Cooler Master Tempest GP27U compatibility issue

KVM Model:

MKAG-8KT3124 (HDMI 2.1 KVM w/EDID & Serial Control - Dual Monitor - Four Computer)

USB Interface (5-gigabit or 10-gigbit):

10-gigabit


Problem:

Cooler Master Tempest GP27U stays asleep after sleep/wake cycle of PC, but only via the KVM.

Other Notes:

Edit: I’ve updated this since my initial post to remove some ambiguities and also some further test cases.

For a while I’ve had a primary LG 27UD59P 27" 4K60 monitor and a secondary LG 22" 1080p TV/monitor. I recently bought a Cooler Master Tempest GP27U (which is a 4K 144/160 Freesync-compatible monitor) to replace the 1080p (and swap around to relegate the LG 27" to be secondary).

The LG 27" + LG 22" setup worked perfectly. The GP27U is a complete nightmare. I wish I’d done more research on it and its various firmware bugs, but the main issue is that it’s slow at detecting a signal. For some reason, this slowness is amplified significantly when running through the L1Techs KVM [Note - any references here are relating to the L1Techs KVM - I am not using any of the KVM/USB features of the GP27U itself].

This manifests as three main issues (pretty much showstoppers):

  1. When the PC is put to sleep, the monitor detects ‘no signal’ for a few seconds, and then starts a timer to sleep after 15 seconds, after which it sleeps with a flashing red power light (all good). When the PC later wakes, the GP27U stays asleep with its power light remaining red and completely refuses to wake up.
  2. When switching resolutions or refresh rates manually in Windows 11, the GP27U takes so long to display an image that the Windows confirmation prompt times-out and subsequently reverts. A second attempt at switching will sometimes succeed.
  3. When rebooting, the GP27U sleeps during the entire boot process, making it impossible to use UEFI BIOS or boot alternate OS. Sometimes the image appears once reaching the Windows lock screen (sometimes not).

The secondary LG 27UD59P 4K60 works fine during all of this.

When the monitor is stuck in this sleep state, switching KVM inputs doesn’t bring it back, nor does power cycling the KVM. The monitor must be fully power cycled (i.e. showing the Cooler Master logo) which takes forever.

My current setup is:

  • Windows 11 24H2
  • Sapphire PURE Radeon RX 7800 XT
  • 7800XT HDMI 1 to KVM HDMI Input 1 Bottom via ultra/8K-certified copper HDMI cable
  • 7800XT HDMI 2 to KVM HDMI Input 1 Top
  • GP27U to KVM HDMI Output Bottom via RUIPRO 3m HDMI 2.1/8K fibre cable
  • LG 4K60 to KVM HDMI Output Top

Things I’ve tried so far, denoted by failures :x: and successes :white_check_mark: :

:x: Different combination of cables and inputs/outputs

I have the following cables available to me:

  • 1.5 metre HDMI Ultra/8K cable that was supplied with the monitor and claims to be “ultra”-certified (it looks reasonable quality with thick cable, and ferrite beads both ends).
  • Several “Cable Matters” HDMI 2.1/Ultra/48Gbps certified 2 metre copper cables.
  • 2no RUIPRO 8K-GEN3/C 3 metre HDMI 2.1/48Gbps fibre cables.

I’ve tried all possible combinations of these cables, swapping out the PC to KVM link and the KVM to PC link separately for each test. Behaviour is the same across all combinations.

All GPU outputs, HDMI 1 and HDMI 2 on the monitor, behave equally.

:x: EDID copy modes
I have tried all three EDID copy modes on the KVM to clone the GP27U using the button next to the input I’m using. No change.

Edit 2024-12-30: see note below for semi-successful test

:x: Disabling/Enabling VRR
Disabled in the monitor as well as enabled in the monitor but enabled/disabled in Windows. No change.

:x: Setting 4K at 60Hz/120Hz/144Hz
Wake issues occur with Windows set to any refresh rate before going to sleep - i.e. waking to a basic 60Hz non-VRR non-HDR signal does not fix this.

I’m reasonably confident it’s not a bandwidth issue because I don’t have any issue running at the max 4K 160Hz with the certified cables (even through the KVM). Whereas I can reliably run into bandwidth/stability issues by putting a cheap AmazonBasics HDMI 2.0 cable and this would show intermittent blanking at 120Hz+ and screen glitches/scrambling in VRR.

:white_check_mark:Cloning the EDID of the LG 4K60 to the input used by the GP27U (partial)
This isn’t ideal since the LG is HDMI 2.0 60Hz, non-VRR, non-HDR, but doing the full EDID clone of the LG (long press with fast-yellow blinking) on the input used for the GP27U, seems to have the GP27U waking up 100% reliably - i.e. in Windows I see 2no 27UD59P (pretty cool feature BTW).

It also switches between say 30Hz and 60Hz refresh quickly, rather than the extended delays as before.

This seems to point to some quirk of handshaking HDMI 2.1 signals right?

Is there any way to feed custom EDIDs to the KVM, i.e. over USB or RS232? I don’t really care about >60Hz, but it would be nice to be able to use it. I might try one of those dummy adapters for headless machines with buggy UEFI that refuse to boot without a monitor - I have one that I bought years ago, but it’s only 1080p.

:white_check_mark:Direct GPU-to-Monitor, without L1Techs KVM
The only thing that works and allows the monitor to work correctly with all capabilities functional is to bypass the KVM. This is across all inputs on the monitor (both HDMI inputs as native HDMI 2.1 links, or the DisplayPort input).

What could the GPU be doing to successfully wake the monitor that the KVM isn’t?

:white_check_mark:Dell DisplayPort to HDMI 2.0 4K60 dongle (partial)
If I plug the HDMI cable for the PC-to-KVM link into a Dell 492-BBXU DisplayPort to HDMI 2.0 adapter, so the KVM is still seeing HDMI input but the PC is outputting to “DisplayPort”.

This is limited to 4K60 since the dongle only has a HDMI 2.0 output. It also has all the DisplayPort niggles I wanted to avoid by going HDMI, like temporary window rearrangement, but at least the monitor does appear to be waking fairly reliably, other than occasionally waking at 1920x1080@60 max, but I assume that’s a quirk of the dongle.

:question:Cable Matters DP 1.4 to HDMI 2.1/8K dongle
I’m waiting on Amazon to deliver this to test.

Other observations
I noticed in HWiNFO64 that under DPMS Modes, there is a difference between the GP27U and the LG 4K60:

  • Standby: Not Supported on GP27U, Supported on LG
  • Suspend: Not Supported on GP27U, Supported on LG
  • Active Off: Supported on GP27U, Supported on LG

I think any differences here are a likely a red-herring though.

I have found an acceptable workaround.

My conclusion is the Cooler Master GP27U is not happy running through the KVM at HDMI 2.1 rates, but is fine at HDMI 2.0. Based on how many people have issues with this monitor even in simple setups, I imagine the issue is with the HDMI implementation in the monitor.

I have dumped the EDID from the monitor, and then using Custom Resolution Utility (CRU) as an EDID editor, edited the HDMI 2.0/2.1 extension block to set “Maximum FRL rate” as “not supported” (the original value was 48Gbps 12x4 lanes) which should be forcing fallback to use TDMS (leaving TDMS character rate at the original value 1200 Mcsc). Prior to setting “not supported”, I tried reducing the max FRL rate as low as 18Gbps, with the same failures. I didn’t bother trying 9Gbps.

I then wrote the custom EDID to a “FUERAN”-branded HDMI 2.1 48Gbps headless/dummy plug from Amazon:

Then I plugged the FUERAN dummy plug into the HDMI Output on the KVM, and used it to store the custom EDID in the KVM (long,long press until rapid yellow blinking) and then plugged the monitor back in its place.

The monitor now wakes and switches resolution reliably. Sure, it’s limited to 4K60, but HDR still works, and there’s no way my GPU could game at 4K@60 let alone higher, high refresh rate desktop just wastes power, and I can still do up to 1440@120, so it’s an acceptable compromise.

Edit: DRM for Prime Video still seems to be working. Also VRR in games, although I had to manually add the Freesync Range to the extension block in the custom EDID since it wasn’t present in the dump :man_shrugging:.