RFC: new design of phosphor-time-manager on sdbusplus

[Mine]

I’d like to discuss the design of phosphor-time-manager with the new
sdbusplus interfaces.

Please see below background and new design, let me know if you have any
comments.
Thanks!
Background

Legacy time-manager expose below interfaces and methods:

   - curr_time_mode
   - curr_time_owner
   - requested_time_mode
   - requested_time_owner
   - GetTime(target)
   - SetTime(target, time)

The implementation of Get/SetTime() will check the current time mode and
owner, and do below:

   - For GetTime(<target>):
      - If target is “bmc”, return BMC time;
      - If target is “host”, return Host time;
      - Else, return error
   - For SetTime(target, time):
      - If target is “bmc”:
         - If mode is NTP, return error
         - If owner is “HOST”, return error
         - Set BMC time
      - If target is “host”:
         - If owner is “BMC”, return error
         - Set Host time
         - If owner is “SPLIT”, save the diff between BMC time and Host
         time;

New Design

Now with sdbusplus, we have EpochTime interface, with elapsed property.

After discussion with Li Yi and Vishwa, we decided to implement as below:

Create two objects:

   - BmcEpoch
   - HostEpoch

They both implements EpochTime interface.

For BmcEpoch:

   - When elapsed() is called, return BMC time;
   - When elapsed(us) is called, use above SetTime(“bmc”) logic

For HostEpoch:

   - When elapsed() is called, return HOST time;
   - When elapsed(us) is called, use above SetTime(“host”) logic.

And there will be no “curr_time_mode/owner” or “requested_time_mode/owner”
properties on DBUS.

--
BRs,
Lei YU
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