MBR hack for serial console

[remodeler]

This is in response to John Baldwin's response to my question about using a
non-legacy IO port for an i386 serial console.

I notice that src/sys/boot/i386/boot0/boot0.S uses a call to the PC BIOS int
14h routine to access the serial console, if SIO is defined. I believe SIO is
defined if make.conf specifies a port for BOOT_COMCONSOLE_PORT (though I'm
probably wrong), in which case the boot0 executable would be made from
src/sys/boot/i386/boot0/boot0sio when recompiling the boot blocks with:

# cd /sys/boot
# make clean
# make
# make install

The 14h pc bios routine is definitely limited to 0x3F8, 0x2F8, 0x3E8, or 0x2E8
because it is called with the DX register set to specify COM1-COM4 (dx set to
0, 1, 2, or 3).

boot2 looks like I might be able to specify the 0xe800 address for the serial
console. src/sys/boot/i386/boot2/sio.S has this definition: .set
SIO_PRT,SIOPRT but I am not sure where SIOPRT is set. I don't see if in any
makefile. I would guess it is being set by BOOT_COMCONSOLE_PORT also?

> if the device is behind a PCI-PCI bridge (which it probably is 
> if it is on bus 3) you have the problem that 0x3f8 is probably not
> in the IO range decoded by the parent bridge

The device is behind a bridge: pcib0 --> pci0 --> pcib3 --> pci3, but AMD
doesn't have a data sheet available on the 'net to tell me what IO range is
decoded by the parent bridge (750SB southbridge chip).

I also notice in the PCI specification this ("I/O Space Address Decoding for
Legacy Devices"):

A function that supports a PC legacy function (IDE, VGA, etc.) is allowed to
claim those addresses associated with the specific function when the I/O Space
(see Figure 6-2) [the configuration space's Command Register layout, bit 0]
enable bit is set.

These addresses are not requested using a Base Address register but are
assigned by initialization software. If a device identifies itself as a legacy
function (class code), the initialization software grants the device
permission to claim the I/O legacy addresses by setting the device’s I/O Space
enable bit.

>  read the current value of the BAR instead and use that address?

If I can set the serial console to a non-legacy port in boot2, that would be
ideal. I don't think the boot0 routine would cause problems; it is just
looking for keystrokes AFAIK. In this approach, I would guess it would make
sense to read the value and set SIOPRT to it in boot2? I think from this point
on, the serial console calls use sio.S, and these functions (sio_init, etc.)
access the hardware directly with assembler out commands rather than using the
PC BIOS routines like boot0. Would something as simple as:

.set SIO_PRT,0xe800

in src/sys/boot/i386/boot2/sio.S work? If it does, I can develop code to read
and set it to the current port instead of hard-coding it.

Thanks again for your help.