Hi Bin,
At present U-Boot supports booting a 64-bit kernel directly. It can also be loaded as a 64-bit payload from EFI. But it cannot be built as a 64-bit boot loader.
I took a bit of a look at this. It looks like we need to stay in 32-bit mode until the FSP is loaded. Also, to get to 64-bit mode I'm pretty sure we need page tables, which means we need somewhere to put them!
Looking at the board_f init sequence, it seems that arch_cpu_init() is the earlist we could switch to 64-bit. We'd need to grab some memory from somewhere to do this - I wonder if this can be CAR? There is no SRAM on x86 chips I think.
For non-FSP devices we don't init the RAM until much later - dram_init(). That means that a significant portion of the init sequence would be 32-bit code. I'm not sure that will work.
I suppose one option is to only go to 64-bit mode when relocating. But then we end up with lots of code that needs to run in 32-bit and 64-bit.
Do you have any ideas on this?
Regards, Simon