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+# Multi-Chassis Support for phosphor-psu-monitor
+
+## Current State
+
+Currently, **phosphor-psu-monitor** operates on a **single chassis**.
+
+- It gets the PSUs device access information (such as PSU I2CBus and I2CAddress)
+  and allowed PSU configurations using interfaces hosted by **entity manager**
+- The phosphor-psu-monitor uses a **fixed object path**, appended with the PSU
+  number, to access **inventory manager**.
+- Example of current PSU path:
+
+```
+/xyz/openbmc_project/inventory/system/chassis/motherboard/powersupply0
+```
+
+## Goal: Support Multi-Chassis PSUs
+
+To support **multi-chassis**, the **phosphor-psu-monitor** requires the
+following changes:
+
+- Define Chassis class
+- Modify the **PSUManager** class to instantiate one **Chassis class** per
+  physical chassis.
+- Find all PSUs associated with each chassis.
+- Update the PSUs status on DBus with appropriate path i.e.
+
+```
+/xyz/openbmc_project/inventory/system/chassis1/motherboard/powersupply1
+```
+
+## Changes Required
+
+### 1. Define Chassis Class
+
+- Chassis class constructor take parameters D-Bus connection object, chassis
+  number, and object path
+- Vector of powerSupply instances
+- Functions to get and add PSUs
+- Function to get chassis status
+
+### 2. Modify PSUManager Class
+
+- Add method to retrieve a list of chassis objects from the **inventory
+  manager** and store them in a vector of chassis. Each chassis object listens
+  for D-BUS events to track chassis PGOOD Status.
+- Retrieve the PSU configurations from **entity manager**
+- Find all PSUs associated with chassis from **inventory manager** (using
+  **[getSubTree]
+  (https://github.com/openbmc/phosphor-power/blob/master/utility.hpp)**) and add
+  them to vector in the chassis object. **getSubTree** is a function interacts
+  with the D-Bus object mapper to retrieve all objects under a specified path
+  that implement interface to certain depth.
+
+### 3. Update Monitoring Logic
+
+- The PSUManager object loops over all **chassis** that are present and have
+  standby power. The **phosphor-psu-monitor** app monitors all PSUs associated
+  with the chassis to:
+  - Monitor Presence
+  - Create appropriate journal entries
+  - Create PELs for faults if the chassis is powered on
+
+### 4. DBus Updates
+
+- Update/publish **PSU sensors, presence and VPD information** on D-Bus.
+- Ensure properties **reflect multi-chassis** support.
+
+Note: The phosphor-psu-monitor logic should remain the same when adding support
+for multi chassis.
+
+### 5. PowerSystemInputs Interface
+
+- TBD
+
+### 6. Error Handling
+
+- Ensure any **error** references the **correct PSU inventory path**, e.g.,
+  - `/xyz/openbmc_project/inventory/system/chassis0/powersupply0`
+  - `/xyz/openbmc_project/inventory/system/chassis1/powersupply3`
+
+### 7. Log Messages
+
+- Update journal messages to reflect **appropriate chassis**.
+
+---
+
+# Summary
+
+**phosphor-psu-monitor** must be enhanced to dynamically:
+
+- Discover chassis, and maintain state of each chassis and react accordingly.
+- Monitor PSUs across all chassis
+- Update DBus and error logs accordingly.