It is possible to use more than 3.5GB RAM on Linux if you don't have any PCI cards installed in the PCI slots. If you need a PCI card then please install it in a PCIe x1 slot with a PCIe x1 to PCI adapter.
ubuntu MATE 16.04.3 LTS PowerPC with more than 3.5GB RAM on an AmigaOne X5000:Spectre660 wrote: If yo use a pciex1 to pci adapter and an 8139 card in a pciex1 slot networking works without the mem=3500M option with additional ram .
Cheers,
Christian