A Linux Fix Is On The Way For Some GPUs Having AMD Smart Access Memory Issue
The quirk is about allowing BAR0 resizing to be done even for PCI Express graphics cards that don't advertise Resizable BAR support for a size large enough to span the entire video memory size but at least larger than the default.
Initially set as a quirk for this handling is the Sapphire Radeon RX 5600 XT Pulse (0x731f device ID) so now the CPU can fully access the BAR. Presumably with time we will see more graphics cards added to this quirk list to workaround problems with other AIB cards while ultimately this should also be addressable by updated video card BIOS too. It's likely other older GPUs are also affected given the AMD Smart Access Memory / Resizable BAR hasn't been a focus until now with the latest Radeon RX 6000 series hardware.
More details on this Linux quirk via this patch series that had been under review for several weeks while now appears ready for mainline. This goes along with other recent work around Linux optimizations for AMD S.A.M. and also restricting by default to Zen 3 and RDNA 2 due to bugs like this Navi 10 issue.