Abstract:
A data link transmitter in a PCI Express device for managing PCI-Express TLPs and DLLPs. The data link transmitter includes a priority system in which a DLLP for initializing flow control has highest priority, and an idle data character has lowest priority. Various embodiments include: a DLLP for power state entrance is lower priority than the DLLP for initializing flow control; a replay TLP for retry buffer re-transmission is lower priority than the DLLP for power state entrance, and a new TLP is lower priority than the replay TLP; an Ack/Nak DLLP is lower priority than the new TLP, a DLLP for updating flow control is lower priority than the Ack/Nak TLP, and a DLLP for acknowledging the DLLP for power state entrance is lower priority than the DLLP for updating flow control; a DLLP for updating flow control is lower priority than the DLLP for power state entrance.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application 61/089,048, filed Aug. 14, 2008, which is hereby incorporated by reference herein for all purposes. 
         [0002]    This application is related to co-pending U.S. patent application Ser. No. [TBD] (VIT. 07-0075-US) entitled PCI-EXPRESS DATA LINK TRANSMITTER EMPLOYING A PLURALITY OF DYNAMICALLY SELECTABLE DATA TRANSMISSION PRIORITY RULES, which is concurrently filed herewith, and which has a common assignee and common inventor, and which is incorporated by reference herein for all purposes. 
     
    
     FIELD OF THE INVENTION 
       [0003]    The present invention relates in general to priority scheduling for serial data link transmission, and particularly within the PCI-Express serial interconnect architecture. 
       BACKGROUND OF THE INVENTION 
       [0004]    In recent years, the PCI-Express architecture has become a widely employed high-speed serial link I/O interconnect designed to transfer data between components within a computer system. PCI-Express is a serial architecture that replaces the parallel bus implementations of the PCI and PCI-X bus specification to provide platforms with greater performance, while using a much lower pin count. The PCI-Express architecture is described in the PCI-Express specifications managed and disseminated by the PCI Special Interest Group (SIG). 
         [0005]    The PCI-Express architecture describes PCI-Express devices as having three discrete logical layers: the Transaction Layer, the Data Link Layer, and the Physical Layer, each having a transmitter and receiver portion. PCI-Express uses packets to communicate information on a serial link between components. The packets include data link layer packets (DLLPs) generated and consumed by the Data Link Layer, and transaction layer packets (TLPs) generated and consumed by the Transaction Layer. The DLLPs are used to manage the serial data link and include various types such as DLLPs related to initialization, flow control on the serial link, power management, and so forth. The TLPs are used to communicate transactions between the components, such as data read and write transactions, as well as certain types of events. 
         [0006]    Multiple DLLPs and TLPs of different types may be scheduled for transmission at the same time. The PCI-Express Data Link Layer includes an arbiter that is in charge of prioritizing the packets for transmission. The arbiter determines which packet to transmit first based on one priority rule. In principle, the priority rule must ensure that all packets for transmission are finished within a regular time and not blocked by other types of packets. The priority rule may affect the transmission performance on a PCI-Express link. For example, assume a scheduled Ack DLLP is stalled waiting for another TLP transmission because the priority rule assigns Ack DLLPs a relatively low transmission priority, and during this time another Ack is scheduled for another received TLP. According to the PCI-Express specification, it is only necessary to transmit the second Ack DLLP, since the information the second Ack DLLP provides will supersede the information in the first Ack DLLP. That is, the Ack DLLPs are “collapsed” into a single Ack DLLP, which is an efficient use of bandwidth. On the other hand, if the priority rule assigns the highest priority to Ack DLLP transmissions, the Ack DLLP will transmit every time for every received TLP, which forfeits the potential benefit of collapsing the Ack DLLPs, which may waste transmission bandwidth. This example provides an illustration of the fact that the one priority rule may affect transmission performance. 
         [0007]    The PCI-Express Base Specification Rev. 1.1 specifies a recommended priority rule of scheduled transmissions, shown here:
       1. Completion of any transmission (TLP or DLLP) currently in progress (highest priority)   2. Nak DLLP transmissions   3. Ack DLLP transmissions scheduled for transmission as soon as possible due to receipt of a duplicated TLP—OR—expiration of the Ack latency timer   4. Flow Control DLLP transmissions (updateFC)   5. Retry Buffer re-transmissions   6. TLPs from the Transaction Layer   7. Flow Control DLLP transmissions (initFC 1  and initFC 2 )   8. All other DLLP transmissions (lowest priority)       
 
         [0016]    The recommended priority rule specified in the PCI-Express Base Specification lacks flexibility when a root complex (RC) is involved in more complicated transaction behavior and the recommended priority rule may result in lower performance in some circumstances. 
       BRIEF SUMMARY OF INVENTION 
       [0017]    In another aspect, the present invention provides a data link transmitter in a Peripheral Component Interconnect (PCI) Express device for managing PCI-Express Transaction Layers Packets (TLPs) and Data Link Layer Packets (DLLPs). The data link transmitter includes a priority system in which a DLLP for initializing flow control has the highest priority, and an idle data character has the lowest priority. In one aspect, a DLLP for power state entrance is lower priority than the DLLP for initializing flow control. In one aspect, a replay TLP for retry buffer re-transmission is lower priority than the DLLP for power state entrance, and a new TLP is lower priority than the replay TLP. In one aspect, an Ack/Nak DLLP is lower priority than the new TLP, a DLLP for updating flow control is lower priority than the Ack/Nak TLP, and a DLLP for acknowledging the DLLP for power state entrance is lower priority than the DLLP for updating flow control. In one aspect, a DLLP for updating flow control is lower priority than the new TLP, an Ack/Nak DLLP is lower priority than the DLLP for updating flow control, and a DLLP for acknowledging the DLLP for power state entrance is lower priority than the Ack/Nak DLLP. In one aspect, an Ack/Nak DLLP is lower priority than the DLLP for power state entrance. In one aspect, a replay TLP for retry buffer re-transmission is lower priority than the Ack/Nak DLLP, a new TLP is lower priority than the replay TLP, a DLLP for updating flow control is lower priority than the new TLP, and a DLLP for acknowledging the DLLP for power state entrance is lower priority than the DLLP for updating flow control. In one aspect, an Ack/Nak TLP, having a SEQ number larger than acknowledged TLPs, is lower priority than the DLLP for acknowledging the DLLP for power state entrance. In one aspect, a DLLP for updating flow control is lower priority than the Ack/Nak TLP, a replay TLP for retry buffer re-transmission is lower priority than the DLLP for updating flow control, a new TLP is lower priority than the replay TLP, and a DLLP for acknowledging the DLLP for power state entrance is lower priority than the new TLP. In one aspect, a DLLP for updating flow control is lower priority than the DLLP for power state entrance. In one aspect, a replay TLP for retry buffer re-transmission is lower priority than the DLLP for updating flow control, a new TLP is lower priority than the replay TLP, an Ack/Nak DLLP is lower priority than the new TLP, and a DLLP for acknowledging the DLLP for power state entrance is lower priority than the new TLP. In one aspect, an Ack/Nak DLLP is lower priority than the DLLP for updating flow control, a replay TLP for retry buffer re-transmission is lower priority than the Ack/Nak DLLP, a new TLP is lower priority than the replay TLP, and a DLLP for acknowledging the DLLP for power state entrance is lower priority than the new TLP. 
         [0018]    In another aspect, the present invention provides a priority method in a data link transmitter of a Peripheral Component Interconnect (PCI) Express device for scheduling PCI-Express Transaction Layers Packets (TLPs) and Data Link Layer Packets (DLLPs). The method includes prioritizing a DLLP for initializing flow control at the highest priority, and prioritizing an idle data character at the lowest priority. 
         [0019]    In another aspect, the present invention provides a priority system established in a data link transmitter of a Peripheral Component Interconnect (PCI) Express device for managing transmission of PCI-Express Transaction Layers Packets (TLPs) and Data Link Layer Packets (DLLPs). The priority system includes a highest priority provided for a DLLP for flow control initialization, a second priority provided for a DLLP for power state entrance, and a lowest priority provided for an idle data character. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a block diagram illustrating a PCI-Express device according to the present invention. 
           [0021]      FIG. 2  is a block diagram illustrating a portion of the PCI-Express device of  FIG. 1 , and particularly the PCI-Express data link layer transmitter of  FIG. 1 , according to the present invention. 
           [0022]      FIG. 3  is a flowchart illustrating operation of the PCI-Express data link layer transmitter of  FIG. 2  according to the present invention. 
           [0023]      FIG. 4  is a flowchart illustrating the programmability of the control register of  FIG. 2  according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    The present specification describes embodiments that, among other things, address the performance problem described above by providing a PCI-Express data link layer transmitter that defines a plurality (seven in one embodiment) of priority rules for scheduling transmissions, i.e., packets. The data link transmitter includes a programmable control register (3-bits in one embodiment) that chooses one of the plurality of priority rules to be used to select the next packet for transmission. Each priority rule may provide improved performance in a corresponding specific transaction pattern. For example, a high-end 3D PCI-Express display card requests a large amount of upstream memory read (MRd) cycles to a PCI-Express Root Complex (RC). The RC needs to transmit back-to-back completion TLPs in order to achieve better transmission performance. Thus, in a transaction pattern with a large amount of upstream MRd cycles, it is advantageous for the data link transmitter to employ one of the plurality of priority rules in which the priority of TLP transmissions is higher than DLLP transmission to increase the likelihood that back-to-back completion of TLPs occurs and minimize the possibility that the back-to-back completion of TLPs is broken by an Ack/Nak DLLP transmission. The ability to select from among multiple priority rules provided by the present invention potentially improves performance over a conventional data link transmitter that employs the single priority rule recommended in the PCI-Express Base Specification. 
         [0025]    Referring now to  FIG. 1 , a block diagram illustrating a PCI-Express device  100  according to the present invention is shown. The PCI-Express device  100  may be included within a PCI-Express root complex, endpoint, switch, or bridge. The PCI-Express device  100  includes a transaction layer transmitter  102  coupled to a data link layer transmitter  104 , and a physical layer transmitter  106  coupled to the data link layer transmitter  104 . The PCI-Express device  100  includes a transaction layer receiver  112  coupled to a data link layer receiver  114 , and a physical layer receiver  116  coupled to the data link layer receiver  114 . The physical layer transmitter  106  and physical layer receiver are also coupled to a PCI-Express serial link  122  upon which they transmit and receive, respectively, PCI-Express packets (DLLPs and TLPs) with another PCI-Express device coupled to the other end of the serial link  122 . 
         [0026]    The PCI-Express specifications specifies the PCI-Express architecture in terms of three discrete logical layers, namely the transaction layer, data link layer, and physical layer, each of which is divided into a transmitter and receiver section. The PCI-Express specifications describe in detail the various functions of these logical layers. Elements  102  through  116  correspond to these logical layers and perform the functions described in the PCI-Express specifications. However, unlike the data link layer transmitter described in the PCI-Express specifications that always selects a packet to transmit using a single priority rule, the data link layer transmitter  104  of  FIG. 1  according to the present invention is modified relative to the data link layer transmitter described in the PCI-Express specifications such that the data link layer transmitter  104  of  FIG. 1  is capable of employing one of a plurality of different priority rules for selecting a packet from among multiple scheduled DLLPs and TLPs, and the particular one of the plurality of priority rules used to select the packet is itself selectable. Advantageously, the particular priority rule used to select the packet is dynamically selectable during operation of the PCI-Express device  100  within a system by programming a control register that specifies the one of the plurality of priority rules to be used. 
         [0027]    Although elements  102  through  116  of  FIG. 1  are described as logical entities, they are embodied in hardware circuits to perform the functions described herein, which according to some embodiments, may be performed in part by stored programs executing on one or more programmable processors. 
         [0028]    Referring now to  FIG. 2 , a block diagram illustrating a portion of the PCI-Express device  100  of  FIG. 1 , and particularly the PCI-Express data link layer transmitter  104  of  FIG. 1 , according to the present invention is shown. The PCI-Express device  100  includes a pool of scheduled packets  202 . The pool of scheduled packets  202  comprises all the DLLPs and TLPs generated by the transaction layer transmitter  102  and data link layer transmitter  104  and that are ready to be transmitted on the serial link  122 . In one embodiment, the pool of scheduled packets  202  comprises a plurality of queues scheduled packets. Each queue holds scheduled packets for a different packet type. Specifically, the different packet types associated with the queues correspond to the various packet types differentiated by the plurality of priority rules discussed below. Thus, for example, according to one embodiment, a different queue exists for each of the packet types specified in Table 1 below, which is effectively a superset of the various packet types specified in Tables 2 through 7 since Priority Rule 1 of Table differentiates two types of packet that are considered as a single type in Tables 2 through 7. In one embodiment, the packets within a queue are transmitted on a first-in-first-out basis relative to packets of the same type. 
         [0029]    The data link layer transmitter  104  includes a plurality of priority rule arbiters  204 - 1 ,  204 - 2 ,  204 - 3 ,  204 - 4 ,  204 - 5 ,  204 - 6 , and  204 - 7 , referred to collectively as priority rule arbiters  204 , or arbiters  204 . Each of the arbiters  204  is capable of viewing the contents of the pool of scheduled packets  202 . Each of the arbiters  204  selects one packet from the pool of scheduled packets  202  for transmission on the serial link  122  according to the priority rule associated with the respective arbiter  204 . According to one embodiment: arbiter  204 - 1  employs Priority Rule 1 of Table 1; arbiter  204 - 2  employs Priority Rule 2 of Table 2; arbiter  204 - 3  employs Priority Rule 3 of Table 3; arbiter  204 - 4  employs Priority Rule 4 of Table 4; arbiter  204 - 5  employs Priority Rule 5 of Table 5; arbiter  204 - 6  employs Priority Rule 6 of Table 6; and arbiter  204 - 7  employs Priority Rule 7 of Table 7. Each arbiter  204  outputs the selected packet on its respective output  224 - 1  through  224 - 7 , referred to collectively as  224 . 
         [0030]    The data link layer transmitter  104  also includes a 7-to-1 multiplexer  206 . The multiplexer  206  receives on its seven inputs the seven outputs  224 - 1  through  224 - 7 . A control register  208  provides a control signal  226  to a control input of the multiplexer  206  to control selection of which input  224 - 1  through  224 - 7  is provided on the multiplexer  206  output as the final granted transmission  212 , i.e., as the packet that will be transmitted on the serial link  122 . In one embodiment, the control register  208  is a 3-bit register capable of holding an encoded value that selects one of the arbiter  204  outputs  224 . In one embodiment, the control register  208  is programmable, as discussed below with respect to  FIG. 4 , to enable dynamic selection of the one of the plurality of priority rules during operation of the PCI-Express device  100 . 
         [0031]    Tables 1 through 7 shown below specify the seven priority rules used by the seven priority rule arbiters  204  of  FIG. 2  according to one embodiment. Definitions of the packet types can be found in PCI Express Base Specification Revision 1.1. The following briefly depicts their meanings.
       1. Flow Control DLLP transmissions, such as initFC 1  and initFC 2 , are those DLLPs for initializing flow control.   2. Power Management DLLP transmissions, such as PM_Enter_L 1 , PM_Enter_L 23  and PM_Active_State_Request_L 1 , are those DLLPs for power state entrance.   3. Flow Control DLLP transmission—UpdateFC is a DLLP for updating flow control.   4. Power Management DLLP transmission—PM_Request_Ack is a DLLP for acknowledging the Power Management DLLP transmissions.       
 
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Priority Rule 1. 
               
             
          
           
               
                 Priority 
                 Packet Type 
               
               
                   
               
               
                 1 
                 Flow Control DLLP transmissions (initFC1 and initFC2) 
               
               
                 2 
                 Power Management DLLP transmissions (PM_Enter_L1, 
               
               
                   
                 PM_Enter_L23 and PM_Active_State_Request_L1) 
               
               
                 3 
                 Ack/Nak DLLP transmissions (caused by receipt of a duplicated 
               
               
                   
                 TLP -OR-receipt of a bad TLP -OR- expiration of the 
               
               
                   
                 Ack latency timer) 
               
               
                 4 
                 Replay TLP transmissions (Retry Buffer re-transmissions) 
               
               
                 5 
                 New TLP transmissions (TLPs from the Transaction Layer 
               
               
                   
                 include Completion TLPs) 
               
               
                 6 
                 Flow control DLLP transmissions (UpdateFC) 
               
               
                 7 
                 Power Management DLLP transmissions (PM_Request_Ack) 
               
               
                 8 
                 Ack/Nak DLLP transmissions (receipt of a TLP whose SEQ 
               
               
                   
                 number is larger than Ack&#39;ed TLP SEQ number) 
               
               
                 9 
                 Transmission Idle (No scheduled transmissions) 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Priority Rule 2. 
               
             
          
           
               
                 Priority 
                 Packet Type 
               
               
                   
               
               
                 1 
                 Flow Control DLLP transmissions (initFC1 and initFC2) 
               
               
                 2 
                 Power Management DLLP transmissions (PM_Enter_L1, 
               
               
                   
                 PM_Enter_L23 and PM_Active_State_Request_L1) 
               
               
                 3 
                 Replay TLP transmissions (Retry Buffer re-transmissions) 
               
               
                 4 
                 New TLP transmissions (TLPs from the Transaction Layer 
               
               
                   
                 include Completion TLPs) 
               
               
                 5 
                 Ack/Nak DLLP transmissions (caused by receipt of a duplicated 
               
               
                   
                 TLP -OR-receipt of a bad TLP -OR- expiration of the Ack 
               
               
                   
                 latency timer -OR- receipt of a TLP whose SEQ number is larger 
               
               
                   
                 than Ack&#39;ed TLP SEQ number) 
               
               
                 6 
                 Flow control DLLP transmissions (UpdateFC) 
               
               
                 7 
                 Power Management DLLP transmissions (PM_Request_Ack) 
               
               
                 8 
                 Transmission Idle (No scheduled transmissions) 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Priority Rule 3. 
               
             
          
           
               
                 Priority 
                 Packet Type 
               
               
                   
               
               
                 1 
                 Flow Control DLLP transmissions (initFC1 and initFC2) 
               
               
                 2 
                 Power Management DLLP transmissions (PM_Enter_L1, 
               
               
                   
                 PM_Enter_L23 and PM_Active_State_Request_L1) 
               
               
                 3 
                 Replay TLP transmissions (Retry Buffer re-transmissions) 
               
               
                 4 
                 New TLP transmissions (TLPs from the Transaction Layer 
               
               
                   
                 include Completion TLPs) 
               
               
                 5 
                 Flow control DLLP transmissions (UpdateFC) 
               
               
                 6 
                 Ack/Nak DLLP transmissions (caused by receipt of a duplicated 
               
               
                   
                 TLP -OR-receipt of a bad TLP -OR- expiration of the Ack 
               
               
                   
                 latency timer -OR- receipt of a TLP whose SEQ number is larger 
               
               
                   
                 than Ack&#39;ed TLP SEQ number) 
               
               
                 7 
                 Power Management DLLP transmissions (PM_Request_Ack) 
               
               
                 8 
                 Transmission Idle (No scheduled transmissions) 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Priority Rule 4. 
               
             
          
           
               
                 Priority 
                 Packet Type 
               
               
                   
               
               
                 1 
                 Flow Control DLLP transmissions (initFC1 and initFC2) 
               
               
                 2 
                 Power Management DLLP transmissions (PM_Enter_L1, 
               
               
                   
                 PM_Enter_L23 and PM_Active_State_Request_L1) 
               
               
                 3 
                 Ack/Nak DLLP transmissions (caused by receipt of a duplicated 
               
               
                   
                 TLP -OR-receipt of a bad TLP -OR- expiration of the Ack 
               
               
                   
                 latency timer -OR- receipt of a TLP whose SEQ number is 
               
               
                   
                 larger than Ack&#39;ed TLP SEQ number) 
               
               
                 4 
                 Replay TLP transmissions (Retry Buffer re-transmissions) 
               
               
                 5 
                 New TLP transmissions (TLPs from the Transaction Layer 
               
               
                   
                 include Completion TLPs) 
               
               
                 6 
                 Flow control DLLP transmissions (UpdateFC) 
               
               
                 7 
                 Power Management DLLP transmissions (PM_Request_Ack) 
               
               
                 8 
                 Transmission Idle (No scheduled transmissions) 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE 5 
               
             
             
               
                   
               
               
                 Priority Rule 5. 
               
             
          
           
               
                 Priority 
                 Packet Type 
               
               
                   
               
               
                 1 
                 Flow Control DLLP transmissions (initFC1 and initFC2) 
               
               
                 2 
                 Power Management DLLP transmissions (PM_Enter_L1, 
               
               
                   
                 PM_Enter_L23 and PM_Active_State_Request_L1) 
               
               
                 3 
                 Ack/Nak DLLP transmissions (caused by receipt of a duplicated 
               
               
                   
                 TLP -OR-receipt of a bad TLP -OR- expiration of the Ack 
               
               
                   
                 latency timer -OR- receipt of a TLP whose SEQ number is 
               
               
                   
                 larger than Ack&#39;ed TLP SEQ number) 
               
               
                 4 
                 Flow control DLLP transmissions (UpdateFC) 
               
               
                 5 
                 Replay TLP transmissions (Retry Buffer re-transmissions) 
               
               
                 6 
                 New TLP transmissions (TLPs from the Transaction Layer 
               
               
                   
                 include Completion TLPs) 
               
               
                 7 
                 Power Management DLLP transmissions (PM_Request_Ack) 
               
               
                 8 
                 Transmission Idle (No scheduled transmissions) 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE 6 
               
             
             
               
                   
               
               
                 Priority Rule 6. 
               
             
          
           
               
                 Priority 
                 Packet Type 
               
               
                   
               
               
                 1 
                 Flow Control DLLP transmissions (initFC1 and initFC2) 
               
               
                 2 
                 Power Management DLLP transmissions (PM_Enter_L1, 
               
               
                   
                 PM_Enter_L23 and PM_Active_State_Request_L1) 
               
               
                 3 
                 Flow control DLLP transmissions (UpdateFC) 
               
               
                 4 
                 Replay TLP transmissions (Retry Buffer re-transmissions) 
               
               
                 5 
                 New TLP transmissions (TLPs from the Transaction Layer 
               
               
                   
                 include Completion TLPs) 
               
               
                 6 
                 Ack/Nak DLLP transmissions (caused by receipt of a duplicated 
               
               
                   
                 TLP -OR-receipt of a bad TLP -OR- expiration of the Ack 
               
               
                   
                 latency timer -OR- receipt of a TLP whose SEQ number is 
               
               
                   
                 larger than Ack&#39;ed TLP SEQ number) 
               
               
                 7 
                 Power Management DLLP transmissions (PM_Request_Ack) 
               
               
                 8 
                 Transmission Idle (No scheduled transmissions) 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE 7 
               
             
             
               
                   
               
               
                 Priority Rule 7. 
               
             
          
           
               
                 Priority 
                 Packet Type 
               
               
                   
               
               
                 1 
                 Flow Control DLLP transmissions (initFC1 and initFC2) 
               
               
                 2 
                 Power Management DLLP transmissions (PM_Enter_L1, 
               
               
                   
                 PM_Enter_L23 and PM_Active_State_Request_L1) 
               
               
                 3 
                 Flow control DLLP transmissions (UpdateFC) 
               
               
                 4 
                 Ack/Nak DLLP transmissions (caused by receipt of a duplicated 
               
               
                   
                 TLP -OR-receipt of a bad TLP -OR- expiration of the Ack 
               
               
                   
                 latency timer -OR- receipt of a TLP whose SEQ number is 
               
               
                   
                 larger than Ack&#39;ed TLP SEQ number) 
               
               
                 5 
                 Replay TLP transmissions (Retry Buffer re-transmissions) 
               
               
                 6 
                 New TLP transmissions (TLPs from the Transaction Layer 
               
               
                   
                 include Completion TLPs) 
               
               
                 7 
                 Power Management DLLP transmissions (PM_Request_Ack) 
               
               
                 8 
                 Transmission Idle (No scheduled transmissions) 
               
               
                   
               
             
          
         
       
     
         [0036]    Referring now to  FIG. 3 , a flowchart illustrating operation of the PCI-Express data link layer transmitter  104  of  FIG. 2  according to the present invention is shown. Flow begins at block  302 . 
         [0037]    At block  302 , the data link layer transmitter  104  is ready to transmit a packet (i.e., DLLP or TLP from the pool of scheduled packets  202 ) on the PCI-Express serial link  122  of  FIG. 1 . Generally, the data link layer transmitter  104  is permitted to transmit at will. However, certain restraints, such as initialization or flow control restraints, may require the data link layer transmitter  104  to wait to transmit a packet. Flow proceeds to block  304 . 
         [0038]    At block  304 , each of the seven arbiters  204  of  FIG. 2  examines the packets scheduled for transmission from the pool  202  of  FIG. 2  and selects one of the packets for transmission. Each arbiter  204  uses its respective priority rule (i.e., one of the priority rules from Tables 1 through 7 above) to select the packet. Each arbiter  204  outputs the selected packet on its respective output  224 - 1  through  224 - 7  of  FIG. 2 , each of which is provided to the multiplexer  206  of  FIG. 2 . Flow proceeds to block  306 . 
         [0039]    At block  306 , the multiplexer  206  selects one of the inputs  224 - 1  through  224 - 7  as the final granted packet for transmission  212 . Specifically, the multiplexer  206  selects as the final granted packet for transmission  212  the one of the packets  224 - 1  through  224 - 7  specified by the output  226  of the control register  208 . Flow ends at block  306 . 
         [0040]    Referring now to  FIG. 4 , a flowchart illustrating the programmability of the control register  208  of  FIG. 2  according to an embodiment of the present invention is shown. Flow begins at block  402 . 
         [0041]    At block  402 , an entity predicts that a particular Priority Rule of Table 1 through 7 will yield the best performance for a mix, or stream, of packet types that the data link layer transmitter  104  of  FIG. 2  will be transmitting on the serial link  122  in the near future. The predicting entity may be system software, such as a device driver executing on the system CPU. The predicting entity may be the paired PCI-Express device coupled to the PCI-Express device  100  of  FIG. 1  by the serial link  122 , such as a video controller, storage device controller, network interface controller, and so forth; or, the predicting entity may be the software controlling the paired PCI-Express device. For example, the PCI-Express device  100  of  FIG. 1  may be a root complex coupled to a PCI-Express video card, and the video controller, or software controlling the video controller, knows that soon the video controller will be requesting the root complex to transmit a large number of Memory Read Completion TLPs to the video controller. Furthermore, the video controller, or software controlling the video controller, knows that one of the particular Priority Rules of Table 1 through 7 will likely yield the best performance for a stream of packets predominated by TLPs, such as Memory Read Completion TLPs. Flow proceeds to block  404 . 
         [0042]    At block  404 , the predicting entity programs the control register  208  of  FIG. 2  with the specific value to select the appropriate Priority Rule that is likely to provide the best performance for the mix of packet type transmissions that the data link layer transmitter  104  will be performing in the near future. Flow ends at block  404 . 
         [0043]    As may be observed from the forgoing, the flexibility provided by the embodiments described herein through the ability to select from among a plurality of priority rules to select a packet to transmit from among multiple DLLPs and TLPs of different types may result in better performance than a PCI-Express data link layer transmitter that always employs the same single priority rule. This may be particularly true in situations in which a PCI-Express Root Complex is involved with complicated transaction streams and/or different types of PCI-Express end-point devices. 
         [0044]    Although various embodiments have been described in which the control register of the PCI-Express data link transmitter is programmed by a CPU connected to the root complex, the PCI-Express data link transmitter may include a PCI-Express slave interface to accept PCI Configuration write cycles to enable other PCI-Express masters to program the control register. Furthermore, although embodiments have been described in which the PCI-Express data link transmitter is within a PCI-Express root complex, the data link transmitter may be employed within any PCI-Express device, including a PCI-Express endpoint, switch, or bridge. Still further, although embodiments have been described in which the data link transmitter may employ a particular number of priority rules (seven according to one embodiment), other numbers of priority rules are contemplated, and the invention is not limited to a particular number. Finally, although embodiments have been described particular priority rules, other priority rules are contemplated, and the invention is not limited to the particular priority rules described. 
         [0045]    While various embodiments of the present invention have been described herein, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant computer arts that various changes in form and detail can be made therein without departing from the scope of the invention. For example, software can enable, for example, the function, fabrication, modeling, simulation, description and/or testing of the apparatus and methods described herein. This can be accomplished through the use of general programming languages (e.g., C, C++), hardware description languages (HDL) including Verilog HDL, VHDL, and so on, or other available programs. Such software can be disposed in any known computer usable medium such as semiconductor, magnetic disk, or optical disc (e.g., CD-ROM, DVD-ROM, etc.). Embodiments of the apparatus and method described herein may be included in a semiconductor intellectual property core, such as a microprocessor core or chipset core (e.g., embodied in HDL) and transformed to hardware in the production of integrated circuits. Additionally, the apparatus and methods described herein may be embodied as a combination of hardware and software. Thus, the present invention should not be limited by any of the exemplary embodiments described herein, but should be defined only in accordance with the following claims and their equivalents. Specifically, the present invention may be implemented within a microprocessor device or chipset device which may be used in a general purpose computer. Finally, those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiments as a basis for designing or modifying other structures for carrying out the same purposes of the present invention without departing from the scope of the invention as defined by the appended claims.