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Updates 2.0.4 issue 153 #164

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Updates 2.0.4 issue 153 #164

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Clarified when support packet must be sent
IainCRobertson Oct 20, 2024
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Merge updates-2.0.4 to branch
IainCRobertson Oct 21, 2024
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Merge updates-2.0.4 to branch
IainCRobertson Oct 24, 2024
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Update payload.adoc
IainCRobertson Oct 25, 2024
9925de0
Further clarifications for issuing support packets
IainCRobertson Oct 25, 2024
ef8860f
Further clarifications for issuing support packets
IainCRobertson Oct 25, 2024
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13 changes: 6 additions & 7 deletions payload.adoc
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Expand Up @@ -235,15 +235,14 @@ instruction context.
[[sec:format33]]
=== Format 3 subformat 3 - Support

This packet provides supporting information to aid the decoder. It is
issued when
This packet provides supporting information to aid the decoder. It must be issued:

* Trace is enabled or disabled;
* The operating mode changes;
* One or more trace packets cannot be sent (for example, due
back-pressure from the packet transport infrastructure).
* Before the first sync packet, in order to ensure the decoder is aware of how the encoder is configured. It is recommended this be sent as soon as the encoder is enabled (*trTeEnable* changes from 0 to 1). This reduces the likelihood of having to generate two packets (support and sync-start) at the point tracing actually starts;
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This seems to change the specification. It was previously required to generate Support on trace enable but now its a recommendation. Its unclear why the clause for generating Support on operating mode is dropped.

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I don't believe it changes the spec, and it is certainly not my intention to do so. Bear in mind that when the spec was ratified, the common control spec didn't exist, and there was no trTeEnable bit defined. It was understood that there was some means of enabling the encoder but the precise mechanism was not defined. There are now several aspects to enabling: trTeEnable is the overall enable, but trTeInstTracing must also be set in order to emit any decodable trace packets (e.g. a sync-start). In the context of this section, the intended original meaning was "before any decodable trace is output" but this was not spelled out beyond "when enabled". But now that the control is defined the original wording is not sufficiently precise. What I'm attempting to do here is clarify that.
It's still mandatory to send a support packet before the first decodable trace packet (i.e. a sync-start). I'm simply recommending that it be sent as early as possible after the encoder is enabled.

* When tracing ceases for any reason (*trTeEnable* set to 0, trace-off trigger, halt, reset, loss of filter qualification, etc.), in order to inform the decoder that the preceding packet reported the address of the final traced instruction;
* If one or more trace packets cannot be sent (for example, due back-pressure from the packet transport infrastructure);
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* If the operating mode of the encoder changes such that the the information output in a previous support packet no longer applies (i.e. if *encoder_mode*, *ioptions*, *doptions* or *denable* change).

The *options* field is a placeholder that must be replaced by an
The *ioptions* and *doptions* fields are placeholders that must be replaced by an
implementation specific set of individual bits - one for each of the
optional modes supported by the encoder.

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