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
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.