Standardized Usage Model

Background

Systems based on the original 1.0 version of the PCI Hot Plug specification implemented hardware and software designs that varied widely because the specification did not define standardized registers or user interfaces. Consequently, customers who purchased Hot Plug capable systems from different vendors were confronted with a wide variation in user interfaces that required retraining operators when new systems were purchased. Furthermore, every board designer was required to write software to manage their implementation-specific hot plug controller. The 1.0 revision of the PCI Hot-Plug Controller (HPC) specification defines:

• a standard user interface that eliminates retraining of operators

• a standard programming interface for the hot plug controller, which permits a standardized hot plug driver to be incorporated into the operating system. PCI Express implements registers not defined by the HPC specification, hence the standard Hot Plug Controller driver implementations for PCI and PCI Express are slightly different.

The following sections discuss the standard user interface.

Standard User Interface

The user interface includes the following features:

• Attention Indicator — shows the attention state of the slot. The indicators are specified to be on, off, or blinking. The specification defines the blinking frequency as 1 to 2 Hz and 50% (+/- 5%) duty cycle. The state of this indicator is strictly under software control.

• Power Indicator (called Slot State Indicator in PCI HP 1.1) — shows the power status of the slot. Power indicator states are on, off, or blinking. The specification defines the blinking frequency as 1 to 2 Hz and 50% (+/- 5%) duty cycle. This indicator is controlled by software; however, the specification permits an exception in the event of a power fault condition.

• Manually Operated Retention Latch and Optional Sensor — secures card within slot and notifies the system when the latch is released

• Electromechanical Interlock (optional) — prevents card being removed from a slot while power is applied.

• Software User Interface — allows operator to request hot plug operation

• Attention Button (optional) — allows operator to manually request hot plug operation.

• Slot Numbering Identification — provides visual identification of slot on the board.

Attention Indicator

As mentioned in the previous section, the specification requires the system vendor to include an Attention Indicator associated with each Hot-Plug slot. This indicator must be located in close proximity to the corresponding slot and is yellow or amber in color. This Indicator draws the attention of the end user to the slot due to the hot plug request having failed due to an operational problem. The specification makes a clear distinction between operational and validation error and does not permit the attention indicator to report validation errors. Validation errors are problems detected and reported by software prior to beginning the hot plug operation. The behavior of the Attention Indicator is listed in Table 17-3 on page 665.

Table 17-3. Behavior and Meaning of the Slot Attention Indicator

Indicator Behavior	Attention State
Off	Normal — Normal Operation
On	Attention — Hot Plug Operation Failed due to an operational problem (e.g., problems with external cabling, add-in cards, software drivers, and power faults)
Blinking	Locate — Slot is being identified at operator's request

Power Indicator

The power indicator simply reflects the state of main power at the slot, and is controlled by Hot Plug software. The color of this indicator is green and is illuminated when power to the slot is "on."

The specification specifically prohibits Root or switch port hardware to change the power indicator state autonomously as a result of power fault or other events. A single exception to this rule allows a platform implementation that is capable of detecting stuck-on power faults. A stuck-on fault is simply a condition in which commands issued to remove slot power are ineffective. If the system is designed to detect this condition the system may override the root or switch port's command to turn the power indicator off and force it to the "on" state. This notifies the operator that the card should not be removed from the slot even though the operator has requested the slot to be powered down. The specification further states that supporting stuck-on faults is optional and if handled via system software "the platform vendor must ensure that this optional feature of the Standard Usage Model is addressed via other software, platform documentation, or by other means."

The behavior of the power indicator and the related power states are listed in Table 17-4 on page 666. Note that V_aux remains on and switch signals are still connected until the retention latch is released or when the card is removed as detected by the Prsnt1# and Prsnt2# signals.

Table 17-4. Behavior and Meaning of the Power Indicator

Indicator Behavior	Power State
Off	Power Off — it is safe to remove or insert a card. All power has been removed as required for hot plug operation. Vaux is only removed when the Manual Retention Latch is released.
On	Power On — removal or insertion of a card is not allowed. Power is currently applied to the slot.
Blinking	Power Transition — card removal or insertion is not allowed. This state notifies the operator that software is currently removing or applying slot power in response to a hot plug request.

Manually Operated Retention Latch and Sensor

The Manual Retention Latch (MRL) is required and it holds PCI Express cards rigidly in the slot. Each MRL can implement an optional sensor that notifies the Hot-Plug Controller that the latch has been closed or opened. The specification also allows a single latch that can hold down multiple cards. Such implementations do not support the MRL sensor.

An MRL Sensor is a Switch, optical device, or other type of sensor. The sensor reports only two conditions: fully closed and open. If an unexpected latch release is detected, the port automatically disables the slot and notifies system software. Note however, that the specification prohibits ports from changing the state of the Power or Attention indicators autonomously.

The switched signals and auxillary power (Vaux) must be automatically removed from the slot when the MRL Sensor indicates that the MRL is open and must be restored to the slot when the MRL Sensor indicates that the latch is re-established. The switched signals are:

• V_aux

• SMBCLK

• SMBDAT

The specification also describes an alternate method for removing V_aux and SMBus power when an MRL sensor is not present. In this case, the PRSNT#1 and PRSNT#2 pins, which indicate whether a card is installed into the slot, can be used to trigger the port to remove the switched signals.

Electromechanical Interlock (optional)

The optional electromechanical card interlock mechanism provides a more sophisticated method of ensuring that a card is not removed when power is still applied to the slot. The specification does not define the specific nature of the interlock, but states that it can physically lock the add-in card or lock the MRL in place.

The lock mechanism is controlled via software; however, there is no specific programming interface defined to control the electromechanical interlock. Instead an interlock is controlled by the same port output signal that enables main power to the slot.

Software User Interface

An operator may use a software interface to request card removal or insertion. This interface is provided by system software, which also monitors slots and reports status information to the operator. The specification states that the user interface is implemented by the Operating System and consequently is beyond the scope of the specification.

The operator must be able to initiate operations at each slot independent of all other slots. Consequently, the operator may initiate a hot-plug operation on one slot using the software user interface or attention button while a hot-plug operation on another slot is in process. This can be done regardless of which interface the operator used to start the first Hot-Plug operation.

Attention Button

The Attention Button is a momentary-contact push-button switch, located near the corresponding Hot-Plug slot or on a module. The operator presses this button to initiate a hot-plug operation for this slot (e.g., card removal or insertion). Once the Attention Button is depressed, the Power Indicator starts to blink. From the time the blinking begins the operator has 5 seconds to abort the Hot Plug operation by depressing the button a second time.

The specification recommends that if an operation initiated by an Attention Button fails, the system software should notify the operator of the failure. For example, a message explaining the nature of the failure can be reported or logged.

Slot Numbering Identification

Software and operators must be able to identify a physical slot based on its slot number. Each hot-plug capable port must implement registers that software uses to identify the physical slot number. The registers include a Physical Slot number and a chassis number. The main chassis is always labeled chassis 0. The chassis number for other chassis' must be a non-zero value and are assigned via the PCI-to-PCI bridge's Chassis Number register ("Introduction To Chassis/Slot Numbering Registers" on page 859).