callback-manager: Add README
Use the design doc as a readme for now to explain what
this application does.
Change-Id: I34f0181552b36530c3db9af6bde50ed1e3e237b5
Signed-off-by: James Feist <james.feist@linux.intel.com>
diff --git a/callback-manager/README.md b/callback-manager/README.md
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+# Callback Manager
+
+Author: James Feist !jfei
+
+Primary assignee: James Feist !jfei
+
+Other contributors: None
+
+Created: 2019-06-26
+
+## Problem Description
+
+We need a centralized location to change the LED state.
+
+## Background and References
+
+[Redfish Health](https://github.com/openbmc/docs/blob/master/designs/redfish_health_rollup.md)
+
+[Sensor Thresholds](https://github.com/openbmc/phosphor-dbus-interfaces/tree/master/xyz/openbmc_project/Sensor/Threshold)
+
+## Requirements
+
+Need to be able to trigger the LED from multiple locations and maintain state.
+
+## Proposed Design
+
+The callback manager can change LED state in the below ways.
+
+1. Monitoring the sensor threshold interface.
+1. Monitoring critical / warning thresholds as Redfish does.
+
+Other interfaces specific interfaces can be added later if it makes sense.
+The reason it was designed this way is Redfish has the idea of a Global
+health state, and a component health state. We need to map these to the LED.
+
+### Thresholds
+
+Warning thresholds will trigger warning (blink green) LED state.
+
+Critical thresholds will trigger critical (blink amber) state.
+
+Callback manager will create associations for redfish for thresholds
+automatically.
+
+### Associations
+
+There are two types of status associations, detailed in the Redfish health
+whitepaper.
+
+1. Global- These are the attributes to the global state (warning / critical).
+This is exposed by the callback manager for ease (xyz.openbmc_project.Inventory.Item.Global,
+path=/xyz/openbmc_project/CallbackManager)
+1. Local- Any other warning / critical association.
+
+If the path for an association is Critical for Global & Local, the LED will be
+in the fatal (amber solid) state.
+
+If the path for an association is in Warning for Global & Critical for local,
+the LED will be in the Critical (amber blink) state.
+
+If the path for an association is in the Warning for Global & not Critical for
+local, the LED will be in the Warning (blink green) state.
+
+## Alternatives Considered
+
+Write the LED manually, this seems very error prone.
+
+## Impacts
+
+Will have to add more associations.
+
+## Testing
+
+Sensor override, then look at LED and Redfish.