Here's a brief explanation of how to get it working for Chromium OS. This is pretty generic and can apply to other distros, though the part about presenting a serial login prompt may differ.
My setup is a generic x86 netbook and an Oxford 950-based mini-PCIe to serial adapter. Here is a snippet from dmesg. Linux sets everything up nicely for this device:
[ 0.676698] Serial: 8250/16550 driver, 4 ports, IRQ sharing enabled
[ 0.678173] serial 0000:04:00.3: PCI INT D -> GSI 18 (level, low) -> IRQ 18
[ 0.678716] 1 ports detected on Oxford PCI Express device
[ 0.678910] ttyS0: detected caps 00000700 should be 00000100
[ 0.679365] 0000:04:00.3: ttyS0 at MMIO 0x50401000 (irq = 18) is a 16C950/954
[ 0.679899] console [ttyS0] enabled, bootconsole disabled
The MMIO base address is important. You can obtain this information a couple of ways, but the easiest is to either look at dmesg or look at /proc/iomem:
localhost ~ # cat /proc/iomem | grep serial
50401000-50401007 : serial
Once you know the MMIO base address, you can add this to your kernel command-line (modify UART settings as needed):
If you want a login prompt to appear, you'll need to add an init service to start one. Here's an example init script, /etc/init/ttyS0:
start on startup
stop on starting halt or starting reboot
exec /sbin/agetty -L -w 115200 ttyS1 linux
Congrats, you have now turned your high-tech mini-PCIe expansion slot into a serial port. Happy hacking!
David Hendricks (dhendrix)
Systems Software Engineer, Google Inc.