AMD CPU and Platfrom Discussion Episode IV: A New Hope is Ryzen

The Something Awful Forums > Discussion > Serious Hardware/Software Crap > AMD CPU and Platfrom Discussion Episode IV: A New Hope is Ryzen

JSON Bourne: Jun 1, 2004

On the topic of PCIE lanes, is there any AM5 motherboard that offers slots for both x16 and x8? I've been looking but the best I can seem to find is an x16 slot and an x4.

I didn't realize this would be a problem when I got a 7800x3d recently, but the motherboard I purchased only does x16 and then 3 PCIE 3.0 x1 slots. I know my use case is stupid, but I have an x8 Mellanox 25gbps card that wouldn't work at x1 for obvious reasons.

# ¿ Apr 21, 2024 15:41

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# ¿ May 21, 2024 07:53

JSON Bourne: Jun 1, 2004

Thanks! I'll take another look at the X670 boards. I get that most people will want more m2 slots these days. A PCIe4 x4 slot should work bandwidth wise, I'll just have to decide if my stupid use case is worth the gamble and spending more money on.

# ¿ Apr 21, 2024 16:10

JSON Bourne: Jun 1, 2004

Wow, thank you all! Appreciate the extra insight here.

hobbesmaster posted:

Physical or electrical?

There are 28 PCIE 5.0 lanes on AM5 CPUs. 4 lanes go to the chipset leaving 24. AMD�s slides call the other 24 lanes �16 graphics, 8 NVMe�. It�s not clear if that is a suggestion or requirement but as you saw motherboards all seem to be doing that.

The 4 PCIE lanes are downgraded to 4.0 lanes for the chipset. The chipsets can be thought of as simple PCIE switches. B650 motherboards have 1, and X670 have two in series. 4x PCIE 4.0 in principle is equivalent to 8x 3.0 so what you want might actually be possible but with the death of SLI it�s questionable if even asrock would bother�

edit: i apologize for ever doubting asrock, their expensive x670e boards have you covered https://pg.asrock.com/mb/AMD/X670E%20PG%20Lightning/index.asp
edit2 no they don�t, goddamnit why can�t you search by motherboard block diagram or something

Edit3: x670e asrock taiichi https://www.asrock.com/mb/AMD/X670E%20Taichi/index.asp#Specification

You're experiencing what I have been going through so many different spec sheets this week. It's been hard to keep it all straight. Thank you for the extra technical detail here.

Klyith posted:

A true x8 lane slot? Not really. The only thing you're going to get is the high-end X670E mobos with 2 x16 PCIe 5.0 slots, which will split the x16 from the CPU into a pair of x8s.

All of the mobos with "x16" slots that are really only x4 or x1 are that way because they hang off the chipset and the chipset link is x4 gen4.

Will the card not work with reduced lanes though? PCIe is supposed to auto-negotiate lanes and stuff. Even at x4 gen3 you're getting 4 GByte/s, which should cover the 25gbit/s of your card. Unless the card is gen2 because it's old, at which point you'd just have to live with reduced speed or get a more modern card.

I should have been more specific, a true slot isn't really a requirement. What clued me into the PCIe lanes potentially being the problem was a log line indicating "NIC supported width: 8, NIC current width 1". It's a not that old of a card, so it is good to know that negotiation should work. Really good to understand about the chipset being a gen4 x4 link, I think I was missing that distinction before.

Combat Pretzel posted:

Yea, seems like best you'll be able to find is a PCIe 3.0 4x hanging off a chipset. Any additional CPU lanes are reserved for the five hundred M.2 slots each higher end mainboard comes with.

Also, I'm running a Mellanox ConnectX-3 in such a slot just fine.

--edit: X670E TUF from Asus here.

Knowing I'm not the only one trying this makes me feel less crazy. I have a ConnectX-6DX so I would hope it would work like yours did.

# ¿ Apr 21, 2024 16:48

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