Lenovo laptop hdmi, Windows 11 no display on dummy plug but ok on external monitor?

[edidcru42]

Hello,

Got a weird situation.

Until now, i've been using fake monitor dummy plugs on vga/dvi/hdmi/displayport with Windows 10/11 successfully on PC/laptops, along with CRU utility.

I've got a Lenovo P1 G7 laptop which has built-in HDMI 2.1 connection. I tried both hdmi 2.0 and 2.1 dummy plugs on it, and Windows 11 does not detect a new display. Perhaps a bad hdmi port on the laptop? I connected up a hdmi external display, and i get new display. Maybe the dummy plugs are bad? I tried them in different laptop/PC combination, and each show the new displays in Windows 11.

What's going on here?

I connected up the port replicator, which has hdmi port on it, to the laptop. Plugged in the same dummy plug, and Windows 11 shows new display.

Seems from isolation that this Lenovo P1 G7 laptop's built-in hdmi, is good, but hdmi dummy plug will not work (for some reason). Well, this is first failure for me in such situation.

Anyone run into compatibility issue on a particular laptop hdmi port with hdmi dummy plug that is known good and works in other latop/PC's hdmi 2.0/2.1 ports?

What could be so unique about the Lenovo P1 G7's hdmi port?


Here the hdmi external display is connected to the laptop's hdmi port (i do have connected to laptop, two dummy DisplayPort's, total of 4 displays, 1, 2, 3 and 4)



Unplugged the external hdmi display, plugged in hdmi dummy plug. Windows 11 does not detect the new display. Windows 11 only has displays 1, 2 and 3.




Thanks!

[ToastyX]

What are the EDIDs on the dummy plugs?

[edidcru42]

(06-24-2025 01:54 AM)ToastyX Wrote:  What are the EDIDs on the dummy plugs?

Here's the screenshot from CRU for the hdmi dummy plug. When plugged into the Lenovo laptop, no display is detected. But when i plug it into some other laptop's hdmi port, new display is detected.




Here's screenshot of external HP Z24i G2 monitor plugged into the same Lenovo laptop's hdmi port, and i get new display from making that connection. But using dummy plug on the same hdmi port, no new display. I tried also copy and paste from HP monitor to the dummy plug, save, reboot. Same result, no display is detected from Lenovo laptop when dummy plug connected to the hdmi port. The graphics card is Intel ARC adapter in laptop.




What's even more strange is, using the same hdmi dummy plug on the Lenovo laptop's docking station which has hdmi port, new display is detected. What........

But if i use the same dummy plug in the hdmi port on the laptop itself, no display. Connect external monitor to same hdmi port, i get display. Urgh...

Thank you.

[ToastyX]

I need the EDID, not a screen shot. Export a file for each dummy plug and post them here.

[edidcru42]

(06-24-2025 03:06 PM)ToastyX Wrote:  I need the EDID, not a screen shot. Export a file for each dummy plug and post them here.

Ok, thanks. Attached is the export of the hdmi dummy plug that is/has been driving me crazy, why it is only not working when plugged into the laptop's built-in hdmi port. Urgh...



Exported file attached here.

Thanks again!

[ToastyX]

It might be a graphics driver bug. The only thing I see that's unusual is the max TMDS clock in the HDMI data block is set to 590 MHz when it shouldn't be more than 340 MHz, but that shouldn't cause it to ignore the dummy plug. None of the resolutions require more than 300 MHz, and it should work even with HDMI 1.4. It also defines 1920x1080 @ 60 Hz as the native resolution while having higher resolutions in the EDID. What resolutions do you need? You might be better off making your own EDID with the resolutions you need and writing it to the dummy plug using EDWriter if it's not write-protected. Maybe fixing the EDID will make a difference. If not, then maybe it's trying to communicate with the dummy plug, but it's not responding like a typical HDMI device would. Also USB-C docks either act as DisplayPort converters, or they use DisplayLink, which acts as its own GPU with its own driver, so that would be handled differently than a direct connection.

[edidcru42]

(06-30-2025 12:24 AM)ToastyX Wrote:  It might be a graphics driver bug. The only thing I see that's unusual is the max TMDS clock in the HDMI data block is set to 590 MHz when it shouldn't be more than 340 MHz, but that shouldn't cause it to ignore the dummy plug. None of the resolutions require more than 300 MHz, and it should work even with HDMI 1.4. It also defines 1920x1080 @ 60 Hz as the native resolution while having higher resolutions in the EDID.

Thanks for the observations!

(06-30-2025 12:24 AM)ToastyX Wrote:  What resolutions do you need? You might be better off making your own EDID with the resolutions you need and writing it to the dummy plug using EDWriter if it's not write-protected.

Only these resolutions, 1920x1080 and 1920x1200. I'll try the EDWriter approach.

(06-30-2025 12:24 AM)ToastyX Wrote:  Maybe fixing the EDID will make a difference.

I see, i need to read up more about what exactly is CRU vs EDWriter. Somehow i thought CRU is to make your own EDID with the resolutions one need, but "does not write it to the dummy plug".

I've successfully used CRU on some DisplayPort dummy plugs that do not show as supporting 1920x1200, but following the post here, i was able to make them work in 1920x1200 resolution. Thanks for that!

https://www.monitortests.com/forum/Threa...0#pid17090

(06-30-2025 12:24 AM)ToastyX Wrote:  If not, then maybe it's trying to communicate with the dummy plug, but it's not responding like a typical HDMI device would.

I hope that isn't the case, but as stated, this would be some "weird" compatibility issue between the Intel ARC Hdmi direct connection in this particular aptop, and/or Intel ARC driver and/or my hdmi 2.0 dummy plug that just happen to expose some sort of incompatibility(s). If this brand of hdmi dummy plug has issue with this laptop hemi direct connection, then i believe my luck would probably be about the same with other (chinese make) dummy plugs.

(06-30-2025 12:24 AM)ToastyX Wrote:  Also USB-C docks either act as DisplayPort converters, or they use DisplayLink, which acts as its own GPU with its own driver, so that would be handled differently than a direct connection.

Ah, you are right, i did recall reading where the DisplayLink was mentioned for this Lenovo USB-C dock for this laptop. Ok, so i shouldn't be comparing the hdmi direct connection from the laptop as apple to apple comparison to hdmi connection at the USB-C dock. Ok.

---

Ah, ok, rats... fineprint on EDWriter...

(Intel is currently not supported)

---

Ok, so if the dummy plug despite showing in the CRU list of displays, but is not showing as "(active)" then there's really nothing that can be done from CRU to make (force) it to be active, other than possibly making customer EDID. Urgh...

[ToastyX]

If the display is not listed as active, then Intel's driver isn't enabling it for some reason. I don't know if CRU would have any effect in that case. You could also try the alternative .exe export method.

I wish Intel was more open to releasing the CUI SDK to the public so I could add Intel support for EDWriter. They have a newer library that is available to the public, but it only supports 12th-gen and newer, so I don't have a way to implement it currently.

If you are familiar with Linux, you can probably write a new EDID using edid-rw: https://github.com/bulletmark/edid-rw

[edidcru42]

(07-01-2025 08:54 PM)edidcru42 Wrote:  Ah, ok, rats... fineprint on EDWriter...

(Intel is currently not supported)

But I do have AMD and Nvidia PCs i can run the EDWriter on against the same hdmi dummy plug. And confirmed from others using the same hdmi dummy plug that it's not write-protected. So will get that tried there and test it in this POS Lenovo laptop.