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PE6 doesn’t work as x8 CPU lanes?

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ZoranC
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2021/06/18 17:16:06 (permalink)
I have been trying to setup X299 Dark with NVMe storage card in PE6 slot and U.2 drive on PU2. CPU is i9-10900X.
 
It is my understanding this slot should get x8 CPU lanes in such setup and that I should be able to bifurcate it. However when I try to do that BIOS sees only one drive as if PE6 is getting only 4 lanes. If I move video card that is x16 when in PE1 to PE6 it too ends up with only 4 lanes even though it should end up with 8.
 
Also, if I benchmark NVMe drive in PE2, PE3, and PE4, all of which use CPU lanes, I get values that are consistent with each other. If I benchmark it in PE5 I get value that is 10-ish % lower when compared to PE2-4, which makes sense because PE5 uses PCH lanes. However, when I benchmark it in PE6 values do not match ones from PE2-4, they match ones from PE5, indicating PE6 is getting PCH, not CPU, lanes.
 
Which is weird because xDevs chart of PCIe mappings indicates only time PE6 ends up with 4 PCH lanes is when one is using older CPUs with insufficient number of CPU lanes, which I am not.
 
So was anyone here able to use PE6 and get 8 CPU lanes?
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    a213m
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    Re: PE6 doesn’t work as x8 CPU lanes? 2021/06/18 18:55:35 (permalink)
    You'll have to install an 16x card to get the PE6 slot to use 8x CPU lanes. Since your NVME card uses 4x lanes, the board borrows them from the PCH. Try shorting PRSNT#1 (first top pin) to PRSNT#2 (bottom 2nd pin from the end) on the motherboard to force the slot to use CPU lanes and cover PRSNT#2 on your NVME card with a capton tape.
     
    Manual, pp. 20 and 27:
     
    PE6,the only x8/x4 slot on the board,receives either x8 lanes from the CPU (Page 26) or x4 from the PCH, depending upon the CPU used and peripheral configuration.
     
    PE6 –x16(Gen3, x8 lanes from CPU or x4 from PCH based on configuration)
    When populated by a video card x8 Gen3 lanes are allocated from the CPU
    When populated by a card using x4 or x1 lanes, and M.2 80mm slot is populated PE6 receives x4 lanes from the PCH

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    ZoranC
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    Re: PE6 doesn’t work as x8 CPU lanes? 2021/06/18 19:01:29 (permalink)
    a213m
    You'll have to install an 16x card to get the PE6 slot to use 8x CPU lanes. Since your NVME card uses 4x lanes, the board borrows them from the PCH.



    As mentioned I -DID- install x16 card (my video card) in PE6 and still I was getting only 4 lanes.
     
    Also one of storage cards I tried is Asus Hyper M.2 x16 Gen2 populated with 4 drives and I would see only one drive, further confirming I am getting only 4 lanes.
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    DEJ915
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    Re: PE6 doesn’t work as x8 CPU lanes? 2021/06/18 21:23:22 (permalink)
    PM1, PM2, PU1 and PU2 steal lanes from the last slot, I'd play around with those and see if you can get it to run x8 again.  I ran through the options before on mine to figure it out because the manual is useless but I've forgotten exactly which combinations does it.
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    ZoranC
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    Re: PE6 doesn’t work as x8 CPU lanes? 2021/06/19 09:34:47 (permalink)
    DEJ915
    PM1, PM2, PU1 and PU2 steal lanes from the last slot, I'd play around with those and see if you can get it to run x8 again.  I ran through the options before on mine to figure it out because the manual is useless but I've forgotten exactly which combinations does it.



    PM1, PM2 and PU1 are empty. I have drive on PU2 but both manual and xDevs imply having drive on PU2 shouldn't affect PE6 if I have CPU that has enough lanes, which I do. None the less I will try again today without drive on PU2. In the meantime I have opened ticket with EVGA.
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    ZoranC
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    Re: PE6 doesn’t work as x8 CPU lanes? 2021/06/20 10:14:50 (permalink)
    I think I’ve figured out what is going on but not why it is going on.
     
    If PU2 is enabled it will take away 4 CPU lanes from PE6. Will remaining 4 PE6 lanes be PCH or CPU depends on how PM2 is configured. If PM2 is configured to use CPU lanes those remaining four PE6 lanes will be PCH ones and, vice versa, if PM2 is configured to use PCH lanes those remaining four PE6 lanes will be CPU ones.
     
    Otherwise PM2’s and PU2’s settings can lead to one of these three layouts:
     
    1. PE6 has only 4 lanes total and they are PCH
    2. PE6 is using CPU lanes but it still has only 4 lanes total
    3. PE6 is using 8 lanes but 4 of them are CPU and other 4 are PCH. In such layout storage card with two drives in bifurcated PE6 results in up to 15% performance difference between drives even though they are in same slot.
     
    So that answers what is going on but not why because my interpretation of PCIe mappings is that this shouldn’t be happening. Otherwise if that is behaving as intended I am able to count only 40 CPU lanes that can be used through slots/ports at the same time which makes me wonder where did remaining 4 go?
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    DEJ915
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    Re: PE6 doesn’t work as x8 CPU lanes? 2021/06/20 10:21:22 (permalink)
    I think the last lanes go to the PM1/PU1 slot if I remember correctly since it just switches between them and you can't disable it which leaves the 5 8x slots with 40 for the 44 total.
    post edited by DEJ915 - 2021/06/20 10:22:29
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    ZoranC
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    Re: PE6 doesn’t work as x8 CPU lanes? 2021/06/20 10:37:35 (permalink)
    DEJ915
    I think the last lanes go to the PM1/PU1 slot if I remember correctly since it just switches between them and you can't disable it which leaves the 5 8x slots with 40 for the 44 total.



    PE1/2 pair shares 16 CPU
    PE3/4 pair shares 16 CPU
    PE5 is 4 PCH so that doesn't count, correct?
    PE6 is 8 CPU if PU2 is disabled and PM2 uses PCH. That brings count to 40 which leaves only PM1/PU1 as possible remaining 4 CPU lanes, like you pointed out. Which would explain it all. Thank you!
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