Abstract:
A distributed power management system for a bus architecture or similar communications network. The system supports multiple low power states and defines entry and exit procedures for maximizing energy savings and communication speed.

Description:
RELATED APPLICATION  
       [0001]     This patent application is a divisional of U.S. patent application Ser. No. 10/335,111 filed Dec. 31, 2002 entitled, “Active State Link Power Management.” 
     
    
     FIELD OF THE INVENTION  
       [0002]     The invention relates to power management of networked devices. Specifically, this invention relates to power management for bus architectures.  
       BACKGROUND  
       [0003]     Power management in modern computer systems plays an important part in conserving energy, managing heat dissipation, and improving system performance. Modern computers systems are increasingly designed to be used in settings where a reliable external power supply is not available making power management to conserve energy important. Even when reliable external power supplies are available careful power management within the computing system can reduce heat produced by the system enabling improved performance of the system. Computing systems generally have better performance at lower ambient temperatures because key components can run at higher speeds without damaging their circuitry. Many computing platforms are constrained by heat dissipation issues including dense servers, DT computers and mobile computers. For mobile computers, energy conservation is especially important to conserve battery power.  
         [0004]     Power management can also reduce the operating costs of a computing system by reducing the amount of energy consumed by a device while in operation. Components of a computer system can be powered down or put in a sleep mode that requires less power than active operation. Computer monitors are often placed in a sleep mode when an operating system detects that the computer system has not received any input from a user for a defined period of time. Other system components can be placed in a sleep or powered down state in order to conserve energy when the components are not in use. The computer system monitors input devices and wakes devices as needed.  
         [0005]     For example, a PCI bus uses a centralized mechanism to determine if the bus is not needed which involves all other devices verifying that they do not need the bus. This system is implemented using out-of-band signaling, thus requiring specialized communication lines in addition to data lines. When the bus is determined not to be needed then the common clock signal is no longer transmitted.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     Embodiments of the invention are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean at least one.  
         [0007]      FIG. 1  is a block diagram of a communication link network.  
         [0008]      FIG. 2  is a diagram of an individual link.  
         [0009]      FIG. 3  is a flow-chart of a procedure for an endpoint to transition a connected lane into an L0s state.  
         [0010]      FIG. 4  is a flow-chart of a procedure for an endpoint to transition a connected lane out of the L0s state.  
         [0011]      FIG. 5  is a flow-chart of a procedure for an intermediate node to transition a connected lane into an L0s state.  
         [0012]      FIG. 6  is a flow-chart of a procedure for an intermediate node to transition a connected lane out of an L0s state.  
         [0013]      FIG. 7A  is a first part of a flow-chart of a procedure for an endpoint to transition a connected link into an L1 state.  
         [0014]      FIG. 7B  is a second part of a flow-chart of a procedure for an endpoint to transition a connected link into an L1 state.  
         [0015]      FIG. 8A  is a first part of a flow-chart of a procedure for an intermediate node to transition a connected link into an L1 state.  
         [0016]      FIG. 8B  is a second part of a flow-chart of a procedure for an intermediate node to transition a connected link into an L1 state.  
         [0017]      FIG. 9  is a flow-chart of a procedure for an endpoint to transition a connected link out of an L1 state.  
         [0018]      FIG. 10  is a flow-chart of a procedure for an intermediate node to transition a connected link out of an L1 state.  
         [0019]      FIG. 11  is a flow-chart of a procedure for determining the enablement of the L0s and L1 states for a network device by power management software.  
         [0020]      FIG. 12  is a block diagram of computer system with a communication link network.  
     
    
     DETAILED DESCRIPTION  
       [0021]      FIG. 1  is a block diagram of an exemplary topology of a communications network. In one embodiment, network  100  includes a root complex  101  that is at the top of the tree type network  100 . Endpoints  103 ,  111 ,  113 ,  115  and  117  represent network devices or resources that communicate over network  100 . Intermediate nodes  105 ,  107  and  109  represent switches or similar network devices for routing data between the endpoints themselves and between the endpoints and root complex  101 . Communication channels or ‘links’  151 ,  153 ,  155 ,  157  and  159  allow the devices at each end to transmit and receive data between them. In one embodiment, network  100  is a set of high speed serial interconnects.  
         [0022]      FIG. 2  is a diagram of an exemplary link  200 . In one embodiment, link  200  connects an ‘upstream’ device  201  with a ‘downstream’ device  203 . An upstream device  201  is a network device that occupies a higher level in the tree topology of network  100  than network device  203  at the other end of connection  200 . For Example, referring to  FIG. 1  intermediate node  107  is an upstream device connected via link  159  to downstream endpoint device  113 . An upstream device  201  in this topology can be an intermediate node or root complex. A downstream device  203  in this topology can be either an intermediate node or an endpoint.  
         [0023]     In one embodiment, link  200  is composed of an upstream lane  207  and a downstream lane  209 . Upstream lane  207  allows downstream device  203  to transmit data to upstream device  201 . Likewise, downstream lane  209  allows upstream device  201  to transmit data to downstream device  203 . Each of these lanes  207  and  209  can be characterized as being composed of a transaction layer (T), link layer (L) and physical layer (P). In one embodiment, the transaction layer manages the translation of read and write requests and data transmission into transaction layer packets (TLPs).  
         [0024]     In one embodiment, the link layer is the physical characterization of a data link layer system. The data link layer system manages error recovery (e.g., initialization of the retransmission of transaction layer packets) and flow control. This is carried out by the transmission of data link layer packets (DLLPs).  
         [0025]     In one embodiment, the physical layer is a set of differential transmit pairs and differential receive pairs. The interconnects of the physical layer transmit dual simplex data on point to point connections that are self clocked. Bandwidth can be adjusted in a linear fashion by increasing the interconnect width and frequency (e.g. adding multiple lanes and increasing the speed of transmission). In one embodiment, network devices include a state machine or similar apparatus for controlling the power levels of the transmit lanes attached to the device. If no power management scheme were implemented in the system, a link would consume the same amount of power regardless of whether it was transmitting data. Because there would be no lower powered state, if a device did not have any data to transmit it would send “idle characters” (e.g., a known set of bits that identifies when the transmission is to be ignored) across the link in order to maintain synchronization with the device at the other end of the link. Without a power management scheme idle characters would be transmitted over these interconnects requiring full power.  
         [0026]     In one embodiment, when in normal operation a link  200  is in an active power state (‘L0’). However, when a network device  203  does not have any data to transmit or is unable to transmit, it is not necessary for upstream lane  207  to remain in an active state. Instead, upstream lane  207  can be placed in a lower power state (‘L0s’) until it is needed to transmit again. In one embodiment, the L0s state is a type of standby state where power is reduced for upstream lane  207 . However, upstream lane  207  is kept in a state such that the lane can return to the active power state L0 which is capable of transmission in a short time. Thus, L0s is a low power state having a low exit latency. In one embodiment, this low power state L0s is implemented in the device physical layer and any lane (upstream or downstream) can be placed in this state. L0s is optimized towards minimal transmission times by minimizing latency for both entry and exit from the low power state. In one embodiment, there is no handshake between the link edges before entering the low-power state. A link edge is a device that communicates at one end of the link.  
         [0027]     In one embodiment, L0s exit latencies may differ significantly depending on whether a reference clock for opposing link edges of a given link is provided from the same source or delivered to each linkage device from a different source. The L0s exit latency depends mainly on the ability of the receiving device to quickly acquire bit and symbol synchronization. In one embodiment, a network device (endpoint, intermediate node or root complex) powers up with the L0s enabled by default if it shares a common reference clock source with the network device on the opposite end of the link (e.g., a common, distributed reference clock configuration). L0s is disabled by default if the network device at the opposite end of a given link has a different asynchronous component reference clock input. Entry into the L0s state is managed separately for each direction (or ‘lane’) of a link. In one embodiment, it is the responsibility of each device at either end of a link to initiate an entry into the L0s state on its transmitting lane. A port (i.e., an interface between a network device and link) that is disabled for the L0s state must not transition its transmitting lanes to the L0s state. It must still however be able to tolerate having its receiver port lanes entering L0s as a result of the device at the other end bringing its transmitting lanes into the L0s state and then later returning to the L0 state.  
         [0028]      FIG. 3  is a flow-chart depicting an exemplary procedure for transitioning a lane connected to either an endpoint  111  or a root complex  101  from an active state L0 to a low power state L0s. In one exemplary embodiment, upstream lane  207  begins in an active state (block  301 ). Endpoint device  111  (or  203 ) determines if any flow control credits are available to allow transmission of data to a recipient network device  107  (or  201 ) (block  303 ). Credits are flow control management devices that are exchanged between network devices that transmit data to one another. A network device limits the incoming data by limiting the number of credits given to other devices. In one embodiment, if endpoint device  111  does not have credits for an intended recipient (e.g., intermediate node  107 ) then it cannot transmit data to that recipient  107 . If flow control credits are available, endpoint device  111  determines if any transactions are scheduled to be transmitted on upstream lane  207  (block  305 ). If there are scheduled transactions and credits available then endpoint device  111  must maintain upstream lane  207  in the L0 state.  
         [0029]     In one embodiment, if there are not any transactions scheduled then endpoint device  111  determines if there are any DLLPs including acknowledgement messages being transmitted or pending transmission (block  307 ) on upstream lane  207 . If there are active or pending transmissions of DLLPs then upstream lane  207  must be maintained in the L0 state. However, if there are no pending DLLPs then endpoint device  111  can transition upstream lane  207  to the L0s state.  
         [0030]     In one embodiment, the identified conditions must be met for a predetermined period, for example two symbol times. Exemplary time periods may be the length of two symbol transmissions, exit latency of the link or well known heuristics for optimizing the time period that maximizes the time period in which the link is in a low power state while reducing the frequency of exits from the low power state. In one embodiment, the transition to the L0s state is executed by the physical layer. Protocol layers are not involved in the transition into or out of the L0s state. In regard to root complex  101 , these rules apply in terms of downstream lanes  209  of root complex  101  that are in links (e.g, link  151  and link  149 ) to each network device on the next lower level of the tree (e.g., devices  103  and  105 ).  
         [0031]      FIG. 4  is a flow-chart of an exemplary procedure for exiting the L0s state to return to a L0 state for an endpoint  111  or root complex  101 . An endpoint  111  or root complex  101  can initiate an exit from the L0s state on its upstream transmit lanes and downstream transmit lanes, respectively.  
         [0032]     In one embodiment, endpoint device  111  starts in the L0s state (block  401 ). Endpoint device  111  checks periodically if it has data to transmit over lane  207  (block  403 ). As long as there is no data to transmit endpoint device  111  maintains lane  207  in the L0s state. If it is detected that it is necessary to transmit data on lane  207 , then endpoint device  111  transitions upstream lane  207  to the active L0 state (block  405 ). This procedure is direction independent, an endpoint uses this procedure for upstream lanes and a root complex uses this procedure for downstream lanes.  
         [0033]     In one embodiment, the transition from the L0s state to the L0 state is not dependent on the status or availability of flow control credits. In this embodiment, the link is able to reach the L0 state and exchange flow control credits across the link. For example, if all credits of a particular type were consumed when the link entered L0s, then any component on either side of the link must still be able to transition the link to the L0 state so that new credits can be sent across the link.  
         [0034]      FIG. 5  is a flow-chart of an exemplary procedure for an intermediate node, such as a switch or similar device, to transition a lane to the L0s state. In one embodiment, an intermediate node  107  in the L0 state (block  501 ) periodically checks if all receiving lanes in either direction, are in the L0s state (block  503 ). If lanes in both directions are in the active L0 state then intermediate node  107  maintains all of its lanes in their current state. In one embodiment, if intermediate node  107  detects that all the receiving lanes in a direction are in the L0s state, then intermediate node  107  determines if there are any transactions scheduled (block  505 ) and if any appropriate flow control credits are available (block  507 ). Finally, if there are not any transactions pending or there are not any credits available intermediate node  107  determines if there are any DLLPs scheduled or pending transmission (block  505 ). If there are no scheduled or pending transmissions in the given direction then intermediate node  107  can transition the lane in the given direction into the L0s state (block  511 ). For example, if all downstream receiving lanes are in L0s then the upstream transmit lane can be transitioned to L0s. In one embodiment, the identified conditions must be met for a predetermined period, for example two symbol times. In another embodiment, a well-known heuristic is used in connection with the above-identified criteria to determine the time of entry into the L0s state.  
         [0035]      FIG. 6  is a flow-chart of an exemplary procedure for an intermediate node  107  to transition a transmitting lane from the L0s state (block  601 ) to the active L0 state. In one embodiment, intermediate node  107  continuously checks receiving lanes to determine if they are in the L0 state (block  603 ). If a change to the L0 state is not detected then intermediate node  107  maintains transmitting lanes in their current state. If a change of receiving lane to the L0 state is detected then the device transitions outgoing lanes in the same direction (e.g., if receiving upstream lane is in the L0 state then all downstream transmitting lanes are transitioned) (block  605 ). Transitioning all transmitting lanes in a direction opposite an active receiving lane reduces the accumulation of latency time for exiting to the L0 state for a packet traversing multiple nodes. For example, from an endpoint node  111  to the root complex  101 . In another embodiment, speed in transmitting data across multiple nodes is traded for power conservation by only transitioning links in the direct path of a packet by examining the packet at each network device to determine the next stop in its path.  
         [0036]     In one embodiment, an L1 state is a low power state that is executed between the protocol layers at the two ends of a link. The protocol layers bring the link into a low power state in an orderly manner, starting by completing any pending transactions between the link edges. Once this is completed, the physical layer is instructed to enter the low power state. This low power state, L1, is optimized for lower power consumption than L0s at the expense of longer entry and exit latencies. In one embodiment, L1 reduces link power beyond the L0s state for cases where very low power is required and longer transition times are acceptable. In one embodiment, support for the L1 state is optional among network devices (e.g., endpoint and intermediate devices). In one embodiment, the handshake mechanisms involves a set of in-band messages. Entry into a low power state L1 is initiated by network devices originating traffic through network  100  (e.g. endpoints).  
         [0037]     In one embodiment, three messages are defined to support the L1 state. An active state request message that is a DLLP, a request acknowledgement message, which is a DLLP, and an active state negative acknowledgement message which is a TLP. Endpoints that are enabled for L1 negotiate to enter the L1 state with the network device on the upstream end of the link.  
         [0038]      FIGS. 7A and 7B  illustrate a flow-chart of an exemplary procedure for an endpoint  111  to transition a link  157  to an L1 state. In one embodiment, this procedure applies to an endpoint  111  in the L0s state (block  701 ). Only endpoints that are enabled for the L1 state carry out this procedure, if endpoint  111  is not enabled it will remain in the L0s state (block  703 ). In one embodiment, endpoint  111  determines if link  157  has been in a L0s state for a predetermined period of time (block  705 ). If endpoint  111  has not been in this state for the predetermined period then it will remain in the L0s state. In another embodiment, well known heuristic devices are also used to determine when to initiate a transition to the L1 state from the L0s state or L0 state. In one embodiment, endpoint  111  then transitions its transmit lane  207  to the L0 state in order to send messages across link  157  (block  706 ). If endpoint  111  has met the predetermined criteria then it blocks the scheduling of new transactions (block  707 ). Endpoint  111  waits to receive acknowledgements for all transaction data transmitted ( 709 ). Once the transmitted data has been acknowledged endpoint  111  sends a request message to upstream device  107  (block  711 ). In one embodiment, endpoint  111  sends the request message continually until it receives a response from upstream device  107 . Endpoint  111  remains in this loop waiting for a response from the upstream device  107 . During this waiting period, the endpoint device  111  must not initiate any transaction layer transfers. However, in one embodiment, endpoint device  111  accepts TLPs and DLLPs from upstream device  107 . It also responds with DLLPs as needed by the link layer protocols. In one embodiment, if endpoint device  111  needs to initiate a transfer on the link for any reason it must first complete the transition to the low power link state. Once in a lower power link L1 state the endpoint device is then permitted to exit the low power link L1 state to handle the transfer. This embodiment involves less complexity and therefor reduces the cost and space required. In another embodiment, endpoint device  111  exits the handshake process in order to transmit the TLP in a more timely fashion.  
         [0039]     In one embodiment, upstream device  107  determines if it is L1 enabled in relation to link  157  from which the request was received (block  713 ). If the upstream device  107  is not L1 enabled then it will transition its transmit lane  209  to the L0 state (block  714 ) and send a negative acknowledgement message to endpoint  111  (block  715 ). Link  157  will then remain in the L0 state (block  716 ). If upstream device  107  does support the L1 state, upstream device  107  determines if it has any transactions scheduled to be transmitted over link  157  to endpoint device  111  which sent the request (block  717 ). If there are transactions scheduled then upstream device  107  will transition its transmit lane  209  to the L0 state (block  714 ) and will send a negative acknowledgement (block  715 ). Subsequently, link  157  remains in the L0 state (block  716 ).  
         [0040]     In one embodiment, upstream device  107  must wait until a minimum number of flow control credits required to send the largest possible packet of a flow control type are accumulated. This allows the network device to immediately issue a TLP after it exits from the L1 state.  
         [0041]     In one embodiment, if no transactions are scheduled, upstream device  107  determines if DLLPs are pending transmission or scheduled for transmission (block  719 ). If DLLPs are pending or scheduled then transmit lane  209  is transitioned to the L0 state (block  714 ) and a negative acknowledgement is sent to endpoint device  111  (block  715 ). Subsequently, link  157  remains in the L0 state (block  716 ). If no DLLPs are pending or scheduled then upstream device  107  blocks the scheduling of transactions (block  721 ). In one embodiment, upstream device  107  waits for the acknowledgement of the last transaction sent (block  723 ) before transitioning to the L0 state (block  724 ) and sending a positive acknowledgement to endpoint device  111  of the L1 request using a DLLP (block  725 ).  
         [0042]     Endpoint  111  and upstream device  107  then transition each lane of the link to the L1 state (block  727  and  729 ). When endpoint device  111  detects the positive acknowledgement DLLP on its receive lanes  209  it ceases sending the request DLLP and disables its link layer and brings its transmit lanes  207  into the electrical idle state L1. Upstream device  107  continuously sends the positive acknowledgement DLLP until it detects that its receive lanes  207  have entered into the L1 electrical idle state. When upstream device  107  detects an L1 electrical idle on its receive lanes  207  it ceases to send the positive acknowledgment DLLP, disables its link layer and brings the downstream lanes  209  into the L1 electrical idle state. In one embodiment, if upstream device  107  for any reason needs to initiate a transfer on link  157  after it sends the positive acknowledgement DLLP, it must first complete the transition to the low power state L1. It can then exit the low power L1 state to handle the transfer once link  157  returns to the L0 state.  
         [0043]     In one embodiment, a transaction layer completion timeout mechanism is used in conjunction with network  100  to determine when a TLP needs to be resent or is not received. This mechanism is not affected by the transition to the L1 state, thus it continues to count. Likewise, in one embodiment, flow control update timers are used in connection with network  100 . These timers are frozen while a link is in the L1 state to prevent a timer expiration that will unnecessarily transition the link back to the L0 state.  
         [0044]      FIGS. 8A and 8B  illustrate a flow-chart of an exemplary procedure for an intermediate node  107  such as a switch or similar device to transition an upstream link  153  into the L1 state. Intermediate node  107  may have an upstream link in a L0s state or L0 state (block  801 ). Intermediate node  107  determines if upstream link  153  supports L1 and if L1 support is enabled (block  803 ). Intermediate node  107  also determines if all downstream links  157  and  159  are in an L1 state (block  805 ) and if any transactions or DLLPs have been scheduled (blocks  805  and  807 ). If there are no scheduled transmissions and the receiving lanes are idle (block  809 ) then intermediate node  107  blocks the scheduling of TLPs (block  811 ). Intermediate node  107  then verifies that the last TLP sent has been acknowledged (block  813 ). In one embodiment, intermediate node  107  must wait until a minimum number of flow control credits required to send the largest possible packet of a flow control type are accumulated. This allows the network device to immediately issue a TLP after it exits from the L1 state. In one embodiment, intermediate node  107  and upstream device  105  transition their transmit links to the L0 state before transmitting messages over link  153 . Intermediate node  107  then sends a request message to upstream device  105  (block  815 ). In one embodiment, intermediate node  107  sends the request message continually until it receives a response from upstream device  105 . Intermediate node  107  remains in this loop waiting for a response from upstream device  105 . Upstream device  105  determines if it supports the L1 state for the port the message is received from and if the L1 state is enabled for the link  153  (block  817 ). If the L1 state is not supported or enabled then upstream device  105  sends a negative acknowledgment to intermediate node  107  (block  829 ). Upon receipt of a negative acknowledgement intermediate node  107  transitions its upstream lane to the L0s state (block  831 ).  
         [0045]     In one embodiment, the upstream device  105  if enabled for the L1 state determines if it has a transaction scheduled or pending (block  817 ) for link  153  or if it has a DLLP scheduled or pending (block  819 ) for link  153 . If either of those conditions are true then a negative acknowledgement is sent (block  829 ) to intermediate node  107 . If there are no scheduled transmissions, then upstream device  105  blocks the scheduling of transactions for that link (block  821 ) and waits for the receipt of the last transaction&#39;s acknowledgment, if necessary (block  823 ). Upon verifying the last transaction is complete the upstream device  105  sends a positive acknowledgment as a DLLP to intermediate node  107  (block  825 ). Intermediate node  107  and upstream device  105  then transition the link to the L1 state (block  827 ) in the same manner as an endpoint and intermediate node.  
         [0046]      FIG. 9  is a flow-chart of an exemplary procedure for an endpoint  111  to transition an upstream link  157  from the L1 state to the L0 state. Unlike the entry protocol the exit protocol does not involve negotiation between the edges of a link. In one embodiment, an endpoint device  111  may have data to transmit while in the L1 state (block  901 ). Endpoint  111  will periodically check for data to be transmitted or otherwise be notified of the need to transmit data (block  903 ). Upon detecting the need to transmit data, endpoint  111  transitions the transmit lane of link  157  to the L0 state (block  905 ).  
         [0047]      FIG. 10  is a flow-chart of an exemplary procedure for an intermediate node or root complex to transition a downstream lane to the L0 state from the L1 state. In one embodiment, upstream device  107  having a downstream link  157  in an L1 state (block  1001 ) periodically checks the receiving lane of link  157  to determine if it has entered the L0 state (block  1003 ). Intermediate node  107  checks each of its receiving lanes to determine if one transitions to the L0 state. If a lane is detected in the L0 state the transmit lane of the same link will be transitioned to the L0 state (block  1005 ). Also, any transmit link that is in the same direction (i.e., downstream or upstream) as the link that is detected in the L0 state is transitioned to the L0 state (block  1007 ). Thus, if the receiving lane of link  157  is detected in the L0 state then intermediate node  107  will transition the transmit lane of link  153 , which is in the same direction (upstream) as the receiving lane that transitioned. Likewise, if receiving lane of link  153  is detected in the L0 state then intermediate node  107  will transition the outgoing (downstream) lanes of links  157  and  159  to the L0 state. In one embodiment, because L1 exit latencies are relatively long, an intermediate node  107  does not wait until its downstream port link has fully exited to the L0 state before initiating an L1 exit transition on its upstream port link. Waiting until the downstream link has completed the L0 transition will cause a message traveling through several intermediate nodes to experience an accumulated latency as it traversed each switch. In one embodiment, an intermediate node  107  initiates an L1 exit transition on its upstream port link after no more than one microsecond from the beginning of an L1 exit transition on any of its downstream port links. In one embodiment, intermediate node  107  does not transition from the L1 state to the L0 state on links that are not in the direction of the message to be transmitted.  
         [0048]     In one embodiment, links that are already in the L0 state do not participate in the exit transition. In one embodiment, downstream links whose downstream network device is in a low power state are also not affect by exit transitions. For example, if an intermediate node with an upstream port in L0s and a downstream network device in a low power state receives a packet destined for the downstream network device in the low power mode the downstream link connecting the intermediate node to the downstream network device will not transition to the L0 state yet. Rather, it will remain in the L1 state. The packet destined for the downstream network device will be checked and routed to the downstream port that shares a link with the downstream device in the low power state. The intermediate node then transitions the downstream link to the L0 state. The transition to the L0 state is thus triggered by the packet being routed to that particular downstream link not by the transition of an upstream link into the L0 state. If a packet is destined for another node then the link to the low power network device would have remained in the L1 state.  
         [0049]      FIG. 11  is a flow chart illustrating the operation of a program that manages the architecture support for the L0s and L1 states in network  100 . This program ensures that no link in network hierarchy  100  enters a lower power state than allowed by a device using it. The program polls each device in network  100  to retrieve its L0s exit latency time (block  1101 ). In one embodiment, component reference clock information is available that can serve as a determining factor in the L0s exit latency value reported by a network device. In one embodiment, reference clock configuration information can also be accessed directly to determine the initial enablement or disablement values for a network device. The program also polls each network device for its L1 exit latency timing (block  1103 ), its L0s latency tolerance (block  1105 ) and L1 latency tolerance (block  1107 ). In one embodiment, isochronous traffic requires bounded service latencies. The distributed power management system may add latency to isochronous transactions beyond expected limits. In one embodiment, the power management system is disabled for network devices that are configured with an isochronous virtual channel. Based on the information retrieved from each network device the program then assigns an active state control value to each device by setting an active link power management support field (block  1109 ). In one embodiment, the active state control value that is assigned to each device is based on the device&#39;s tolerance in comparison to the accumulative latency of the path between the device and another device such as an endpoint or root. In this embodiment, the device is enabled for the L0s or L1 state if the accumulated latency along the path is lower than the acceptable latency for the device. In one embodiment, this value labels each network device as supporting both the L0s and L1 state, either state separately or neither state. Thus, the power management software enables or disables each port of a component by setting a support field associated with that network device. The power management software can be implemented with a basic input output system (BIOS) for use with legacy operating systems or as a program that runs under or as part of an operating system.  
         [0050]     Table I is an exemplary embodiment of an encoding scheme for an active link power management support field. In one embodiment, this field is stored in a storage device (e.g., a register, eeprom, or similar device) associated with an endpoint device or intermediate node device by the power management program.  
                           TABLE I                       Field   Read/Write   Default Value   Description                   Active State Link   RO   01b   00b-Reserved       PM Support       or   01b-L0s supported               11b   10b-Reserved                   11b-L0s and L1                   supported                  
 
         [0051]     Table II is an exemplary encoding of L0 exit latencies for endpoint devices and intermediate nodes devices to be reported or monitored by the power management program.  
                           TABLE II                       Field   Read/Write   Default Value   Description                   L0s Exit Latency   RO   N/A   000b-less than 64 ns                   001b-64 ns-128 ns                   010b-128 ns-256 ns                   011b-256 ns-512 ns                   100b-512 ns-1 μs                   101b-1 μs-2 μs                   110b-2 μs-4 μs                   111b-Reserved                  
 
         [0052]     Table III is an exemplary encoding of L1 exit latencies for endpoint devices and intermediate node devices to be reported or monitored by the power management program.  
                           TABLE III                       Field   Read/Write   Default Value   Description                   L1 Exit Latency   RO   N/A   000b-less than 1 μs                   001b-1 μs-2 μs                   010b-2 μs-4 μs                   011b-4 μs-8 μs                   100b-8 μs-16 μs                   101b-16 μs-32 μs                   110b-32 μs-64 μs                   111b-L1 transition not                   supported                  
 
         [0053]     Tables IV and V are an exemplary encoding of endpoint latency tolerances. Endpoints devices report or store a value indicating latency the endpoint devices can absorb due to transition times from the L0s and L1 states to the L0 state for an associated link: Power management software, using the latency information reported by all components in the hierarchy can enable the appropriate level of active link power management support by comparing exit latencies for each given path from the root to endpoint against the acceptable latency that each corresponding endpoint can withstand.  
                           TABLE IV                       Field   Read/Write   Default Value   Description                   Endpoint L0s   RO   N/A   000b-less than 64 ns       Acceptable           001b-64 ns-128 ns       Latency           010b-128 ns-256 ns                   011b-256 ns-512 μs                   100b-512 ns-1 μs                   101b-1 μs-2 μs                   110b-2 μs-4 μs                   111b-More than 4 μs                  
 
         [0054]    
       
         
               
               
               
               
             
           
               
                 TABLE V 
               
               
                   
               
               
                   
               
               
                 Field 
                 Read/Write 
                 Default Value 
                 Description 
               
               
                   
               
             
             
               
                 Endpoint L1 
                 RO 
                 N/A 
                 000b-less than 1 μs 
               
               
                 Acceptable 
                   
                   
                 001b-1 μs-2 μs 
               
               
                 Latency 
                   
                   
                 010b-2 μs-4 μs 
               
               
                   
                   
                   
                 011b-4 μs-8 μs 
               
               
                   
                   
                   
                 100b-8 μs-16 μs 
               
               
                   
                   
                   
                 101b-16 μs-32 μs 
               
               
                   
                   
                   
                 110b-32 μs-64 μs 
               
               
                   
                   
                   
                 111b-More than 4 μs 
               
               
                   
               
             
          
         
       
     
         [0055]     In one embodiment, multi-function endpoints are programmed with different values in their respective support fields for each function. In one embodiment, a policy is used that multi-function devices will be governed by the most active common denominator among all of its active state functions based on:  
         [0056]     whether functions in non-active states are ignored in determining the active state link power management policy;  
         [0057]     whether any active state functions have their active state link power management disabled resulting in the entire network device being disabled;  
         [0058]     if at least one of the active state functions is enabled for L0s only then the active state link power management is enabled for the L0s state only, for the entire component;  
         [0059]     if the other rules do not apply then the active state link power management is enabled for both L0s and L1.  
         [0060]     In one embodiment, network devices are able to change their behavior during runtime as devices enter and exit low power device states. For example, if one function within a multi-function component is programmed to disable active state link power management, then active state link power management will be disabled for that network device while that function is in the active state. If the network device transitions to a non-active state then the active state power management will be enabled to at least support the L0s state if all other functions are enabled for active state link power management.  
         [0061]     In one embodiment, network devices, including endpoint and intermediate devices also have low power states. Network devices in an active state can conserve power using the power management scheme for their associated links. Even if the network devices are in an active state, power savings can be achieved by placing idle links in the L0s and L1 states. This allows the hardware network devices autonomous dynamic link power reduction beyond what is achievable by software only control of power management.  
         [0062]      FIG. 12  is a block diagram of a computer system encompassing the power management scheme. In one embodiment, the system includes a central processing unit  1201 , graphics port  1207  and memory device  1205  connected to the root complex  1203 . The root complex is connected to switches  1209  and  1211 . Switch  1209  is further coupled to switch  1221  and PCI bridge  1213 . PCI bridge  1213  allows communication between the network and a set of PCI devices  1215 . Switches  1221  and  1211  allow endpoint devices  1223  and  1217  and legacy endpoint devices  1225  and  1219 , respectively, to communicate with other devices on the network. In one embodiment, endpoints can be peripheral cards such as audio cards modems, or similar devices. Endpoints can also include docks and cables connecting consumer devices or systems to the network  100 .  
         [0063]     In another embodiment, the network can be adapted for use in a network routing device or other specialized system. In this system, power management is optimized to support peer to peer data transmission. A network router may have multiple communication devices at endpoints in a tree hierarchy that need to forward packets to one another. This system would also include a specialized processor (e.g., an application specific integrated circuit) for facilitating the forwarding of network traffic and for implementing security protocols.  
         [0064]     In one embodiment, the power management scheme is used in connection with the PCI Express® standard. All PCI Express® compatible components support an L0s power state for devices in an active state. All PCI Express® devices must report their level of support for link power management and include an active state link power management support configuration field. PCI Express® components also report L0s and L1 exit latencies. Endpoints in the PCI Express® system must also report worst-case latency that they can withstand before risking data corruption, buffer overruns or similar problems.  
         [0065]     In one embodiment, the power management system is a distributed system that uses in-band messaging in order to manage power usage. The distributed system dynamically analyzes the activity on the link to determine the transition policy. The system considers hierarchy performance and power tradeoffs when enabling low power states. Depending on in-band messaging reduces the complexity of the architecture and consequently reduces the space requirements for the system because specialized command and control lines are not needed. At least two states are defined in this system to allow the system a choice in trade offs between power reduction and performance. The L0s state allows some power savings while minimizing performance loss. The L1 state allows greater power savings at greater potential loss of performance. The power management system can be used in connection with any serial interconnect system, especially high speed serial interconnect systems. Exemplary devices that the power management system can be used with include I/O chipsets, graphics accelerators, interconnect devices and similar devices.  
         [0066]     Another embodiment would implement the distributed power management system in software (e.g., microcode or higher level computer languages) in each device on the network. A software implementation may be stored on a machine readable medium. A “machine readable” medium may include any medium that can store or transfer information. Examples of a machine readable medium include a ROM, a floppy diskette, a CD-ROM, an optical disk, a hard disk, a radio frequency (RF) link, etc.  
         [0067]     In the foregoing specification, the invention has been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes can be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. One of ordinary skill in the art, for example, would understand that the power states described could be replaced with any number or type of power states (e.g., different levels of active power states for high speed and low speed transmissions) while remaining within the scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.