PCIe 2 x16 in PCIe 3 x8?

[WeekendWarrior]

Is there a reasonable way (not a big kludge) to put a PCIe 2 x16 card into a PCIe 3 x8 slot while still getting the full bandwidth of the x16 connector? Given that PCIe 3 has approximately twice the bandwidth of PCIe 2, this seems technically plausible. But the need for this may be very limited or it may only work through a lot of MacGuyver activity.

Any advice would be appreciated --
Dave

 

[Fritz]

I too would like to know.

 

[canta]

technically NO!

since the spec is different
well, it start duirng initialization to know how many lanes and pcie version, and start to do negotiation after that
pcie 3.x supports 2.X( down level)

the only way is using pcie 2.X <=> pcie 3.X bridge (hardware) that would be do as a middle person. I do not see this product at all since this is a waste on money ..

 

[pricklypunter]

Not happening. As soon as enumeration begins, the interface will report PCI-e ver 2.x and the controller will drop into running that version across all lanes it is responsible for, not only those that your card slot is tapping. In order to use PCI-e 3.0, everything connected to the controller must be ver 3.0 compliant. As canta said, you would need a bridge device of some sort, which kind of defeats the purpose of having PCI-e ver 3.0 to begin with

 

[WeekendWarrior]

@canta - agreed that a bridge or some sort of magic is needed and that PCIe 3 is backwards compatible with PCIe 2. But that doesn't mean that it is a waste of money.

A use case where this question applies is if someone has three PCIe 2.0 x16 cards but single-processor systems support (at most) 40 PCIe 3 lanes. Therefore we can't plug all 3 cards into any single-processor system without some magic. But the aggregate bandwidth of the 40 PCIe 3.0 lanes is more than enough to support 48 PCIe 2.0 lanes.

I've heard that some motherboards extend PCIe slots using a 'switch' - which might be fine for this application because not all PCIe 2.0 x16 cards would be communicating at all times - but I don't know much about such switches.

 

[WeekendWarrior]

Not happening. As soon as enumeration begins, the interface will report PCI-e ver 2.x and the controller will drop into running that version across all lanes it is responsible for, not only those that your card slot is tapping. In order to use PCI-e 3.0, everything connected to the controller must be ver 3.0 compliant. As canta said, you would need a bridge device of some sort, which kind of defeats the purpose of having PCI-e ver 3.0 to begin with

Thanks for your more detailed response - do you see a path to addressing the use case I describe below? Thanks.

 

[cesmith9999]

Yes it is a waste of money.

For the cost of the bridge card, you would not be too far off just upgrading the card to a PCIe 3.0 model.

PLX makes PCIe bridge chips (seen on a lot of upper end enthusiast MB). They typically add ~100 to the cost of the MB.

In the long run, it would cost you less and cause less pain to upgrade to better MB/cards than to try and shoehorn old technology into new technology.

Chris

 

[pricklypunter]

Honestly, no. I agree that, technically speaking, it's feasible, but would basically involve hacking your own controller together and then trying to overcome the mechanical and electrical issues of introducing that as a bolt on back to your main board. I think if that functionality were to be included in the design spec to begin with, it would probably limit exploring other advanced features that are more useful in the field, hence no main boards available that natively support it. That's not to say that a smaller manufacturer wouldn't produce such a board for a specific niche market, but you would have to go some way to convince them that it was worth their efforts. If it were simple and straightforward to achieve, I think the far east factories would be churning out adapters by the thousands

 

[WeekendWarrior]

Thanks all for your comments. Unfortunately the specific card I want to use (several of) only comes in PCIe 2 although other manufacturers make similar cards at 2-3x the price (due to a steep discount on the PCIe 2 card). Thus I would want to make the PCIe 2 card(s) work even if doing so cost several hundred dollars more.

A somewhat different approach to solving this problem is the PLX chips @cesmith9999 mentioned above. @Patrick discussed this subject some time ago (not surprising that he's ahead on that).

Avago and PLX - Future of PCIe?

Moreover, a motherboard with one or two PLX chips can be had for far less than going with the PCIe 3 cards:
ASRock X79 Extreme11, first LSI SAS2308 onboard SAS/SATA Preview

So that may be the solution.

 

[canta]

Thanks all for your comments. Unfortunately the specific card I want to use (several of) only comes in PCIe 2 although other manufacturers make similar cards at 2-3x the price (due to a steep discount on the PCIe 2 card). Thus I would want to make the PCIe 2 card(s) work even if doing so cost several hundred dollars more.

A somewhat different approach to solving this problem is the PLX chips @cesmith9999 mentioned above. @Patrick discussed this subject some time ago (not surprising that he's ahead on that).

Avago and PLX - Future of PCIe?

Moreover, a motherboard with one or two PLX chips can be had for far less than going with the PCIe 3 cards:
ASRock X79 Extreme11, first LSI SAS2308 onboard SAS/SATA Preview

So that may be the solution.

that using pcie switch(multiplexer)... basically mux-demux to extend from limited pcie lanteto more lanes

the issue is timing and some card would not work as expected,
I know HBA card SAS would not work since need dedicate B/W and lanes.


this is the diagram:


I know they have pcie to pci bridge by using pcie 1 lane.

how about buying motherboard that has pcie3 16x lane that can stepdown to pcie2 16x. or pcie3 16x to another pcie3X 16x. Mostly I know from 16 lane to 8 lanes or 8 lanes to 4 lanes, NOT 16 lanes to 16 lanes
I think this is a simple solution for current motherboard is buy 16X of pcie3 lanes and install your card

I has one AMD borad with plx switch/multiplexer, and does not work for card that need dedicated B/W or data consuming.
it works well on Intel NIC cards and usb3.0 card.

 

[cesmith9999]

Thanks all for your comments. Unfortunately the specific card I want to use (several of) only comes in PCIe 2 although other manufacturers make similar cards at 2-3x the price (due to a steep discount on the PCIe 2 card). Thus I would want to make the PCIe 2 card(s) work even if doing so cost several hundred dollars more.

What cards are you trying to use? what is special about them?

Chris

 

[WeekendWarrior]

I was looking at a 6-port 10G SFP+ card: Six Port Fiber 10 Gigabit Ethernet PCI Express Server Adapter Intel® 82599ES based: Silicom Connectivity Solutions

 

[cesmith9999]

If those are the cards you are looking at. Most likely there is already a PCIe bridge chip on that card. It looks like that card is basically 3 * (2 * 10 GB) Intel cards rolled up into one.

So you are thinking of adding another bridge to a card that has a bridge chip on it. doing that would add even more latency to your PCIe bus.

I do not have that card specifically. so I cannot talk definitively, but that is how most NICs that have more than 2 ports are built in the past.

Chris

 

[canta]

If those are the cards you are looking at. Most likely there is already a PCIe bridge chip on that card. It looks like that card is basically 3 * (2 * 10 GB) Intel cards rolled up into one.

So you are thinking of adding another bridge to a card that has a bridge chip on it. doing that would add even more latency to your PCIe bus.

I do not have that card specifically. so I cannot talk definitively, but that is how most NICs that have more than 2 ports are built in the past.

Chris

totally true,

I se pe210g6sp19 card, use 3X 82599ES (dual port 10g ethernet).
the plx multiplexer/switch(bridge) must be on on the board
this provice 16X lanes of pcie 2.x to 3 Intel chipsets (dual ports)...
i would assume 16x to 3 of 8x lanes

 

[cesmith9999]

the next question is what are you doing that would require that much bandwidth (3 * (6 * 10 GB ports))?

Chris

 

[Diavuno]

He's going to man in the middle the internet, all of it.

 

[WeekendWarrior]

the next question is what are you doing that would require that much bandwidth (3 * (6 * 10 GB ports))?

Chris

My use case was a DIY 10G switch. But MITM might be interesting also.

 

[pricklypunter]

NSA telecommuter