On Fri, 2013-11-29 at 12:44 -0500, Kevin O'Connor wrote:
Do you need debug output from 16bit mode or 32bit segmented mode? The post and boot phases are all 32bit code so typical boot time debugging shouldn't be impacted. Gerd's cbmem debugging code uses this approach.
I don't need debug output. But an INT 10h implementation like sgabios¹ would be really useful on Quark, because there's normally no real VGA output (unless you can connect mini-PCI one, which is what I've done so far).
However, I note that sgabios already talks about having support for SMM traps to talk to an EFI console, so perhaps that's the way forward. The SMM side can be provided either by UEFI in the CONFIG_CSM case, or by SeaBIOS itself when it's native. You've already been looking at something very similar to this anyway, right?
I can live with that. Perhaps I should make it work in 16-bit mode but *only* if the appropriate BAR has been put in a memory hole below 1MiB.
That should be okay, but would it ever actually be mapped below 1Meg? Where would it be mapped to: 0xa0000-0xc0000?
Hm, I suspect we *could* set up the A and B segments to map to MMIO space and then set the BAR for the appropriate UART to be there. But an sgabios implementation is going to need *memory* in the A segment, and play tricks to spot when the application/OS has written there and output the appropriate changes to the serial port. So I'm not sure it's stunningly useful to do so.