Slot Power Limit Control

Software is responsible for determining the maximum amount of power that an expansion device is allowed to consume. This power allocation is based on the power partitioning within the system, thermal capabilities, etc. Knowledge of the system's power and thermal limits comes from system firmware. The firmware or power manager (which receives power information from firmware) is responsible for reporting the power limits to each expansion port.

Expansion Port Delivers Slot Power Limit

Software writes to the Slot Power Limit Value and Slot Power Limit Scale fields of the Slot Capability register to specify the maximum power that can be consumed by the device. Software is required to specify a power value that reflects one of the maximum values defined by the specification. For example, the electromechanical specification defines maximum power listed in Table 15-1.

Table 15-1. Maximum Power Consumption for System Board Expansion Slots

	X1 Link		X4/X8 Link	X16 Link
Standard Height	10W (max)	25W (max)	25W (max)	25W (max)	40W (max)
Low Profile Card	10W (max)		10W (max)	25W (max)

When these registers are written by power budget software, the expansion port sends a Set_Slot_Power_Limit message to the expansion device. This procedure is illustrated in Figure 15-3 on page 563.

Expansion Device Limits Power Consumption

The device driver reads the values from the Slot Power Limit and Scale fields to verify that the power available is sufficient to operate the device. Several conditions may exist:

• The power available is the power required to operate the device at full capability. In this case, the driver enables the device by writing to the configuration Command register, permitting the device to consume up to the amount of power specified in the Power Limit fields.

• The power available is sufficient to operate the device but not at full capability. In this case, the driver is required to configure the device such that it consumes no more power than specified in the Power Limit fields.

• The power available is < the power required to operate the device. In this case, the driver must not enable the card and must report the inadequate power condition to the upper software layers, which ideally would inform the end user of the power-related problem.

• The power available exceeds the maximum power specified by the form factor specification. This condition should not occur. However, if it does, the device is not permitted to consume power beyond the maximum permitted by the form factor.

• The power available is < the lowest value specified by the form factor specification. This is a violation of the specification, which states that the expansion port "must not transmit a Set_Slot_Power_Limit Message which indicates a limit that is lower than the lowest value specified in the electromechanical specification for the slot's form factor." See Table 15-1 on page 562.

Some devices implemented on expansion devices may consume less power than the lowest limit specified for the form factor. Such devices are permitted to discard the information delivered in the Set_Slot_Power_Limit Messages. When the Slot Power Limit Value and Scale fields are read, these devices return zeros.