Power requirements and PCIe slots

Question

I’m putting an idea out to see if anyone has some data to back a theory.

I’ve been talking with @Spanner about some problems I was having with a card, and, after looking over everything I’ve done, I’m wondering if a 75W slot is *required* rather than recommended, which would mean you need to use a PCIe x16 slot.

One issue I had with the card was a small v shaped cut-out in the full height PCIe back-plate, which meant that the card wouldn’t sit fully into any PCIe x16 slot that I had (photo attached so you can see what I mean). This meant I had to use a 8x or 4x slot.

Looking through the PCIe spec, only 16x slots are supplied with 75W, meaning that the 8x and 4x slots I tried were limited to 25W (which I confirmed), which I’m thinking could be the cause of the problems I saw.

I have a 2nd hand machine turning up next week which I can test this theory on more easily (eBay still is useful for some things), but I’m wondering whether the PCIe card should be sized for a 16x slot in order to ensure the 75W capability, or if there should be an optional 6 pin power connector which would allow a PSU to supply 75W, and allow the card to fit into a 4x or larger slot.

Anyone have thoughts on this?

alsutton · Answer

I’ve been through three machines and here’s the summary;

Dell T5810; Recognised the card in the 16x slot, but it was unreliably recognised (i.e. couldn’t guarantee it’d be there on every boot)

Lenovo 720s: Failed to recognised the card in the 16x slot. The physical fit was extremely tight (i.e. 1mm longer and it wouldn’t have fitted, 1mm shorter and there’d be no problem).

Supermicro X10DRU-i+: Recognised there was a card, but wouldn’t interact with it (lcpci -tv entry existed, but was blank)

During a successful boot on the T5810 I managed to get the firmware on the card upgraded to the latest before moving to the different machines, and all of the machines were booting with `intel_iommu=off pcie_aspm=off` in the kernel command line.

I’m going to call it a day here, hopefully the next SDK release will make things a little more consistent for my use cases.

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