In a move sure to elicit plenty of criticism, GIGABYTE have begun to roll out new motherboard BIOS revisions for AMD 300-series and 400-series motherboards which explicitly disable PCI-Express 4.0 operation. The BIOSes are part of an update implementing AGESA 18.104.22.168 ABB, a much-discussed AMD firmware revision which improves Destiny 2 compatibility with 3rd Generation Ryzen CPUs (among other fixes).
Prior to Zen 2's launch we reported AMD's intention to disable PCIe 4.0 operation when Ryzen 3000-series CPUs were paired with older 400-series and 300-series motherboards. At the time they stated:
[..] Users today may find a PCIe® 4.0 option available in their pre-X570 motherboards. However, users should expect this option to be disabled when final retail BIOSes are released to implement full performance and stability for new 3rd Gen Ryzen™ processors. As pre-X570 motherboards were not designed with PCIe® 4.0 in mind, their designs may be incapable of running PCIe® 4.0 signaling with the requisite stability and performance. To ensure a reliable and consistent experience in the field, PCIe® 4.0 will not be an option ultimately available to pre-X570 motherboards. Users may continue with a beta BIOS if they desire, but performance and stability cannot be guaranteed.
It's important to realise that, if the AGESA is responsible for this aspect of the platform's operation, GIGABYTE haven't been given a choice. Similarly, to ensure a consistent experience that emphasises stability, the onus is on AMD to disable PCIe 4.0 across legacy motherboard hardware if they have even a whiff of a problem. While there have been no reported issues with PCIe 4.0 operation in tandem with supported peripheral hardware, the population size of such systems is probably terribly small and as yet are unlikely to tax the specification's full potential.
Other motherboard manufacturers such as ASRock, ASUS and MSI are all likely to follow suit in the near future as AGESA 22.214.171.124 ABB is rolled out across the Ryzen ecosystem. It may be tempting to jump ship for another manufacturer in protest at GIGABYTE's actions, but all hands will be tied by the same enforcement of standards.
Any PR damage here is self-inflicted due to poor messaging in the lead-up and following 3rd Gen. Ryzen's launch. Motherboard manufacturers hedged despite AMD making their position clear, while AMD themselves were hit by media 'confirmation' of support long before the wraps were taken off the platform and confusion surrounding platform backwards compatibility. Whatever the reasons, it's the Ryzen brand as a whole which will pay the penalty in the medium term, as well as those who invested in 400-series motherboards and other PCIe 4.0 hardware just prior to the 3000-series launch.