Can the CM4 PCIe support two devices?

[dan_rhodes]

PCIe is new to me and I'm trying to figure out if you can have more than one device on the CM4 PCIe bus. All of the carrier boards I've seen only have one device, so you can't have two GbE along with an M.2 drive or similar.

[Gavinmc42]

There are some PCIe switches on Jeff's pages
https://pipci.jeffgeerling.com/

[fanoush]

You need PCIE switch for that. I tried two different PCIE switches (based on ASM1184e and Pericom PI7C9X2G) and both worked, the 1x PCIE 2.0 bandwidth is shared of course so more PCIE devices communicating concurrently is slower, in reality it worked very well - tried reading from NVME and usb 3.0 SSD and it run full speed with one drive and half speed with reading two drives concurrently - more info here viewtopic.php?p=1827474#p1827474

Currently CM4 cannot boot directly from NVME drive when the drive is behind switch, needs small microsd card for that.

Not sure if anyone designed CM4 carrier board with PCIE switch chip directly on board to provide more slots but it is definitely possible.