More than 4 serial ports under Linux

(Originally posted 24 Oct 09, lost in server mishap, found in Google's cache of this page)

So at work I am using a PCI104 octal serial port board. It's pretty cool that Linux now supports those OOB, but I had problems; I only saw the first two ports!

After doing a bunch of research; I finally found the problem. I had assumed it was something with the chipset itself. However, it is a problem with the default kernel build from RHEL/CentOS. They only allow up to four by default! To get more (up to 32 with the RHEL/CentOS kernel), you have to add to the command line in GRUB:

8250.nr_uarts=12

Again, that can be up to 32. I chose 12 because "traditionally" the mobo could have up to four. That made the two on the mobo ttyS0 and ttyS1, so the octal card has ttyS4 to ttyS11. So ttyS2 and ttyS3 are unused. A quick check with dmesg | grep ttyS will show them being allocated.

Side note: You can check how many the default is by doing grep CONFIG_SERIAL_8250 /boot/config-`uname -r` and looking for CONFIG_SERIAL_8250_RUNTIME_UARTS. CONFIG_SERIAL_8250_NR_UARTS is the maximum you can have without rebuilding the kernel.

Maybe I'll get inspired and blog sometime about the cool stuff I did with udev so that I can configure a box and then the software would just "know" where which serial port the (external device) was on by calling /dev/projectname/extdevice .