Help debugging kernel together with X

[Fernando Herrero Carrón]

Hi list,

I would like to ask for some help debugging the kernel.

Here is the problem:

I have a computer about five years old with an on-board graphics card
(SiS 661). I am trying to install an ATI Radeon 128 on the AGP port.
FreeBSD (FreeBSD 7.2-STABLE #10: Tue May 26 15:08:39 CEST 2009) is able
to start DRM and AGP without trouble. However, whenever I start X (X.Org
X Server 1.6.1; Release Date: 2009-4-14; Build Date: 11 May 2009
12:03:27PM) the system freezes with a blank screen with some apparent
noisy green dots on it (reproducible, however). It does not panic, nor
reboot.

Now I suspect there is some problem on the AGP bridge driver since linux
can run X without trouble (i.e. hardware and BIOS are ok). I have found
someone reporting problems with this bridge long time ago with no
answers (http://www.nabble.com/agp-on-sis-661-td1446998.html).

So here I am trying to figure out where the system freezes. I have
recompiled my kernel adding the following options:

options         KDB
options         DDB
options         MP_WATCHDOG
options         SW_WATCHDOG

I am able to enter the debugger with ctrl+alt+esc from a console. So
what I am trying to do is to have watchdogd time out and the debugger
make a dump:

ddb script kern.enter.watchdog=call doadump; reboot

However, when I start X no timeout occurs, and the system stays freezed.
Am I missing something on the workings of watchdogd?

Thanks a lot for your help!

Fernando