by January 3, 2001 0 comments

A fully capable hotplug system includes hotplug system hardware (server),
hotplug-aware software and operating system (OS) support, hotplug-capable device
drivers, and hotplug-aware PCI user interface (see picture below). The hardware
and software components work together to ensure that any hotplug activity
occurring on the PCI bus is benign to other devices on the PCI bus. The vast
majority of industry-standard PCI adapters can be made hotplug aware through the
installation of hotplug device drivers. PCI adapter vendors usually provide
these device drivers.

System hardware

the PCI hotplug specification identifies the technical requirements of a hotplug
capable system, it doesn’t specify the implementation of this technology. In
the specific implementation that Compaq developed, the PCI hotplug hardware
isolates a single PCI slot from all other devices on the PCI bus.

The PCI hotplug system hardware performs two main functions: Powers down a
single adapter slot, allowing insertion and removal of adapters; and protects
the system and other adapters from the electrical effects of hot-plug

Hotplug electronics consist of two separate elements: the hotplug controller
and the slot-specific power control. The hotplug controller is designed to
manage the following components:

PCI Bus The controller communicates with isolation devices on the PCI bus to
electrically isolate a single PCI slot from the rest of the system. Slot
isolation permits the insertion or removal of an adapter without interruption to
the server or other active adapters.

Power The controller receives a command from the OS to power up or power down
a single PCI slot. To perform this function, the controller uses the
slot-specific power control. The slot-specific power-control electronics allow
proper power sequencing on the PCI bus and guarantee safe control of power to
individual PCI adapters.

Slot LED indicators The hot-plug controller also governs the slot LEDs. In
one specific implementation of hotplug hardware, each slot has a green and an
amber LED to indicate slot status. The green LED indicates power to the slot and
flashes while performing a power state change; the amber LED indicates that the
slot requires attention.

PCI hotplug button This feature is available only on one specific vendor’s
PCI hotplug servers. The button is pressed to signal the software to initiate a
power state change. While the button is more convenient, the same functionality
can be provided through the software interface. Each slot has its own button to
indicate which slot is to be addressed by the supporting software.

Compatibility issues

PCI hotplug technology addresses compatibility concerns by using standard PCI
adapters. A hotplug system requires a hotplug platform, a hotplug OS, and
hotplug adapter drivers. A system can include any combination of hotplug and
conventional versions of each of these components, including a mix of both
hotplug and conventional adapter drivers. However, a particular adapter can be
hotplugged only if all three components for that adapter support hotplug

A hotplug platform supports loading a conventional OS. The system behaves as
a conventional system if no hotplug software is loaded. Hotplug OSs are designed
to load and execute on any platform. If no hotplug controller is found on the
platform, then the OS will not permit the user to perform any hotplug operations
at the user interface.

Hotplug OSs generally require driver modifications to support full hotplug
capability. However, as with any driver revision, the OS will often support
previous generations of drivers. Furthermore, in some cases, the new driver can
be loaded onto the previous version of the OS. If a conventional driver is
loaded onto a hotplug OS, or vice versa, the driver will continue to have the
same capability it always had in the conventional application. However, the
adapter cannot be hotplugged unless both the driver and the OS support the
hotplug operation.

Thus, most industry-standard PCI adapters can be used without any
modifications. For industry-standard PCI adapters to have hotplug functionality,
however, new features must be added to the device drivers.

Naren NG is product manager, Proliant Servers, Compaq India

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