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I have PCIe x4 NIC plugged into PCIe x16 slot. I have read, that even though it should run in x4 mode, it's not guaranteed: the network card can run in backward compatibility 1x mode. How to verify this under Linux?

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I got it, it's in the logs:

0000:00:01.0: eth2: (PCI Express:2.5GB/s:Width x1) 00:15:17:96:79:57
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Just curious - what's the difference? Not like you would saturate x1 mode? –  Karolis T. Jun 23 '09 at 12:45
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I thought the same thing - even allowing for adding 100% in overheads a 1Gbit network card would only just saturate the 2Gbit maximum expected throughput (2.5Gbit signal rate) of a single PCIe v1 channel. A 10Gb NIC, or a card with multiple 1Gb interfaces, or some other high-speed standard I'm unaware of, could be a different matter though. –  David Spillett Jun 23 '09 at 12:55
    
x1 width is a bottleneck for two 1GBit interfaces. It's described here: osdir.com/ml/linux.drivers.e1000.devel/2007-07/msg00139.html –  ctinnist Jun 23 '09 at 14:14
    
PS. Yes, the card has 2 interfaces. –  ctinnist Jun 23 '09 at 14:15
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