DellEMC S6000 xcvrd support

[paavaanan]

- What I did

• Added xcvrd support for DellEMC S600 switches.

- How I did it

• get_transceiver_change_event() in spfputil.py check for default presence registers and if any state-change happens on ports, the difference is updated and propgrated to xcvrd.

- How to verify it

• sfputil show eeprom

- Description for the changelog

- A picture of a cute animal (not mandatory but encouraged)

[lguohan]

where is the sleep?

[lguohan]

code is still buggy.

[jleveque]

Please note that the function signature for get_transceiver_change_event() should be as follows:

def get_transceiver_change_event(self, timeout=0):

It should accept an optional timeout value. If the value is 0, it should block indefinitely until an event is received, otherwise, it should block until the timeout expires. Please see the comments in the API definition; https://github.com/Azure/sonic-platform-common/blob/92b54b1984db0b71196e4fe68cc5a09796fd185c/sonic_sfp/sfputilbase.py#L1097

[paavaanan]

Joe, I agree with the standards, but in S6000 we are using polling method so we used a 1 second delay in while loop if no change in transceiver status. Instead, if an OIR change is detected we return updated port_dict to xcvrd. So, the core need for timeout variable is achieved without using that in the code. The code block indefinitely until an OIR change is detected without timeout variable. I modified the function definition as per the API standard.

[paavaanan]

Joe, added timeout support in xcvrd. Here we used time.time() library which is which is subject to variance as per system clock adjustments. This helps us to detect modified time variations. We handle timeout value and update port_dict within the given timeout value. Only incremental updates are sent to xcvrd. We sleep if there is no change with 1 second granularity..

[jleveque]

@lguohan: Please review again.