Debugging

This page describes the process of obtaining diagnostic information from SeaBIOS and for reporting problems.

Diagnostic information

SeaBIOS has the ability to output diagnostic messages. This is implemented in the code via calls to the "dprintf()" C function.

On QEMU these messages are written to a special debug port. One can view these messages by adding '-chardev stdio,id=seabios -device isa-debugcon,iobase=0x402,chardev=seabios' to the QEMU command line. Once this is done, one should see status messages on the console.

On coreboot these messages are generally written to the "cbmem" console (CONFIG_DEBUG_COREBOOT). If SeaBIOS launches a Linux operating system, one can obtain the cbmem tool from the coreboot repository and run "cbmem -c" to view the SeaBIOS diagnostic messages.

Additionally, if a serial port is available, one may compile SeaBIOS to send the diagnostic messages to the serial port. See the SeaBIOS CONFIG_DEBUG_SERIAL option.

Trouble reporting

If you are experiencing problems with SeaBIOS, it's useful to increase the debugging level. This is done by running "make menuconfig" and setting CONFIG_DEBUG_LEVEL to a higher value. A debug level of 8 will show a lot of diagnostic information without flooding the serial port (levels above 8 will frequently cause too much data).

To report an issue, please collect the serial boot log with SeaBIOS set to a debug level of 8 and forward the full log along with a description of the problem to the SeaBIOS mailing list.

Timing debug messages

The SeaBIOS repository has a tool (scripts/readserial.py) that can timestamp each diagnostic message produced. The timestamps can provide some additional information on how long internal processes take. It also provides a simple profiling mechanism.

The tool can be used on coreboot builds that have diagnostic messages sent to a serial port. Make sure SeaBIOS is configured with CONFIG_DEBUG_SERIAL and run the following on the host receiving serial output:

/path/to/seabios/scripts/readserial.py /dev/ttyS0 115200

Update the above command with the appropriate serial device and baud rate.

The tool can also timestamp the messages from the QEMU debug port. To use with QEMU run the following:

mkfifo qemudebugpipe
qemu -chardev pipe,path=qemudebugpipe,id=seabios -device isa-debugcon,iobase=0x402,chardev=seabios ...

And then in another session:

/path/to/seabios/scripts/readserial.py -nf qemudebugpipe

The mkfifo command only needs to be run once to create the pipe file.

When readserial.py is running, it shows a timestamp with millisecond precision of the amount of time since the start of the log. If one presses the "enter" key in the readserial.py session it will add a blank line to the screen and also reset the time back to zero. The readserial.py program also keeps a log of all output in files that look like "seriallog-YYYYMMDD_HHMMSS.log".

Debugging with gdb on QEMU

One can use gdb with QEMU to debug system images. To do this, add '-s -S' to the QEMU command line. For example:

qemu -bios out/bios.bin -fda myfdimage.img -s -S

Then, in another session, run gdb with either out/rom16.o (to debug bios 16bit code) or out/rom.o (to debug bios 32bit code). For example:

gdb out/rom16.o

Once in gdb, use the command target remote localhost:1234 to have gdb connect to QEMU. See the QEMU documentation for more information on using gdb and QEMU in this mode.

When debugging 16bit code it is necessary to load the 16bit symbols twice in order for gdb to properly handle break points. To do this, run the following command objcopy --adjust-vma 0xf0000 out/rom16.o rom16offset.o and then run the following in gdb:

set architecture i8086
add-symbol-file rom16offset.o 0

To debug a VGA BIOS image, run gdb out/vgarom.o, create a vgaromoffset.o file with offset 0xc0000, add use the gdb command add-symbol-file out/vgaromoffset.o 0 to load the 16bit VGA BIOS symbols twice.

If debugging the 32bit SeaBIOS initialization code with gdb, note that SeaBIOS does self relocation by default. This relocation will alter the location of initialization code symbols. Disable CONFIG_RELOCATE_INIT to prevent SeaBIOS from doing this.