interrupt it with "echo -n g >/dev/ttyS1").
The Ugly:
* USB 3.0 doesn't seem to work - xhci doesn't start correctly:
[ 10.163137] xhci_hcd 0000:02:00.0: xHCI Host Controller
[ 10.185235] xhci_hcd 0000:02:00.0: new USB bus registered, assigned bus number 3
[ 10.230163] xhci_hcd 0000:02:00.0: Host took too long to start, waited 16000 microseconds.
[ 10.238481] xhci_hcd 0000:02:00.0: startup error -19
[ 10.322217] xhci_hcd 0000:02:00.0: USB bus 3 deregistered
[ 10.346872] xhci_hcd 0000:02:00.0: remove, state 1
dpkg: error: unable to create new file '/var/lib/dpkg/status-new': Read-only file system