> From: Bin Meng <bmeng...@gmail.com> > Date: Sat, 1 May 2021 19:45:02 +0800 > > On Fri, Apr 30, 2021 at 7:22 PM Andre Przywara <andre.przyw...@arm.com> wrote: > > > > Hi, > > > > We now see the first Allwinner devices [1] having DRAM located above > > 4GB in address space (4GB DRAM starting at 1GB). After one fix[2] > > this works somewhat fine, but the sun8i-emac network device is still > > limited to 32-bit DMA addresses. With U-Boot relocating itself (plus > > stack and heap) to the end of DRAM, it now runs completely beyond 4GB > > on those machines, so not giving pure 32-bit addresses for buffers > > anymore. > > In Linux we handle this easily by just keeping the default DMA > > mask at 32 bits, and letting the DMA framework deal with the nasty > > details. > > > > I was wondering how this should be handled in U-Boot? The straight > > forward solution would be: > > - Let the driver allocate the RX and TX buffers separately, placing them > > below 4GB in the address space (using lmb_reserve(), I guess?) > > - Use those RX buffers and hand the addresses back to the upper layers. > > - We already copy TX packets, so this would also be covered, in this > > situation. Other drivers might need to introduce copying. > > > > This sounds like a common problem, so I was wondering if there is a > > more generic solution to this? Maybe there are already platforms or > > devices affected? Or should the whole heap and stack be moved below 4GB > > (if this is easily possible)? > > My understanding is that the relocated address of U-Boot should be > below 4GB then there is no problem for the 32-bit DMA. I thought this > is a rule to be followed by every board, but this is not the case on > your board?
Yes, that was my impression as well. And I think that would work fine on this board as there is plenty of DRAM below 4GB. And this can be achieved by implementing the board_get_usable_ram_top() function. As I indicated in my reply, some care is needed in the EFI subsystem, but there already is a solution for that. There is CONFIG_EFI_LOADER_BOUNCE_BUFFER, but that might not actually be needed in this case. By default the EFI subsystem will mark all conventional memory above "ram_top" as EFI_BOOT_SERVICES_DATA. So EFI applications uch as OS loaders will not allocate that memory until they've called ExitBootServices() at which point U-Boot will be completely out of the picture. > > In our case we make the buffers part of our priv struct, so should > > there be an option to let the priv_auto allocation come from below 4GB? > > > > Grateful for any input on this! > > Regards, > Bin >