Will my PCIe configuration work?

[Messju]

Hey, I was always a little confused when it comes to PCIe-lanes and how they work.

I'll quickly give you an example of what I'd like to do.

Hardware used: AMD Ryzen 9 7950X, ProArt X670E-CREATOR

By specification, the Ryzen provides 24 usable PCIe Gen5 lanes and the chipset of the mainboard 12 PCIe Gen4 lanes.

The mainboard provides 4 M.2 slots, two of which are connected with PCIe Gen5 and two with Gen4.

I also have a simple PCIe Gen4x4 Dante card which I'd like to use.

Is it possible for me to populate ALL of those slots at once, regarding that I use 16 lanes for my graphics card already? In theory the lanes provided by processor and mainboard should sum up to a total of 36 lanes.

Adding up all the lanes I'd like to use [16(Graphics)+4(M.2)+4(M.2)+4(M.2)+4(M.2)+4(Dante)] I get 36 lanes as well. Will it work?

[Zando_]

From my digging: 

 

If you do NOT put an SSD in M.2_3, it looks like you should have x4 lanes at PCIEX16_3 for the Dante card. If you have populated it, it'll either have 0 lanes, or you have to set it in BIOS to share x2/x2 between the PCIe slot and the SSD. If you install the card in PCIEX16_2, PCIEX16_1 will drop to x8 lanes, which shouldn't be an issue for most GPUs, so that's likely the better solution if you want all the SSDs. 

 

[Messju]

Hey, wow, thank you for this amazing reply!

 

 

A few quick questions though: Am I going to lose bandwidth if I populate the M.2_3 x2 slot with a PCIe Gen3 SSD? Theoretically the two lanes should have enough throughput for Gen3 speeds but I guess there is no way of converting four lanes of PCIe Gen3 into zwo PCIe Gen4 lanes on an M.2 formfactor, right?

Will the SSD (Samsung 970 evo) work at all with only two lanes assigned?

And where did those four PCIe lanes go that I am missing?

[Zando_]

58 minutes ago, Messju said:

Am I going to lose bandwidth if I populate the M.2_3 x2 slot with a PCIe Gen3 SSD?

Yes. Roooughly half but I don't know if the scaling is perfectly linear.

59 minutes ago, Messju said:

Theoretically the two lanes should have enough throughput for Gen3 speeds but I guess there is no way of converting four lanes of PCIe Gen3 into zwo PCIe Gen4 lanes on an M.2 formfactor, right?

Not that I know of unfortunately.

1 hour ago, Messju said:

Will the SSD (Samsung 970 evo) work at all with only two lanes assigned?

Yep! I ran my 970 Evo in the M.2 slot on my X99 Classified for a bit, that was a 1st gen X99 board with only a PCIe 3.0 x2 M.2 slot, 0 issues other than the bandwidth loss.

1 hour ago, Messju said:

And where did those four PCIe lanes go that I am missing?

Those go to the chipset. CPU has 24 lanes, 16 to main slot(s), 4 for an M.2 drive (usually), 4 for the chipset. All the chipset devices/lanes share that bandwidth back to the CPU.

[Messju]

14 hours ago, Zando_ said:

Those go to the chipset. CPU has 24 lanes, 16 to main slot(s), 4 for an M.2 drive (usually), 4 for the chipset. All the chipset devices/lanes share that bandwidth back to the CPU.

Really? I thought the CPU has 24 usable lanes, 28 in total (see https://www.amd.com/de/product/12151) and thus those four lanes go to the chipset. But I could be mistaken as well.

 

However: Thank you for your great advice and those fast replies! I really appreciate it.

[Zando_]

3 hours ago, Messju said:

Really? I thought the CPU has 24 usable lanes, 28 in total (see https://www.amd.com/de/product/12151) and thus those four lanes go to the chipset. But I could be mistaken as well.

No, you are correct. Has 28 lanes, 24 usable, Anandtech says most are thrown to one of the M.2 slots: https://www.anandtech.com/show/17585/amd-zen-4-ryzen-9-7950x-and-ryzen-5-7600x-review-retaking-the-high-end/3. I'd assume that's the case on this board, it likely has 2 M.2 slots using CPU lanes. Ahh yep, I'm dumb, says so right here:

M.2_1 and M.2_2 are via the CPU.