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The NVIDIA Bluefield-3 spec sheet lists the following as its PCIe spec:

32 lanes of PCIe Gen 5.0
PCIe switch bi-furcation of up to 16 downstream ports

Would this mean the device can run on anything in between PCIe 5 x2 port and PCIe 5 x32 port?

I thought PCIe switch bi-furcation meant splitting a port into multiple ports. For example, bifurcating x16 slot would result in two x8 slots.

What does it mean that a PCIe device supports "bifurcation"?

lol
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  • I very much doubt there’s such a thing as PCIe 5 x32. Other than that, it may be correct. – Daniel B Jan 26 '23 at 19:54
  • x32 does seem to be a thing on some motherboards: https://www.supermicro.com/en/products/motherboard/x13sew-tf – lol Jan 26 '23 at 20:30
  • Not really though. That’s a riser slot, not something to plug add-in cards into. – Daniel B Jan 26 '23 at 20:58
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    _"Would this mean the device can run on anything in between PCIe 5 x2 port and PCIe 5 x32 port?"_ - almost any PCIe device would. You can plug anything anywhere as long as it physically fits and it should work. Some motherboards come with x16-length slots wired for x4 or x1. Some come with slots that are open at the end, allowing for insertion of cards longer than the slot. Bandwidth of a true x16 card will be limited, but it should be functional. – gronostaj Jan 26 '23 at 22:08

2 Answers2

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PCIe bifurcation is dividing the PCIe slot in smaller chunks/branches. Example : a PCIe x8 card slot could be bifurcated into two x4 chunks. Another example : a PCIe x16 into four x4, or two x8, or one x8 and two x4 etc etc.

PCIe Bifurcation does not decrease speed but only splits/bifurcates lanes. In order to use bifurcation, the motherboard must support it as well as the BIOS. Each lane still works at its own speed.

For your motherboard, it has 32 lanes in all which can be split into at most up to 16 groups of lanes. In other words, the motherboard can support up to 16 devices. This number does not normally include the CPU lanes which come in addition.

For more information, see for example the article PCI Express Bifurcation in the 100G Era.

harrymc
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  • I am specifically interested in what bifurcation would mean in a spec sheet of a device. I have edited the question to make it clear. – lol Jan 26 '23 at 20:46
  • Yes, this is answered above. Your motherboard has 32 lanes in all, which can be split into at most up to 16 groups of lanes. In other words, the motherboard can support up to 16 devices. This number does not normally include the CPU lanes which come in addition. – harrymc Jan 26 '23 at 20:51
  • I'm not sure if it does. I'm asking in the context of a device. I don't have a motherboard. I have a device and its spec sheet (the lines quoted in the question). I want to find out if I should plug it into a pcie 5 x16 slot of the motherboard that I'm looking to buy. – lol Jan 26 '23 at 21:12
  • Looking at your device, I see that it supports up to 16 cores, meaning that each one will use 2 data lanes. You might be able to split the lanes differently than evenly, but answering that requires studying the manual. Another question is of lanes used by other devices than CPUs, which also requires study. – harrymc Jan 26 '23 at 21:20
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The intent of offering many port configuration options .. is to support the PCIe lanes being assigned to multiple Downstream ports by the embedded PCIe Switch. For example if you want to connect many NVMe controllers / SSDs, this is supported. ( But you can't create sixteen Host Ports. ) BF-2 also presented 32 PCIe lanes; but BF-3 can be configured to present twice the number of PCIe ports. ( Up to 16. )
When you asked "If I should plug it into a PCIe 5.0 x16 Slot ..." the answer is pretty much always yes. Since the most common configuration ( of the DPU ) is where those first sixteen PCIe Lanes are presenting the BF-3 ( switch ) Upstream Port.
On the question of "How to configure / assign the other sixteen PCIe Lanes to present one or more Downstream Switch Ports ..." it is probably best to ask NVIDIA - whether that ( switch configuration ) can be set by users, or whether it would need some kind of special firmware version.
They also have the BF-3 DPU User Guide online at NVIDIA BlueField-3 DPU User Guide

LarryC
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