Re: Debugger / Emulator / Logic Analyzer for EBSA285 clone board

[Russell King - ARM Linux Admin]

Nicolas Pitre writes:
> The best strategy is still probably to have a solid knowledge of what
> should be going on and carefully placed printk's to a serial port.

Indeed yes.  Over the 4 complete machine architecture ports that I have
personnally done myself (A5000, EBSA110, RiscPC, EBSA285), I have not
had to resort to any other equipment other than a separate machine to
log stuff that can be printed out from the serial port.

My general reasoning is this:  A serial port is a very simple thing to
talk to.  You can talk properly to a serial port in just 6 instructions,
and therefore it is very easy to prove that code correct.

Once you've done that, you can build other routines to print hex numbers
etc around that, which can be run independent of the target system to
ensure correctness.  You then place the two together, and you have a way
of getting binary information out of the target system, as well as strings.

As a result, with some strategically placed "debugging points" to trigger
messages, you can quickly get a good idea where something is not working
correctly during bootup of the Linux kernel.  One thing that I have commonly
done when faced with a kernel which won't boot is to place a series of these
throughout start_kernel(), and then progressed down through the code until
I find the bit of code which is hanging the kernel.

Of course, once the kernel has booted up and you have a functional printk,
you can use the normal printk() instead of the above routines to display
your debug messages.

Obviously, there are other ways to debug a kernel, but something like kdb,
remote gdb or a logic analyser, I don't think would be as good as the above
method for a kernel which locks solid on boot.

However, I would agree with anyone who says that the above isn't particularly
a "good way" of debugging.

I hope this gives a few useful pointers.
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