Minor clean-up of PSA implementation code

[jafingerhut]

I wanted to create an issue to record some thoughts for small clean-up kinds of changes to the PSA implementation code, while reading and reviewing it. All of the comments are for code in the file targets/psa_switch/psa_switch.cpp.

Right now the field psa_ingress_input_metadata.ingress_timestamp is assigned a value in several different places. It seems cleaner to do so exactly once, in the method ingress_thread(), just before performing the ingress_parser processing. That is the time that the PSA spec says should be recorded, regardless of whether the packet is new from a port, resubmitted, or recirculated. By recording it earlier in other places, there might be significant packet queueing latency between when it is recorded, and when parsing begins.

Filling in the value of egress_timestamp should be done similarly: in one place in the code, not several, just before performing the egress_parser processing.

Right now when a packet is recirculated, there is code that explicitly assigns values to psa_ingress_input_metadata.ingress_port and to psa_ingress_input_metadata.packet_path. It seems those code be removed, since ingress_thread will assign the same values to those fields, copied from the corresponding fields of psa_ingress_parser_input_metadata.

TBD: For a recirculated packet, when it goes through the ingress_thread() method after being recirculated, does the assignment port_t ingress_port = packet->get_ingress_port() return PSA_PORT_RECIRCULATE? It should, but if it does that, how?

[jafingerhut]

@peteli3 I wanted to record these comments somewhere, but I do not think they necessarily need to be done in the current PR you are working on for PSA resubmit.

[jafingerhut]

Regarding my TBD question above, I have confirmed by running the test program psa-recirculate-no-meta-bmv2.p4 with a simple test packet, and console logging enabled with the --log-console option, that after a recirculate operation is done on the packet, the log line prints

Processing packet received on port 0

just like it did when the packet first arrived. I would expect that after correction, it should print:

Processing packet received on port 4294967290

where 4294967290 is the decimal value of the hex 0xfffffffa == PSA_PORT_RECIRCULATE.