Here are some of the problems I have encountered:

1) CPU core voltage adjustments don't always apply

2) Setting the boot performance mode to " http://www.ebay.com/sch/i.html?_nkw=max+battery" rel="nofollow - Max Battery " (the feature description states that this is the recommended setting for BCLK overclocking) sometimes causes the system to boot into Windows (10) using a very low multiplier (no software overclocking utilities are running)

3) The UEFI (BIOS) setup GUI freezes often, even when everything is running at stock, and most often when clicking the "back" button to exit a particular section, even after doing a proper reset operation as described in the manual

4) Even when core voltage adjustments work only a minimal amount of overclocking is possible.  I can barely get my  http://shop.lenovo.com/us/en/desktops/ideacentre/700/?sb=:000001C9:000161BA:" rel="nofollow - i5-6400  to hit 3.9GHz (which is also unstable), and this remains true even when I UNDERCLOCK my memory (which has been tested extensively in another board and found to be perfectly fine)

Please tell me that you guys working on improving the BIOS as right now it's almost unusable.  We really need there to be at least one DDR3 Skylake board that has decent BCLK overclocking capabilities.

By the way, do you think you could add a "fixed" voltage mode for vcore adjustments instead of (or in addition to) just an offset?  And how about Load-Line Calibration?

Thanks :)

Sure:

Asrock Z170 Pro4/D3 running BIOS v1.74

Intel i5-6400 (cooled with a Noctua NH-C12P SE14)

Kingston KHX1866C10D3/4G (8GB kit, dual channel, slots 1,3)

MSI nVidia GTX970 Gaming

Corsair HX650W PSU

+ a couple of Intel SSDs and WD Green drives

An update:

After finally managing to get a vcore adjustment to stick, and get some other appropriate settings dialed in without a UEFI freeze, I successfully booted into Windows 10 at 4GHz.  It was Prime95 stable for an hour, after which I also ran some 3D benchmarks and even played Fallout 4 for a while.  So we know this board can overclock at least a bit.  However some significant issues remain:

For some reason I can't boot Linux when overclocking, even when the overclock seems 100% stable in all other respects.  Weird.

The system wouldn't come out of "sleep" mode, and it took several reboot attempts to bring it up again. Maybe I shouldn't be expecting proper power management behaviour in this scenario, but I've overclocked every system I've ever owned and never had to sacrifice such features.  However if this was the only problem I think I could live with it.

I thought perhaps that the "Boot performance mode" might be in play here (it sounds like it keeps the CPU multiplier low until the OS boots) so I tried setting it to "Max battery" again but observed the same behaviour as before: Windows boots with the multiplier permanently locked to a low value (right now my i5-6400 is running at 1200MHz and wont budge from there even when running Prime95).

So, vcore adjustments can work (albeit very intermittently), stable overclocking is possible (when the stars align), but there are all sorts of pretty major peripheral bugs and usability issues.

Do the Asrock engineers peruse these forums, or should I be emailing them this sort of information directly?

 

(EDIT: have now used the support request form relay this information as well)

Thanks again.

Done :)

The Max Battery option is supposed to do what you are experiencing. It keeps the core multipliers at a fixed setting so they don't interfere with a BCLK OC.

Enabling the Max Battery option is itself interfering with the overclock by virtue of locking the multiplier to a low value.  Yes, that is appropriate behaviour, but only up until the point where the OS has booted. But for me it remains locked to x8 even after Windows has booted.  This means that a 4GHz OC becomes a 1.2GHz underclock (while the baseclock remains at 150MHz).

Setting it back to "Max Non-Turbo Performance" gets me back to 4GHz as a result of the multiplier being locked at x27, which is where it should be.

"Max battery mode will set CPU ratio as x8 until OS handoff"

Given the description, and the fact that it is the recommended setting for BCLK overclocking, one would have to conclude that this is a bug, right?

Why bother with the setting then?  My thinking is that it might solve some intermittent boot issues and help facilitate higher overclocks (by putting less initial strain on the system), so it seems like a useful feature.  As such I'd like to see it working properly :)

Recall that C States are not active when doing a BCLK OC, since they do just that, vary the multiplier among other things. Back when over clocking was done mainly by the BCLK, disabling C States was a common practice for stability.

Sure.

Sometimes video cards have problems with coming out of Sleep, and a PSU can too. Did you check the Windows Event Log for any clues?

I haven't checked the log yet.  But the system has no trouble coming out of sleep when I'm not overclocking.

What are your basic settings for your 4GHz OC? BCLK, multiplier for All Cores?  Multi Core Enhancement should be Disabled. Do you have any special BCLK options (some boards do) that may make a BCLK OC more stable?

The BIOS on this board is sparsely populated with settings compared to some other Asrock Z170 boards.  I've disabled things like Speedstep and Turbo Boost obviously.  I keep the RAM within spec.  I keep the vcore at or close to 1.325v. About the only "special" BCLK OC-related option is the one that doesn't work properly: "Boot performance mode".

Linux would boot for you when you are not using your BCLK OC?

Yes, it boots fine when everything is running at stock speeds.  It also wont boot with the "Max Battery" option, which as described above seems to result in a permanent (for the boot cycle at hand anyway) underclock.

The Linux (Ubuntu 15.10) boot issue doesn't appear to be stability-related since even the tiniest of overclocks (110MHz BLCK for example) stops it from booting (hangs with a flashing cursor after grub with no useful information logged).  This suggests that the issue might be related to bad handling of a scenario involving disabled CPU features or something like that.

Knowing that Skylake support was basically sort of hacked together for 15.10 I decided to update it to kernel 4.3, but the boot failure persists in an identical fashion. Even a daily build of 16.04 behaved in precisely the same way.

Again all it takes to make it all work again is to set the BCLK back to 100MHz.  This is getting to be a bit of a deal-breaker.  As it stands it would have made more sense to get a cheaper board with a low-end chipset to pair with something like an i5-6600.  Roughly the same outlay, roughly the same performance (as my 4GHz i5-6400 OC) but without all the drama.

Still hoping the situation can be salvaged with a BIOS update of course.

I've also tried the automagical Sky OC "press X to overclock" feature (which seems to lock the OC to a speed that while higher than base is still less than the CPU's turbo boost speed? :/) and it doesn't behave any differently from a manual OC.  I suspected that this would be the case but I tried it anyway.

Vcore adjustments seem to be more reliable in the sense that they actually stick but since the problem was intermittent before only further testing will confirm an improvement.

I haven't tried OC'ing beyond 4GHz on the new BIOS yet to see if there are any stability improvements but it's kind of a moot point anyway if I can't boot into Ubuntu (I do most of my gaming in Windows, but most of my really CPU intensive tasks in Ubuntu).  There's probably just the right kernel argument (or combination thereof) out there somewhere to serve as a workaround but I haven't found it yet.

At this point I'd like to encourage anyone who stumbles across this thread via a google search to sign up and report on their own experiences with BCLK OC'ing on this same motherboard as it would be profoundly useful to know whether or not I am the only one having these problems.  Please :)

I will of course wait to hear back from Asrock support before I decide what to do moving forward.

Thanks for the input so far everyone :)

Dang, ya know what? Right after I edited my post above, nolapic, I had a brain storm. Maybe Go into the BIOS, disable HPET, and retry.

Good thinking 99.  But no cigar unfortunately.

Thanks for your efforts nonetheless :)

It scared me a couple of times when trying to find the optimum UEFI settings but nothing I could not eventually resolve.

I think I have flashed the bios at least 10 times. 

Have not tried skyoc yet, I'm using iGPU and frankly it runs every game I throw at it with decent framerate at 1920x1200 with low or medium quality in game settings

Bios 2.80 allowed me to set a BLCK of 103Mhz while the latest 3.10 is not stable past 102Mhz but the difference is minimal in games. 

It is a lot more important to try and keep the HD530 at its maximum speed without throttling.