Re: Remote Serial Protocol -- reply to '?' when target running

Gareth McMullin
2016-08-09T19:15:31Z

On Wed, Aug 10, 2016 at 5:33 AM, Michael Eager <eager@eagerm.com> wrote:
>
> When gdb connects to a remote target, it asks about
> features, and then sends a '?' packet, asking why the
> target is stopped. When the target connection is through
> a JTAG pod, the pod may be able to respond to the feature
> request, but not be able to determine what the current state of
> the target is, likely because it is not stopped. What should a
> JTAG pod reply in this case?
>
> The GDB Remote Protocol doc doesn't seem to be entirely consistent.
>
> The description for '?' is
> Indicate the reason the target halted. The reply is the
> same as for step and continue.
>
> The description for Stop Reply Packet says the following:
> The ‘C’, ‘c’, ‘S’, ‘s’, ‘vCont’, ‘vAttach’, ‘vRun’, ‘vStopped’,
> and ‘?’ packets can receive any of the below as a reply. Except
> for ‘?’ and ‘vStopped’, that reply is only returned when the
> target halts.
>
> This suggests that the target can reply to '?' even if it is not
> stopped, which seems to contradict the description for '?'. But
> none of the listed replies seem to be appropriate. None of the
> replies look like "Don't bother me, I'm busy".
>
> How should the JTAG pod respond? What do most do in this situation?

I can't comment on others, but the Black Magic debug project that I maintain [1]
implements the extended-remote prototcol, and initially responds to
'?' with 'W00' indicating
the target exited (there is no attached target). The user initiates a
scan for connected
targets with 'monitor jtag_scan' from GDB, and then uses the GDB
'attach' command
to connect, sending the 'vAttach' packet. This interrupts the target
and the reply comes
back as 'T05'.

[1] https://github.com/blacksphere/blackmagic