The Power Budgeting Elements

Figure 15-2 illustrates the concept of Power Budgeting for hot plug cards. The role of each element involved in the power budgeting, allocation, and reporting process is listed and described below:

• System Firmware Power Management (used during boot time)

• Power Budget Manager (used during run time)

• Expansion Ports (ports to which card slots are attached)

• Add-in Devices (Power Budget Capable)

System Firmware — System firmware, having knowledge of the system design, is responsible for reporting system power information. The specification recommends the following power information be reported to the PCI Express power budget manager, which allocates and verifies power consumption and dissipation during runtime:

• Total system power available.

• Power allocated to system devices by firmware

• Number and type of slots in the system.

Firmware may also allocate power to PCI Express devices that support the power budgeting capability configuration register set (e.g., a hot-plug device used during boot time). The Power Budgeting Capability register (see Figure 15-1) contains a System Allocated bit that is intended to be set by firmware to notify the power budget manager that power for this device has been included in the system power allocation. Note that the power manager must read and save power information for hot-plug devices that are allocated by the system, in case they are removed during runtime.

The Power Manager — The power manager initializes when the OS installs, at which time it receives power-budget information from system firmware. The specification does not define the method for communicating this information.

The power budget manager is responsible for allocating power for all PCI Express devices. This allocation includes:

• PCI Express devices that have not already been allocated by the system (includes embedded devices that support power budgeting).

• Hot-plugged devices installed at boot time.

• New devices added during runtime.

Expansion Ports — Figure 15-2 on page 561 illustrates a hot plug port that must have the Slot Power Limit and Slot Power Scale fields within the Slot Capabilities register implemented. The firmware or power budget manager must load these fields with a value that represents the maximum amount of power supported by this port. When software writes to these fields the port delivers the Set_Slot_Power_Limit message to the device. These fields are also written when software configures a card that has been added during a hot plug installation.

The PCI Express specification requires that:

• Any downstream port of a Switch or a Root Complex that has a slot attached (i.e., the Slot Implemented bit within its PCI Express Capabilities register is set) must implement the Slot Capabilities register.

• Software must initialize the Slot Power Limit Value and Scale fields of the Slot Capabilities register of the Switch or Root Complex Downstream Port that is connected to an add-in slot.

• The Upstream Port of an Endpoint, Switch, or a PCI Express-PCI Bridge must implement the Device Capabilities register.

• When a card is installed in a slot, and software updates the power limit and scale values in the Downstream port of the Switch or Root Complex, that port will automatically transmit the Set_Slot_Power_Limit message to the Upstream Port of the Endpoint, Switch, or a PCI Express-PCI Bridge on the installed card.

• The recipient of the Message must use the value in the Message data payload to limit usage of the power for the entire card/module, unless the card/module will never exceed the lowest value specified in the corresponding electromechanical specification.

Add-in Devices— Expansion cards that support the power budgeting capability must include the:

• Slot Power Limit Value and Slot Limit Scale fields within the Device Capabilities register.

• Power Budgeting Capability register set for reporting power-related information.

These devices must not consume more power than the lowest power specified by the form factor specification. Once power budgeting software allocates additional power via the Set_Slot_Power_Limit message, the device can consume the power specified, but not until it has been configured and enabled.

Device Driver— The device's software driver is responsible for verifying that sufficient power is available for proper device operation prior to enabling it. If the power is lower than that required by the device, the device driver is responsible for reporting this to a higher software authority.