PCIe lanes

[Revanth A]

Hai,

I have a network interface card (Optical) and it has PCIe x8 Lanes, I know that this can be inserted to PCIe x16 slot and can work fine. but now consider iam designing a cpu which has PCIe x2 lanes pins and if i route the signals of the CPU to the network interface card that have  (Here routing means that taking the 2 lanes of the CPU to the first 2 lanes of the network interface card) , will it work properly or it need all 8 lanes to work properly? (look in to the image for better understanding? 

 

[FloRolf]

It depends. Theoretically it works but you might be bandwidth limited. A pcie 3.0 x2 slot can only do 2gbit/s 

[Revanth A]

2 minutes ago, FloRolf said:

It depends. Theoretically it works but you might be bandwidth limited. A pcie 3.0 x2 slot can only do 2gbit/s 

@FloRolf Thanks for the comment! Not considering the limitation in bandwidth it should work fine right?

[FloRolf]

1 hour ago, Revanth A said:

@FloRolf Thanks for the comment! Not considering the limitation in bandwidth it should work fine right?

I guess it also depends on the device. For example Nvidia doesn't allow their gpu's to run at anything lower than 8x (iirc). Maybe the NIC has similar limitations. 

 

Just out of curiosity, how are you designing your own cpu? And why? 

[Revanth A]

imx6 is a cpu that has 2 lanes of PCIe, and in FPGA we can create logic for those. Any way I'll chck with my NIC manufacturer.

[GoldenLag]

29 minutes ago, Revanth A said:

imx6 is a cpu that has 2 lanes of PCIe, and in FPGA we can create logic for those. Any way I'll chck with my NIC manufacturer.

wouldnt you want to use at least 1 of those lanes for other IO?

[Revanth A]

2 minutes ago, GoldenLag said:

wouldnt you want to use at least 1 of those lanes for other IO?

I has already one USB, Ethernet and GPIOS , iam using this CPU for specific task/application so that wont be an issue!

[beersykins]

4 hours ago, FloRolf said:

A pcie 3.0 x2 slot can only do 2gbit/s 

I assume you mean 2 GB/s which is 16 gbit

[Alex Atkin UK]

NICs I believe can be a curiosity when it comes to this, especially multi-NIC cards.

 

I believe some connect each port as its own NIC with its own PCIe lanes, so if you have less lanes, ports are missing entirely.  Others reduce the bandwidth as they presumably have some sort of PCIe bridge inbetween.

 

What always puzzled me is why PCIe switch cards aren't more common.  This could be extremely useful for a router or NAS, where you might only need 1Gig to the host PC but want to plug in several devices and let them communicate with each other at Gigabit speeds.  Its basically how many routers work internally.

 

You'd think at least some enterprising appliance manufacturer would do this with their PCB design rather than slapping on five Intel NICs.