I've got a board based on the Vortex86DX CPU which is an x86 based microcontroller. It has five 16550A UARTS, four (COM1-COM4) of which are at the usual io port addresses (0x3f8, 0x2f8, 0x3e8, 0x2e8) but the fifth (COM9) is at 0x010, irq 9.
The first four UARTS are detected, I have no problem with them. The problem is that I can't get the fifth one (COM9) to work in Linux. COM9 works in a stand alone DOS application, but I can't even detect it from Linux.
What I've done so far:
added my extra UART to /drivers/tty/serial/8250/
enabled DEBUG_AUTOCONF in /drivers/tty/serial/8250/8250.c so I can see when and which addresses are being probed.
editted /arch/x86/kernel/setup.c because io port 0x010 overlapped an existing region reserved for dma1
Result: the kernel failed to probe the UART at address 0x010
So I wrote an io port probing utility to directly read from io port 0x010, and I could not detect any 16550A registers in the 0x010 to 0x017 region. It appears there is nothing there. I have been trying to write and read back data from the scratch register (offset +7), but I get nothing at address 0x017.
The datasheet for the CPU doesn't say much. It indicates there is a control bit in a Southbridge register that does "something" for COM9. I wrote a PCI utility that wrote to this bit, but it didn't seem to enable the COM port or change anything when I did the io port probing.
Any help would be appreciated at this point!