I'm using several PCIe 3.0 extension cards (GPUs and Infiniband interconnects). I'm wondering how lanes are actually managed and if I may optimize my devices by changing ports or by using some adpaters (16x -> 8x). Intel Haswell-EP may manage 40 lanes PCIe 3.0. On Intel's schematics, the PCIe 3.0 controller seems to be split in two x16 and one x8 sub-bridges.
Are all devices connected to a main PCIe bridge (and quantity of lanes automatically négociatiated for each device), or do the motherboard connect the devices directly to one of the supposedly 3 sub-bridges 16x, 16x and 8x (quantity of lane are then negociated for each of those sub-bridges)?
I do not have a direct access to the motherbard to see how devices are connected, but I suspect that the lanes of the supposedly 8x sub-bridge are not utilized. Also, I would like to know if by using a 16x to 8x adapter, I could harness more lanes and increase my total PCIe bandwidth (eventough the maximum theoretical bandwidth would be divided by two for that device).