[clug-talk] serial console help

lewkor@telus.net lewkor at telus.net
Sun Sep 25 23:33:51 PDT 2005

Quoting Gustin Johnson <gustin at echostar.ca>: 
> I am running a qemu session with -nographic which means that there is no  
> graphic output.  Control of the VM occurs across a virtual serial connection 
> (/dev/ttyS0 to the guest, and either stdio or a pty on the host).  The  
> problem is when I run vim somefile I get some garbled characters and the  
> following sprayed across much of the screen: 
> serial8250: too much work for irq4 
> I have not had much experience with serial consoles (except as a user of the 
> UofC's DevlNET, which I kinda miss), so any help would be appreciated.   
> The output of setserial: 
> setserial -g /dev/ttyS* 
> /dev/ttyS0, UART: 16450, Port: 0x03f8, IRQ: 4 
> I have also played with various rate (aka speed) settings from 9600 to 
> 115200 
You may have to go down even lower baud rates to get this working.  As you can 
see qemu thinks the chip is a 16450.  That chip had no fifo, so on receiving 
the data has to come slow enough that the CPU can get the data out of the 
buffer before the next byte otherwise the chip will raise an overrun flag 
indicating it could not keep up to the data stream.  The first thing that I'd 
try is 2400 baud (see comment below). 
> I have not pleased the google gods with the correct prayer incantation. 
> I have also been to: 
> http://www.faqs.org/docs/Linux-HOWTO/Serial-HOWTO.html 
> The entry I have in /etc/inittab is 
> S0:123456:respawn:/sbin/mgetty -x1 -b -r -s 38400 ttyS0 
Try 2400 baud here.  If that works then work up to 3600, 4800, 9600, 19200.  I 
don't think that you will get to 19200 (just a gut feeling). 
> and in /boot/grub/menu.lst 
> # Serial console setup 
> serial --unit=0 --speed=38400 
> # now setup terminal as both serial line (/dev/ttyS0) and regular monitor 
> # console (/dev/tty0) depending on where you press a key 
> terminal --timeout=15 serial console 
> and the kernel option: 
> console=ttyS0 
> Of course when using ssh to connect everything works as expected. 
> Id should also be noted that regular console use appears to work normally  
> (copying, moving, running ifconfig, ip route etc). 
> Thanks in advance. 

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