Dynamic Link Throttling

Root Port supports dynamic link throttling as a mechanism to help lower the overall component power, ensuring that the component never operates beyond the thermal limit of the package. Dynamic link throttling is also used as a mechanism for ensuring that the ICC_max current rating of the voltage regulator is never exceeded. The target response time for this particular usage model is < 100 µs.

If dynamic link throttling is enabled, the link will be induced by the Root Port to enter TxL0s and RxL0s based on the throttle severity indication received. To induce the link into TxL0s, new TLP requests and opportunistic flow control update will be blocked. Eventually, in the absence of TLP and DLLP requests, the transmitter side of the link will enter TxL0s.

The periodic flow control update, as required by the PCI Express Base Specification is not blocked. However, the flow control credit values advertised to the component on the other side of the link will not be incremented, even if the periodic flow control update packet is sent. Once the other component runs out of credits, it will eventually enter TxL0s, resulting in the local receiver entering RxL0s.

Each of the Root Ports receives four throttle severity indications; T0, T1, T2, and T3. The throttling response for each of the four throttle severity levels can be independently configured in the Root Port TNPT.TSLxM register fields. This allows the duty cycle of the Throttling Window to be varied based on the severity levels, when dynamic link throttling is enabled.

A Throttling Window is defined as a period of time where the duty cycle of throttling can be specified. A Throttling Window is sub-divided into a Throttling Zone and a Non-Throttling Zone. The period of the Throttling Zone is configurable through the TNPT.TT field. Depending on the throttle severity levels, the throttling duration specified by the TNPT.TT field will be multiplied by the multipliers configurable through TNPT.TSLxM.

The period of the Throttling Window is configurable through the TNPT.TP field. The Throttling Window is always referenced from the time a new Throttle State change indication is received by the Root Port or from the time the throttling is enabled by the configuration register. The Throttling Window and Throttling Zone timers continue to behave the same as in L0 or L0s even if the link transitions to other LTSSM states, except for L1, L23_​Rdy and link down. For L1 case, the timer is allowed to be stopped and hardware is allowed to re-start the Throttling Window and the corresponding Throttling Zone timers on exit from L1.