You can build the make the kgdb8250 driver as a kernel module
"8250_kgdb.ko" or as a built-in. Either way you must pass the
parameter kgdb8250=<args described
below> to configure the I/O driver. Alternately if
you used the kgdb8250 kgdb I/O driver as a built-in you can
configure it at run time by using the echo command to send the
configuration into the sysfs parameters. Note that when using
sysfs, you omit the "kgdb8250=" For example:
kgdb8250=ttyS<n>,<baud rate>
You can also return kgdb back to the unregistered state by using:
echo "" > /sys/module/8250_kgdb/paramaters/kgdb8250
When loading the 8250_kgdb.ko module or using the kgdb8250 driver as a built in, the configuration takes one of two forms.
Full serial address specification:
kgdb8250=<io|mmio|mbase>,<address>[/<regshift>],<baud rate>,<irq>
The values io or mmio
refer to if the address being passed next needs to be memory
mapped (mmio) or not. The mbase (short for
membase) parameter is only used if mapbase is set to zero for uart
definition, which is the case for some ppc boards. The
address must be passed in hex and is the
hardware address and will be remapped if passed as
mmio. An optional
regshift value can be given to express
address spreading of the 8250
registers. regshift just as the succeeding
baud rate and irq values
are base-10. The supported values for baud
rate are 9600,
19200, 38400,
57600, and 115200.
To specify the values of the serial port at boot:
kgdb8250=io,3f8,115200,3
On certain non x86 archs it might look something like:
kgdb8250=mmio,0xff5e0000,115200,74
Simple configuration:
kgdb8250=<tty_name>,<baud rate>
The simple configuration can be used so long as you are not trying to perform "early" debugging on a platform that supports early debugging. Early debugging allows you to debug the kernel prior to console_init() and the tty driver registration. In the case of the simple configuration the address, port type and irq information can be obtained dynamically by the kgdb I/O driver after the tty's are registered. Here is an example of the simple configuration parameter for ttyS0:
kgdb8250=ttyS0,115200
If you do use the full serial address configuration and the kgdbwait parameter to boot your kernel, on some architectures it may be able to stop before console output has started in which it might look like the target machine is hung when in reality it is just waiting for the debugger.
Finally, it should be noted that you should not try to multiplex a standard serial console and use the kgdb8250 driver at the same time if you have a single serial port IE: "console=ttyS0,11520 kgdb8250=ttyS0,115200". You should use the kgdboc and not the kgdb8250 driver if this is what you want to do.