Item: Add description for associations involving Bmc

Change-Id: I9aa0fe58c5ab0c47f168c1340fd8c328ad7f6e9e
Signed-off-by: Sui Chen <suichen@google.com>
diff --git a/yaml/xyz/openbmc_project/Inventory/Item/Bmc.interface.yaml b/yaml/xyz/openbmc_project/Inventory/Item/Bmc.interface.yaml
index 43ae62e..0472bab 100644
--- a/yaml/xyz/openbmc_project/Inventory/Item/Bmc.interface.yaml
+++ b/yaml/xyz/openbmc_project/Inventory/Item/Bmc.interface.yaml
@@ -1,2 +1,15 @@
 description: >
-    Implement to provide BMC attributes.
+    Implement to provide BMC attributes. Example attributes are
+    the resource utilization of the BMC itself (BMC's CPU and memory,
+    etc).
+
+associations:
+    - name: monitored_by
+      description: >
+          Objects that implement Bmc can implement the
+          'monitored_by' association to link to utilization sensors
+          for indicating the utilization sensors are for the BMC and
+          not the host.
+      reverse_names: monitoring
+      required_endpoint_interfaces:
+        - xyz.openbmc_project.Sensor.Value
diff --git a/yaml/xyz/openbmc_project/Inventory/Item/README.md b/yaml/xyz/openbmc_project/Inventory/Item/README.md
index 237e4b0..a66e675 100644
--- a/yaml/xyz/openbmc_project/Inventory/Item/README.md
+++ b/yaml/xyz/openbmc_project/Inventory/Item/README.md
@@ -1,3 +1,15 @@
+# Representing Inventory Items on DBus
+
+## Overview
+
+Inventory refers to physical objects. Some of the objects can be physically
+accessed, such as a board, a chassis and a drive. Some objects are embedded in
+other objects, such as a BMC embedded in a board, or a core in a CPU. The DBus
+interfaces in this directory describe the DBus representation of those physical
+objects. A graph consisting of Inventory objects as vertexes and Association
+as edges can be constructed that represents the relationship between different
+parts (such as chassis, drive, sensors, etc) of the system.
+
 # Inventory Association
 
 DBus objects with xyz.opebmc_project.Inventory.Item.${ItemType} interface might
@@ -7,3 +19,4 @@
 Document ObjectMapper association forward and reverse names as follows:
 * cpu and PCIe slots: [associated_pcie_slots, upstream_processor]
 * cpu and core: [containing, contained_by]
+* Sensor.Value and Bmc: [monitoring, monitored_by]
diff --git a/yaml/xyz/openbmc_project/Sensor/Value.interface.yaml b/yaml/xyz/openbmc_project/Sensor/Value.interface.yaml
index 77cef58..dd80468 100644
--- a/yaml/xyz/openbmc_project/Sensor/Value.interface.yaml
+++ b/yaml/xyz/openbmc_project/Sensor/Value.interface.yaml
@@ -99,4 +99,12 @@
       reverse_names:
           - sensors
       required_endpoint_interfaces:
-          - xyz.openbmc_project.Inventory.Item
\ No newline at end of file
+          - xyz.openbmc_project.Inventory.Item
+    - name: monitoring
+      description: >
+          Sensors may monitor the BMC's resource utilization and implement
+          an associatoin to the Bmc item.
+      reverse_names:
+          - monitored_by
+      required_endpoint_interfaces:
+          - xyz.openbmc_project.Inventory.Item.Bmc