mpc8xx-2.2.13 kernel hangs during boot.

Alan Mimms alan at
Tue Feb 1 14:42:58 EST 2000

On Tue, 01 Feb 2000, Brendan John Simon wrote:
> Alan Mimms wrote:
> > > > We use a serial connection to debug our applications with GDB all the time
> > > > using stubs that are part of our "OS".  It certainly is possible.  We put the
> > > > devices on the end of a 24 port Livingston Portmaster terminal server and then
> > > > setup the ports as raw data ports to which we connect with GDB running on our
> > > > main build/debug box.  That way the user can debug something in another room or
> > > > across the planet very easily through the network.
> > >
> > > Cool.  A little overkill never hurt anybody :)
> > > I'll assume one build/debug box connected directly to the target via a serial cable
> > > or ethernet for now.
> >
> > I see what you mean, but I still think it's a necessity.  It's not overkill if
> > it means I don't have to wander across the hall every 5 minutes to dink with the
> > platform or sit in a room full of LOUD fans to work and debug.
> I was being a bit sarchy.  It is not overkill.  Infact I will probably setup some kind of
> server for compile and debug too so some other engineers can work on linux from there
> MS-Win machines (poor buggers).  I have cross-compiled Insight to work with the cygwin
> environment so that might be an option with gdbserver.  I may even compile the entire
> tool chain for cygwin and see if I can compile the kernel and apps from the MS-Win
> machines (I'm just asking for trouble aren't I).  If it works I will ask Dan to put the
> tools on the ftp server.
> > > > I debugged my initial port of the linux kernel to our platform using just this
> > > > kind of interface.  You have to commandeer the serial hardware away from the
> > > > driver - I simply didn't tell the kernel there was a device there and
> > > > configured it "manually" from the GDB nub initialization function which I
> > > > called early in main.c.  This works fine for KERNEL debugging.
> > >
> > > When you say configured manually what exactly do you mean.
> > > Is this in the kernel itself ?
> > > When you say main.c do you mean init/main.c in the linux kernel source tree ?
> > > Has this got anything to do with the kgdb option in the "make config" process ?
> > > Can ethernet be used to debug the kernel or is a serial port the only or best way to
> > > go ?
> >
> > I DO mean init/main.c.  I mean "manually" in the sense that the low level
> > driver I used in my gdb nub has an "init" call made from init/main.c to
> > setup the UART registers properly (baud rate, etc.).  The nub UART driver simply
> > polls the UART and does simple one byte read/write operations to it.  The nub
> > has the necessary (simple) hex/unhex conversion routines for the GDB serial
> > protocol.  Doesn't use anything else so you can theoretically debug the
> > kernel's version of printf with breakpoints and everything without screwing up
> > the debugger by debugging code it depends upon.
> What is "nub" ?
> Have you have written the code to interpret the gdb serial protocol or are you using code
> from gdb itself ?
> Are your suggestions taken from the gdb howto or documentation ?

The "nub" is the thing that interprets the GDB serial protocol (which is dead
simple BTW).  It gets called on any PowerPC exception and knows how to interact
with the context of the exception, letting GDB grab data from it and from other
machine state (RAM contents).  GDB can also change the state of registers from
the exception and say "GO", or whatever.  If you look in the GDB sources, there
are three or four of these implemented for many processors.  I looked at a few
of them then wrote my own for PowerPC.  The hard part was the wrapper in
assembly language to gobble up the exception state and store it in the GDB
prescribed register status block format.  That's pretty much what is being done
in Linux already when it calls the code in traps.c.

> Thanks,
> Brendan Simon.
Alan Mimms     Packet Engines, Inc.     Spokane, Washington [99214-0497]
  USA, Earth, Sol, Milky Way, The Local Group, Virgo Supercluster, U0
Despite the cost of living, have you noticed how popular it remains?
  -- Steven Wright?

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