Z790 Dark Hero - GPU only using 8x PCI lanes

[mrosenbu]

New 14900k build here.  Coming from a 12900k + Maximus VIII mobo + 4 x m.2 drives + 4090.

New build boots fine.  M.2 drives are installed on the mobo in M2_2 through M2_5 slots (leaving M2_1 free for 16x lanes for the 4090).  No other peripherals are installed (though I do have 2 SATA cables plugged in, albeit with no drives plugged in - option value and all).

the 4090 is showing in GPU-z as running at 8x instead of 16x (the old build was indeed running at 16x).

Are any of the other M2 slots using the GPU lanes?  To be clear, M2_1 is free and open (that the slot right above the first GPU PCI-E slot, which is secured via a separate heatsink / cover - I never removed that).

any thoughts would be helpful - thank you.

[achugh]

Hi @mrosenbu first of all congratulation on your new build. I have the same board as you (see my signature for my build details).

Do you have anything put into the 2nd PCIEX16 slot even if you have not connected any wire? The moment any PINS in this slot are pushed, the board detects that this slot is in use and will switch to X8/X8 mode where both the PCIEX16 slots will now run at X8 speeds.

The 16 PCIe GEN5 lanes are divided as follows (see PAGE XV of your motherboard manual for details).

Since you have nothing attached to M.2_1 the only reason will be if you put anything in your PCIEX16(G5)_2 slot. You can also see https://rog-forum.asus.com/t5/intel-700-600-series/gpu-stuck-in-x8-no-nvme-drive-no-pcie-devices-plu... recent post with very similar question about a different board.

I hope this helps you resolve your issue.

[mrosenbu]

Thanks for the reply Achugh!  Nothing is in the 2nd PCIE slot either - haven’t even touched it.  I made sure to read page 15 of the manual to ensure I wasn’t doing anything that would interfere with getting all 16 lanes assigned to the 4090.  Also, I moved this card from another rig and it indeed registers as 16x in that one.

[achugh]

What is the exact make and model of this RTX 4090 card that you moved into your new build?

Also when you go into BIOS and connect a USB stick, pressing F12 will save a screenshot to the USB drive which you can post here. Keep in mind that the screenshot will be saved as BMP file which you can double click to open in Windows using the default Photos app and then press CTLR+S to SAVE AS JPEG file. These JPEG files can then be easily posted as I am posting mine below.

Can you please share the following screenshots from your BIOS?

OnBoard Device ConfigurationPCH Configuration\PCI Express ConfigurationSA Configuration\PCI Express Configuration

[mrosenbu]

Hi Achugh - thanks for offering to assist (it's REALLY appreciated).  Here are the screenshots you requested.  I see the PCIe bandwidth bifurcation configuration setting was set to AUTO (as you'll see in my screenshots below), which as I understand it is the correct value it should be set to to enable 16x for the 4090.  I am curious about what I saw in the PCI Express Configuration screen - how both the M2_1 and M2_2 NVME drive slots are listed.  Makes me wonder if the M.2 drive in M2_2 is indeed the one using the other PCI lanes (reminder that I do not have an M2 drive in M2_1 as these screenshots demonstrate).  Thanks man and let me know what you think.

P.S. - I forgot to say that I'm running an MSI Suprim X AIO cooled 4090

[achugh]

Hi @mrosenbu ,

Thank you for sharing this information. The BIOS by default shows all the devices. This is why you see M.2_1. If you look at your screenshot again, you will notice that in GREY TEXT it does say Not Present and then underneath is a dropdown set to AUTO. This mean when you install something in future, the BIOS will try to auto detect it. This is how to read this information and your setup is correct here.

If you look at Intel's i9-14900K CPU, you will find that this CPU support 16 lanes of PCIe GEN 5 which could be split into TWO 8 lanes of PCIe GEN 5 to make a total of 16 PCIe GEN 5 lanes. On top of these 16 lanes there are 4 lanes of PCIe GEN 4 available as well. Refer https://www.intel.com/content/www/us/en/products/docs/processors/core/core-14th-gen-desktop-brief.ht...

The M.2_2 is fully dedicated to using these 4 PCIe lanes running at a maximum of GEN 4. See below.

It is for this reason in the BIOS under ADVANCED > System Agent (SA) Configuration > PCI Express Configuration menu when you click on the drop down you will NOT see it support Gen5 as a speed.

The 16 GEN 5 lanes are shared between the TWO PCIEX long slots where you install GFX card or expansion cards and the M.2_1 slot with M.2_1 slot having the preference if you have something installed in the 2nd long PCI slot.

Can I suggest you try the following?

• Change the PCIe Bifurcation configuration back to AUTO the way it is in your screenshot above.
  • This is because my understanding is that only AUTO gives you full 16 lanes. The other two options in the dropdown are only helping you split the 16 lanes to be divided into X8 and X8 where you decide where the other half will go i.e. M.2_1 (referred as storage like I have done since I have a drive in my M.2_1) or to the PCIEX16(G5)_2 slot incase you are trying to use two cards.
• Under BIOS under ADVANCED > System Agent (SA) Configuration > PCI Express Configuration where you see PCIEX16(G5)_1 LINK SPEED dropdown is set to GEN4, change this value to GEN5.
• Now press F10 to SAVE and EXIT
• As soon as your system boots, press DEL to enter back into BIOS and when you go back to BIOS under ADVANCED > System Agent (SA) Configuration > PCI Express Configuration screen, do you see X16 in GREY TEXT for your PCIEX16(G5)_1 where you see X8 shown in the screenshot above?

Finally, I will say that I would not worry too much about these lanes. This is because there are no PCI GEN 5 cards out yet. 

Do you have HWiNFO installed? It is a FREE software that gives you hardware information. In my case when I click on my GFX card, I see 

Here as you can see, even though I am sending only X8 GEN5 lanes, the card is getting full X16 speed at GEN 4. This is because 1 GEN5 lane is equal to 2 GEN 4 lanes. So when I select GEN 5 for my PEG slot and send X8 lanes the card still gets the maximum it can support.

NOTE: I am using an ASUS GFX card (see my signature) so my card is able to negotiate things better with this board. I have seen here that using a non ASUS GFX card causes issues where people have to go into BOOT Configuration and ENABLE CSM (Compatibility Support Module) just to boot properly. You have not provided what card you are using so it is possible that you may need to CLEAR CMOS in case you make a change and the system does not boot.

[mrosenbu]

Ok - I changed my PCIEX16(G5)_1 LINK SPEED value to Gen5, saved, rebooted, went back into the BIOS and it's still at x8.  I also included a screencap of HWInfo, which also confirms that I'm at @ 8x 4.0.  

I thought you might have seen my edit - I'm running an MSI Suprim Liquid X 4090 (have had it since March of 2023).

Isn't it odd that the M.2_2 drive is listed on this screen though it supposedly has nothing to do with the link speed of the PCIEX16 slot?  I just might take out that M2 drive to see if that's the culprit.

Any other thoughts?  Again, my sincere thanks for you trying to help here.  I'll be sure to pay it forward when the opportunity arises.

[mrosenbu]

Also, this section of the Dark Hero manual just makes me believe that M2_2 is sharing PCI lanes from the GPU.  Why would M2_2 be listed under the Intel Core Processors section if it wasn't using lanes also used by the GPU?

[achugh]

Hi @mrosenbu M.2_2 is not sharing lanes with GPU. The reason it is listed under CPU is because it is getting the 4 PCIe GEN 4 lanes directly from the CPU. This is why it is on the TIER 1 speed page in BIOS.

Look at the 3rd bullet point in your screenshot. It is telling you what is being shared i.e. M.2_1 is sharing with the 2 full length PCI 16 slots (in terms of the length i.e. the PEG slot and the 2nd long PCI slot for expansion cards).

[achugh]

Hi @mrosenbu ,

The reason M.2_2 is shown on this screen is because these are the devices that are connected to CPU so to me it all makes sense that it is here. Rest of the PCI Express Devices are connected to the Z790 Chipset and hence they are shown on a different screen. Finally, the traditional SATA devices are shown under PCH Storage as a 3rd screen so as not to confuse legacy devices. This 3 screen approach at least to me makes it clear the 3 tiers of storage available. The highest tier is this screen connected to CPU, the next tier are the other M.2 devices as they are also running on high speed PCI Express lanes although via Z790. Finally the slowest Tier 3 are the SATA devices. I hope this helps you understand how the ASUS development team has segregated the information into various screens and why.

Next, thanks for sharing the HWiNFO screen. From this screen, the VIDEO BUS SPEED is EXACTLY the same as mine.

Video Bus: PCIe v4.0 x16 (16.0 GT/s) @ x8 (2.5 GT/s)

Actually, I never paid much attention till your post and as I did some research it seems something is not correct for both you and me. According to Wikipedia page at https://en.wikipedia.org/wiki/PCI_Express we should both see the following

Video Bus: PCIe v4.0 x16 (16.0 GT/s) @ x8 (32.0 GT/s)

Also, this old page showed up in my search results about this issue https://www.hwinfo.com/forum/threads/pcie-v3-0-x16-8-0-gb-s-x16-2-5-gb-s.2393/ 

So at this time it seems both you and I need some help from others. I will so some research on my end. Thank you for helping me spot this issue.