Still confused about M2 PCIe Lane allocations

[wildman]

I have tried to understand how this works in order to establish if the x16 PCIe for GPU gets reduced to x8 but I am still massively confused as there doesn't seem to be empirical confirmation and all I have really seen is a lot of anecdotal info based primarilly on peoples interpretation of information they have seen.

So I am looking at getting this motherboard

Asus ROG Strix X670E-F Gaming WIFI (Socket AM5) DDR5 ATX Motherboard

Order Asus ROG Strix X670E-F Gaming WIFI (Socket AM5) DDR5 ATX Motherboard now online and benefit from fast delivery.

www.overclockers.co.uk

pairing it with a 7950X3D, but I want toi be able to also run for M2 drives on the strix 2 of them are PCIe5.0 and 2 are 4.0

I downloaded the Asus manual for the strix here https://rog.asus.com/motherboards/rog-strix/rog-strix-x670e-f-gaming-wifi-model/helpdesk_manual/ and read it and I don't see any mention or reference of what happens to the PCIe lanes if you use 4 x M2 drives and run an PCIe 5.0 GPU

looking at the specs

Strix PCie 5.0 x16_1 runs off chipset and the M2_1 and M2_2 looks like it runs off the CPU

Ryzen 7950CPU supports up to 24 PCIe 5.0 lanes.

So my assumption is as follows:

GPU x16
M2_1 x4
M2_2 x4

total = 24 PCIe 5.0 lanes = all used and no downgrade to x8 on x16_1

the other 2 are PCIe 4.0 so I aslo assume that these will have no impact on the 16_1 PCIe 5.0 slot but would effectively mean if I put something into the x16_2 slot it would run at x4 as that slot is from the chipset and the m2_3 and m2_4 slots are also off the chipset and X670E AMD chipset only supports 12 PCIe 4.0 lanes.

Did a bit more reading and found this

The AMD X670E chipset​

The X670E is AMD’s most powerful AM5 chipset for Ryzen 7000 series processors. Under the hood, it’s a chipset made of not one but two smaller chips that communicate between themselves via four PCI Express 4.0 lanes. By design, the X670E chipset supports 24 direct processor PCI Express 5.0 lanes:

• 16 lanes go to the graphics slot(s) - you can use one graphics card in x16 mode or two in x8 mode.
• 4 lanes are used for a fast NVMe solid-state drive.
• 4 lanes are general purpose lanes (GPP), meaning they are free and can be used independently as the motherboard manufacturer sees fit.

Besides the direct processor PCIe 5.0 lanes, the X670E also offers 12 PCIe 4.0 lanes and 8 PCIe 3.0 lanes. Manufacturers can use the PCIe 4.0 lanes to implement other fast components on their motherboards, like Ethernet, Wi-Fi, or additional NVMe (SSD) slots. PCIe 3.0 ports can be used for lower-speed components such as SATA ports (hard disk drives).


Another key aspect of the X670E chipset is that it also supports a maximum of two SuperSpeed USB ports at 20 Gbps and twelve SuperSpeed USB 10 Gbps ports.

source:- https://www.digitalcitizen.life/x670e-x670-b650e-b650-chipsets/

so having basically theorcrafted all this out based on the specs and some additional reading is my conclusion below correct ?

I can get an Asus Strix X670E AMD motherboard and run 4 x M2 drives and a GPU and they will all have the correct number of lanes allocated to them with none of them being downgraded to either x4 or x8 as the lane allocation for 5.0 and 4.0 from chipset and CPU seems to be handled correctly in that motherboad to allow for this kind of scenario.

Flipside being if I try and introduce any other PCIe 5.0 dependant piece of hardware I am going to take a hit either on the GPU or one of the M2 PCie5.0 drives. PCIe 4.0 would still have x4 lanes left so potentially scope for additional cards up to x4 lanes of PCIe 4.0

sorry for long post but I am really trying to get my head around this properly before I start pulling the trigger on any purchases

 

[Tetras]

All AM5 CPUs in theory can support 2x M.2 drives (up to PCI-E 5.0 speed) with zero impact on the PCI-E graphics lanes. I think a lot of the confusion comes from Intel boards, because 12th-14th gen CPUs do NOT support any PCI-E 5.0 M.2 drives without stealing them from the graphics lanes.

That said, not all boards make those 8 lanes from the CPU available, so there are still AM5 boards that share lanes between the M.2 slots and PCI-E graphics. If the board has more than 2x PCI-E 5.0 M.2 slots, it will also have to share lanes because the CPU doesn't have that many lanes to spare.

With the X670E-F it has 1x 16 lane PCI-E 5.0 slot for the graphics and uses the full 8 lanes available from the CPU for the M.2, so there is no need to steal PCI-E 5.0 lanes from anywhere.

The X670E-E is different, if you use all 3x PCI-E 5.0 M.2 slots, you WILL lose 8 lanes, because it is trying to offer more PCI-E 5.0 lanes than are available with AM5 CPUs.