Re: gdbserver, remote serial protocol and endian issues

[Daniel Jacobowitz <dmj+@andrew.cmu.edu>]

On Mon, Apr 08, 2002 at 04:33:01PM +0100, Paul Bartlett wrote:
> Hi Daniel,
> 
> > I don't agree.  Target registers are in 
> > target-endianness when you read them off 
> > the stack; they should be in target endianness 
> > in memory.  GDB has 'set endian little' and 
> > 'set endian big', and the stub should just pass 
> > them along however it gets them.  gdbserver is 
> > also meant to run in a native configuration, 
> > where compile-time checks can tell you the 
> > endianness.
> 
> Well, maybe I'm guilty of not considering the
> general case - haven't thought it through yet.

As soon as you try looking through stack frames, you realize that
keeping registers in target byte order is a lot simpler for the rest of
GDB.

> Our application connects to the H-UDI (JTAG)
> port of the SH4 for high speed (~20 MBits/s) 
> access through the back door into the chip.
> 
> The physical implementation includes a 32 bit 
> shift register. What we notice is that if we 
> change endianness then we have to endswap the 
> 32 bit quantity that appears in the register 
> before transferring it to the target machine 
> reg. - it never actually gets into memory.

That's a property of the board, right?  It expects this shift register
to be in its own endianness... or is something else going on?

> > My first impression is that gdbserver is the 
> > wrong tool for the job. Gdbserver is meant as 
> > a remote stub for Unix-like systems.  You're 
> > not running it on a Unix-like system; you're 
> > using it as a proxy, right? Since GDB already 
> > has a stub to speak to the hardware monitor 
> > on SH4 boards, as far as I'm aware.  
> > This sounds like a job for rproxy 
> > (http://world.std.com/~qqi) instead.
> 
> Maybe we re-invented the wheel, some. Our
> implementation runs on a box equipped with
> cpu/flash/ram and ethernet and jtag interfaces.
> It does implement the remote protocol in much
> the same way that rproxy does, but also adds
> facilities like posix i/o through the debug
> port.
> 
> Works well. We've used it for debugging both
> 'bare machine' embedded and rtos based apps. 
> (I also brought up a linux kernel with it).

Nice job, although I'm not sure gdbserver is the best base for it; like
I said, it's generally Unix-based.  However, with the other
organizational cleanup I've been adding to it lately, I think there's a
clean place to integrate this sort of thing now.

I don't suppose you're thinking of contributing it?  The POSIX I/O
stuff especially would be nice to have in gdbserver.

-- 
Daniel Jacobowitz                           Carnegie Mellon University
MontaVista Software                         Debian GNU/Linux Developer