On Thu, Dec 12, 2019 at 10:38:29PM +0000, Ard Biesheuvel wrote: > > > > Patch 5 is to make sure that we don't wind up with a >4G framebuffer on > > > > a 32-bit kernel that can only address 4G. I'm not sure if this can > > > > practically happen on anything that we can run a 32-bit kernel on, but > > > > UEFI specs the framebuffer physical address to be 64-bit even on 32-bit > > > > systems, so I figured we might as well cover this edge case. > > > > > > > > > > This cannot happen. The 32-bit kernel can only run on 32-bit firmware, > > > which cannot create mappings above 4 GB, so in that case, fb_base is > > > guaranteed to be 32-bit addressable. > > > > > Is that still true with PAE? > > Yes > > > i.e. is it conceivable that some system > > uses the 36-bit address space to map PCI memory to above 4G physical > > addresses, or can PAE only be used with actual RAM? > > > > PAE can be used with anything, but UEFI won't map anything in places > where it cannot access it itself. (UEFI mandates a 1:1 mapping) > At least on my system, the framebuffer isn't mapped by UEFI though -- it's not part of the EFI memory map.
