Minor ETB and ETM bugfixes and doc updates

- ETB
    * report _actual_ hardware status, not just expected status
    * add a missing diagnostic on a potential ETB setup error
    * prefix any diagnostics with "ETB"
 - ETM
    * make "etm status" show ETM hardware status too, instead of
      just traceport status (which previously was fake, sigh)
 - Docs
    * flesh out "etm tracemode" docs a bit
    * clarify "etm status" ... previously it was traceport status
    * explain "etm trigger_percent" as a *traceport* option

ETM+ETB tracing still isn't behaving, but now I can see that part of 
the reason is that the ETB turns itself off almost immediately after
being enabled, and before collecting any data.


git-svn-id: svn://svn.berlios.de/openocd/trunk@2790 b42882b7-edfa-0310-969c-e2dbd0fdcd60

doc/openocd.texi

@@ -4891,7 +4891,7 @@ Displays information about the current target's ETM.
 @end deffn
 
 @deffn Command {etm status}
-Displays status of the current target's ETM:
+Displays status of the current target's ETM and trace port driver:
 is the ETM idle, or is it collecting data?
 Did trace data overflow?
 Was it triggered?
@@ -4904,19 +4904,43 @@ When the configuration changes, tracing is stopped
 and any buffered trace data is invalidated.
 
 @itemize
-@item @var{type} ... one of
+@item @var{type} ... describing how data accesses are traced,
+when they pass any ViewData filtering that that was set up.
+The value is one of
 @option{none} (save nothing),
 @option{data} (save data),
 @option{address} (save addresses),
 @option{all} (save data and addresses)
 @item @var{context_id_bits} ... 0, 8, 16, or 32
 @item @var{cycle_accurate} ...  @option{enable} or @option{disable}
-@item @var{branch_output} ...  @option{enable} or @option{disable}
+cycle-accurate instruction tracing.
+Before ETMv3, enabling this causes much extra data to be recorded.
+@item @var{branch_output} ...  @option{enable} or @option{disable}.
+Disable this unless you need to try reconstructing the instruction
+trace stream without an image of the code.
 @end itemize
 @end deffn
 
-@deffn Command {etm trigger_percent} percent
-@emph{Buggy and effectively a NOP ... @var{percent} from 2..100}
+@deffn Command {etm trigger_percent} [percent]
+This displays, or optionally changes, the trace port driver's
+behavior after the ETM's configured @emph{trigger} event fires.
+It controls how much more trace data is saved after the (single)
+trace trigger becomes active.
+
+@itemize
+@item The default corresponds to @emph{trace around} usage,
+recording 50 percent data before the event and the rest
+afterwards.
+@item The minimum value of @var{percent} is 2 percent,
+recording almost exclusively data before the trigger.
+Such extreme @emph{trace before} usage can help figure out
+what caused that event to happen.
+@item The maximum value of @var{percent} is 100 percent,
+recording data almost exclusively after the event.
+This extreme @emph{trace after} usage might help sort out
+how the event caused trouble.
+@end itemize
+@c REVISIT allow "break" too -- enter debug mode.
 @end deffn
 
 @subsection ETM Trace Operation