maybe irrelevant, but I have just noticed this line in the dmesg (kernel 4.16, running on C3600) "runtime IRQ mapping not provided by arch" drivers/pci/setup-irq.c: pci_dbg(dev, "runtime IRQ mapping not provided by arch\n"); It's not exactly clear to me, googling I have found this https://patchwork.kernel.org/patch/9775659 what do you think?
