How to use mini-PCIe Serial port

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David Hendricks

Jul 2, 2010, 6:33:58 PM7/2/10
Some folks doing bring-up work are using mini-PCIe cards to get serial connectivity. This is useful for situations where you might not have networking or VGA available and during *very* early debugging.

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.

Nick Sanders

Aug 18, 2010, 3:46:05 PM8/18/10
to David Hendricks,
You can edit the kernel commandline in the build at

Or, if you are running a legacy bios, you can edit sdb12:/syslinux/usb.A.cfg
on a prebuilt image.

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David Hendricks

Jun 2, 2011, 11:27:45 PM6/2/11
to Chromium OS dev
For those who do a lot of serial debugging, I updated and improved these instructions to include needed Linux configuration changes and some other ideas for how to find alternate IO and MMIO addresses.

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