Though you might not know it from a cursory reading of the IPMI spec, entities are an important part of IPMI. They define what a sensor (and in OpenIPMI, a control) monitors (or controls). They are, in essence, the physical parts of the system. For instance, if a system has a temperature sensor on the processor and another temperature sensor on the main board, the temperature sensors will be attached to the processor entity and board entity, respectively. This way, you can tell what the sensor monitors.
Entities are defined by two numbers, the entity id and the entity instance. The entity id defines the type of thing, such as a power supply, processor, board, or memory. The entity instance defines the instance of the thing. For instance, a system may have 4 DIMMs. Each of these DIMMs would be the same entity id (memory), but they would each have a different instance. From now on these are referred to as (<entity id>,<entity instance>). For example, entity (3,1) would be the first processor in the system.
IPMI defines two types of entities: system-relative and device-relative. System-relative entities are unique throughout the entire system (the domain, in OpenIPMI terms). Thus if sensors on different MCs referred entity (3,1), they would all be referring to exactly the same physical thing. System-relative entity instances are defined to be less than 96.
Device-relative entities are unique on the management controller that controls them. These entity’s instances are numbered 96-128. These are referred to using their channel and IPMB address in the form r(<channel>,<IPMB>,<entity id>,<entity instance>-96). For instance, if an MC at address C0h had a sensor on channel 0 that monitored entity id 3, instance 97, that would be r(0,C0,3,1)
Entities may or may not have specific information describing them. Entities that do have specific infor- mation describing them have device locator records.
Entity IDs defined by IPMI are:
description The entity id is not used. Something else? It’s hard to understand why the entity id wouldn’t be known, but this is defined by the spec. A processor A disk or disk bay A peripheral bay