On Wed, Mar 25, 2020 at 03:51:36PM +1100, Alexey Kardashevskiy wrote: > >> This is for persistent memory which you can DMA to/from but yet it does > >> not appear in the system as a normal memory and therefore requires > >> special handling anyway (O_DIRECT or DAX, I do not know the exact > >> mechanics). All other devices in the system should just run as usual, > >> i.e. use 1:1 mapping if possible. > > > > On other systems (x86 and arm) pmem as long as it is page backed does > > not require any special handling. This must be some weird way powerpc > > fucked up again, and I suspect you'll have to suffer from it. > > > It does not matter if it is backed by pages or not, the problem may also > appear if we wanted for example p2p PCI via IOMMU (between PHBs) and > MMIO might be mapped way too high in the system address space and make > 1:1 impossible.
How can it be mapped too high for a direct mapping with a 64-bit DMA mask?