Update: We spoke with AMD representatives, who confirmed that 300- and 400-series AM4 motherboards can support PCIe 4.0. AMD will not lock the out feature, instead it will be up to motherboard vendors to validate and qualify the faster standard on its motherboards on a case-by-case basis. Motherboard vendors that do support the feature will enable it through BIOS updates, but those updates will come at the discretion of the vendor. As mentioned below, support could be limited to slots based upon board, switch, and mux layouts.
AMD set enthusiasts' hearts alight today when it announced its 7nm Ryzen 3000-series processors. These new processors come bearing the 7nm process, the world's smallest for desktop PCs, which will lead to reduced power consumption and heat, but they also support PCIe 4.0, giving AMD yet another advantage over Intel's lineup.
At its CES keynote, AMD reiterated its guarantee of support for the Socket AM4 motherboards until 2020, so the new Ryzen processors will be backward compatible with the existing motherboards, but with a caveat: AMD says you will lose support for PCIe 4.0 on its older platforms.
But after speaking with several motherboard vendors here at CES 2019, we've learned that many of them have successfully tested PCIe 4.0 on 300- and 400-series AMD motherboards, meaning that the feature could be enabled with a simple BIOS update, at least partially.
Our sources tell us that after unlocking the feature via a BIOS update, the older motherboards supply a PCIe 4.0 x16 connection to the first slot on the motherboard, but the remainder of the slots revert to PCIe 3.0 signaling rates. That's because any trace routing on the motherboard that exceeds six inches requires newer redrivers and retimers that support PCIe 4.0's faster signaling rates. That means the PCIe slot nearest to the CPU will easily support PCIe 4.0, while the other slots, including M.2 ports, will run at a PCIe 3.0 signaling rate.
The 500-Series chipsets will consume more power (~15W) than the 28nm chipsets (~8W) used on current AM4 motherboards, but that's because the 500-series chipsets also support PCIe 4.0. We weren't told the specific lane allocations of the new chipset, but those faster lanes will be useful for numerous types of secondary I/O devices.
We're also told by motherboard vendors that AMD will stop using ASMedia to design its chipsets. But AMD says that it will continue to use ASMedia, but the new chipsets will be "more nuanced." That could mean that AMD will design parts of the chipset and simply license key IP blocks, like USB 3.1, from ASMedia.
Even though multiple board partners have tested PCIe 4.0 on previous-gen chipsets, it remains to be seen if AMD will allow them to expose that functionality via BIOS updates. Our sources tell us that AMD can simply lock out that feature and that the fate of PCIe 4.0 support on 300- and 400-series motherboards haven't been communicated to them yet.
We'll update as we learn more.