Re: ethercons: ethernet console driver for 5-current

On Mon, 20 Oct 2003, Steve Kargl wrote:

> On Mon, Oct 20, 2003 at 12:13:27PM -0400, Robert Watson wrote:
> > 
> > I had a fair amount of time over the last week running in disconnected
> > operation, and realized I had too many cables under my desk, so I spent a
> > bit of time exploring the FreeBSD console code.  After reading a FREENIX
> > paper this summer on a Linux ethernet console driver, I took a pass at
> > implementing ethernet console support for FreeBSD.
> 
> This looks very interesting!  Can we run ddb over the ethercon to debug
> a wedged machine? 

Not currently.  In the current implementation, the ethernet console picks
up its input, and generates output, using the ethernet layer of the
network stack.  Since the debugger suspends scheduling, this means
interrupt threads, netisrs, etc, aren't running, so for now, ethercons is
disabled when "in the debugger" (db_active != 0).  This permits other
console devices, such as serial console, to be used for the debugger,
however, at the same time. 

To support ethernet debugging, the debugger would need to be able to drive
polling of the network interface in an interrupt-thread-free environment,
and reproduce more of the lower level network code (i.e., not use mbufs,
etc).  This is feasible to do, but would probably require adding new
interfaces to the ethernet driver, and supporting only ethernet cards that
had these additional debugging interfaces.  Compared to serial console,
you'd also have a lot more situations where the driver/hardware state
would be sufficiently inconsistent as to make debugging network-related
crashes difficult.  On the other hand, Darwin runs quite well with a
network debugger; I believe they have a fairly complex UDP/IP
implementation in the network debugger, although I haven't inspected it. 
Apple has the advantage, though, of providing very few ethernet drivers.

So I'm happy to look at it, but the level of time investment to get to
network debugging from the current (and pretty simple) ethercons device
will be fairly high.  I know Jonathan Lemon was looking at network console
and debugging code previously, but I don't have copies of his patches.  If
I had to guess, I'd assume he had modified the if_fxp driver, and perhaps
others, to provide an appropriate polled interface for use with a
debugger, but I don't know for sure.  If someone has copies of these
patches, I'd be happy to take a look at them. 

Robert N M Watson             FreeBSD Core Team, TrustedBSD Projects
robert_at_fledge.watson.org      Network Associates Laboratories