Abstract:
A USB port transmitter includes a plurality of arbiters, each employing a distinct priority rule to select one USB transmission from among multiple scheduled USB transmissions based on their types. A selector selects one of the arbiters to select the one USB transmission from among the multiple scheduled USB transmissions. A programmable storage element controls the selector to select the one arbiter. In one embodiment, at least a first arbiter prioritizes header/data packets higher than link commands, and at least a second arbiter prioritizes link commands higher than header/data packets. In one embodiment, at least one arbiter prioritizes flow control and power management link commands higher than header/data packets. In one embodiment, at least a first of the arbiters prioritizes USB LGO_Ux link commands higher than USB LAU/LXU link commands, and at least a second arbiter prioritizes USB LAU/LXU link commands higher than USB LGO_Ux link commands.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates in general to priority scheduling for serial data link transmission, and particularly within the USB architecture. 
       BACKGROUND OF THE INVENTION 
       [0002]    In recent years, the USB architecture has become a widely employed high-speed serial cable bus designed that supports data exchange between a host computer and a wide range of simultaneously accessible peripherals. The attached peripherals share USB bandwidth through a host-scheduled, token-based protocol. More specifically, the USB 3.0 architecture, specified in the USB 3.0 Specification, Revision 1.0, Nov. 12, 2008, managed and disseminated by the USB Implementers Forum, Inc., includes highly desirable features over previous USB architectures, including the SuperSpeed protocol. 
         [0003]    According to the USB 3.0 Specification, a USB port transmitter can transmit various types of transmissions, including link commands, header packets, data packets, SKP ordered sets, Training Sequence (TS) ordered sets, and Bit Error Rate Test (BERT) ordered sets. USB devices transmit these transmissions to one another on serial links that connect ports of the USB devices. When multiple link command, header packet, data packet, and SKP ordered set transmissions are scheduled to be transmitted by a USB port in the U0 operational state (the normal operational state), an arbiter is needed to be in charge of prioritizing the transmissions to be transmitted on the USB link to which the port is connected. Sections 10.7.5 and 10.7.9 of the USB 3.0 Specification include FIGS. 10-16 and 10-18, respectively, which describe the functional requirements of a USB port transmitter state machine. From these Figures, a priority rule of scheduled transmissions that may be inferred from these sections of the USB 3.0 Specification is as follows:
       1. Link Command transmissions (All Link Commands) and Skip Ordered Set (if required)—highest priority   2. Header Packet transmission (Transaction Packet (TP), Link Management Packet (LMP), and Isochronous Timestamp Packet (ITP)) and Skip Ordered Set (if required)—next highest priority   3. Data Packet transmissions and Skip Ordered Set—lowest priority       
 
         [0007]    The various types of transmissions referenced above are defined in the USB 3.0 Specification in the Terms and Abbreviations section on pages 2-1 to 2-7. 
         [0008]    A USB port arbiter that determines which scheduled transmission to transmit first based on the single priority rule inferred from the USB 3.0 Specification may not exhibit good performance and transmission behavior in some circumstances. 
       BRIEF SUMMARY OF INVENTION 
       [0009]    In one aspect, the present invention provides a Universal Serial Bus (USB) port transmitter for transmitting USB transmissions on a USB link. The USB port transmitter includes a plurality of arbiters, each configured to employ a distinct priority rule to select one USB transmission from among multiple scheduled USB transmissions based on the types of the multiple scheduled USB transmissions. The USB port transmitter also includes a selector, coupled to the plurality of arbiters. The selector is configured to select one of the plurality of arbiters to select the one USB transmission from among the multiple scheduled USB transmissions to transmit on the USB link. In one embodiment, a programmable storage element provides a value to the selector to control the selector to select the one of the plurality of arbiters. In one embodiment, the distinct priority rule employed by at least a first of the plurality of arbiters prioritizes header/data packet type transmissions higher than link command type transmissions, and the distinct priority rule employed by at least a second of the plurality of arbiters prioritizes link command type transmissions higher than header/data packet type transmissions. In one embodiment, the distinct priority rule employed by the at least a second of the plurality of arbiters prioritizes flow control and power management link command type transmissions higher than header/data packet type transmissions. In one embodiment, the distinct priority rule employed by at least a first of the plurality of arbiters prioritizes USB LGO_Ux link command type transmissions higher than USB LAU/LXU link command type transmissions, and the distinct priority rule employed by at least a second of the plurality of arbiters prioritizes USB LAU/LXU link command type transmissions higher than USB LGO_Ux link command type transmissions. 
         [0010]    In another aspect, the present invention provides a method for a USB port to transmit USB transmissions on a USB link. The method includes selecting one priority rule from among a plurality of distinct priority rules. Each of the plurality of distinct priority rules includes a distinct priority for each of a plurality of USB transmission types. The method also includes using the selected one of the plurality of distinct priority rules to select one USB transmission from among multiple scheduled USB transmissions for transmission on the USB link. 
         [0011]    In another aspect, the present invention provides a USB port transmitter for transmitting USB transmissions on a USB link. The USB port transmitter includes a first plurality of arbiters, each configured to employ a distinct priority rule to select one USB link command transmission from among multiple scheduled USB link command transmissions based on the types of the multiple scheduled USB link command transmissions. The USB port transmitter also includes a first selector, coupled to the first plurality of arbiters. The first selector is configured to select one of the first plurality of arbiters to select the one USB link command transmission. The USB port transmitter also includes a second plurality of arbiters, each configured to employ a distinct priority rule to select one USB transmission from among multiple scheduled USB transmissions based on the types of the multiple scheduled USB transmissions. The multiple scheduled USB transmissions include the one USB link command transmission selected by the first selector and multiple header/data packet transmissions. The USB port transmitter also includes a second selector, coupled to the second plurality of arbiters. The second selector is configured to select one of the second plurality of arbiters to select the one USB transmission from among the multiple scheduled USB transmissions to transmit on the USB link. 
         [0012]    In another aspect, the present invention provides a method for a USB port to transmit USB transmissions on a USB link. The method includes selecting one priority rule from among a first plurality of distinct priority rules. Each of the first plurality of distinct priority rules includes a distinct priority for each of a plurality of USB link command transmission types. The method also includes using the selected one of the first plurality of distinct priority rules to select one USB link command transmission from among multiple scheduled USB link command transmissions. The method also includes selecting one priority rule from among a second plurality of distinct priority rules. Each of the second plurality of distinct priority rules includes a distinct priority for each of a plurality of USB transmission types. The method also includes using the selected one of the second plurality of distinct priority rules to select one USB transmission from among multiple scheduled USB transmissions to transmit on the USB link. The multiple scheduled USB transmissions include the selected one USB link command transmission and multiple header/data packet transmissions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a block diagram illustrating a USB device according to the present invention. 
           [0014]      FIG. 2  is a block diagram illustrating a portion of the USB device of  FIG. 1 , and particularly the USB port transmitter of  FIG. 1 , according to the present invention. 
           [0015]      FIG. 3  is a flowchart illustrating operation of the USB port transmitter of  FIG. 2  according to the present invention. 
           [0016]      FIG. 4  is a flowchart illustrating the programmability of the first-level control register and the second-level control register of  FIG. 2  according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    The present specification describes embodiments that potentially provide improved performance and transmission characteristics by providing a USB port transmitter that defines a plurality of priority rules for scheduling transmissions. Furthermore, the USB port transmitter may employ a multi-level approach to the plurality of priority rules. The USB port transmitter includes one or more programmable control registers that choose from among the plurality of priority rules to be used to select the next transmission to transmit on the USB link. Each priority rule, or combination thereof, may provide improved performance in a corresponding specific transmission pattern. The ability to select from among multiple priority rules provided by the present invention potentially improves performance over a conventional USB port transmitter that employs a single priority rule, such as the priority rule inferred above from the USB 3.0 Specification. 
         [0018]    Referring now to  FIG. 1 , a block diagram illustrating a USB device  100  according to the present invention is shown. The USB device  100  may be included within a USB host, USB hub, or USB peripheral device. The USB device  100  includes a USB protocol layer  108  in communication with a USB link layer  112 . The USB link layer  112  is in communication with a USB physical layer  114 . The USB device  100  includes a USB port  106 , which includes a port transmitter  104  and port receiver  102 . The link layer  112  comprises a logical portion of the USB port  106 , and the physical layer  114  comprises a physical portion of the USB port  106 . The USB protocol layer  108  and USB link layer  112  collectively generate USB transmissions (i.e., USB ordered sets, link commands, and header/data packets) to be transmitted by the USB port transmitter  104  to its link partner at the other end of the USB link  122 . Advantageously, the USB port transmitter  104  of  FIG. 1  according to the present invention is modified relative to a conventional USB port transmitter  104  such that the USB port transmitter  104  of  FIG. 1  is capable of employing one of a plurality of different priority rules for selecting a transmission from among multiple scheduled transmissions, and the particular priority rule used to select the transmission is itself selectable. Advantageously, the particular priority rule used to select the transmission is dynamically selectable during operation of the USB device  100  within a system by programming a control register that specifies the priority rule to be used. 
         [0019]    Although some of the elements 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. 
         [0020]    Referring now to  FIG. 2 , a block diagram illustrating a portion of the USB device  100  of  FIG. 1 , and particularly the USB port transmitter  104  of  FIG. 1 , according to the present invention is shown. The USB device  100  includes a pool of scheduled USB SKP ordered set (SKP-OS), header/data packet, and special link command transmissions  202 . The USB special link command type includes the LUP link command described in sections 7.2.2.2 and 7.5.6.1 of the USB 3.0 Specification, which is sent by an upstream port every 10 microseconds when there are no packets or other link commands to be transmitted, and which may only be transmitted in the U0 operational state. The USB special link command type also includes the LDN link command described in Engineering Change Notice of the USB 3.0 Specification in Apr. 4, 2009, which is sent by a downstream port every 10 microseconds when there are no packets or other link commands to be transmitted, and which may only be transmitted in the U0 operational state. USB SKP ordered set and header/data packet transmissions are also described in detail in the USB 3.0 Specification. The USB device  100  also includes a pool of scheduled USB normal link command transmissions  232 . The USB normal link command transmission types include the LGOOD_n, LBAD, LCRD_x, LGO_Ux, LAU, LXU, and LPMA link commands described in detail in the USB 3.0 Specification. In one embodiment, LRTY type link commands inherit the same priority as their associated header packet re-transmissions. The pools of scheduled transmissions  202  and  232  comprise the USB transmissions that are ready to be transmitted on the USB link  122 . In one embodiment, each pool of scheduled transmissions  202  and  232  comprises a plurality of queues of scheduled transmissions each of a different transmission type. Specifically, the different transmission types associated with the queues correspond to the various transmission types differentiated by the plurality of priority rules discussed below. Thus, for example, according to one embodiment, a different queue exists within the pool  232  for each of the normal link command transmission types specified in Tables 3/4 below, and a different queue exists within the pool  202  for each of the USB transmission types specified in Tables 1/2 below, except that there is not a queue for the normal link command transmission type, since the queues for the normal link commands are within the pool  232  and the granted normal link command  258  is received from the first-level multiplexer  236 . In one embodiment, the transmissions within a queue are transmitted on a first-in-first-out basis relative to transmissions of the same type. 
         [0021]    The USB port transmitter  104  includes a plurality of first-level priority rule arbiters  234 - 3  and  234 - 4 , referred to collectively as first-level priority rule arbiters  234 , or first-level arbiters  234 . Each of the first-level arbiters  234  is capable of viewing the contents of the pool of scheduled normal link command transmissions  232 . Each of the first-level arbiters  234  selects one transmission from the pool of scheduled normal link command transmissions  232  according to the priority rule associated with the respective first-level arbiter  234 . According to one embodiment: first-level arbiter  234 - 3  employs Priority Rule 3 of Table 3 and arbiter  234 - 4  employs Priority Rule 4 of Table 4. Each first-level arbiter  234  outputs the selected normal link command on its respective output  254 - 3  and  254 - 4 , referred to collectively as  254 . 
         [0022]    The USB port transmitter  104  also includes a 2-to-1 first-level multiplexer  236 . The first-level multiplexer  236  receives on its two inputs the two outputs  254 - 3  and  254 - 4 . A first-level control register  238  provides a first-level control signal  256  to a control input of the first-level multiplexer  236  to control selection of which input  254  is provided on the first-level multiplexer  236  output as the granted normal link command transmission  258 . In one embodiment, the first-level control register  238  is a 1-bit register capable of holding a value that selects one of the first-level arbiter  234  outputs  254 . In one embodiment, the first-level control register  238  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 USB device  100 . 
         [0023]    The USB port transmitter  104  includes a plurality of second-level priority rule arbiters  204 - 1  and  204 - 2 , referred to collectively as second-level priority rule arbiters  204 , or second-level arbiters  204 . Each of the second-level arbiters  204  is capable of viewing the contents of the pool of scheduled transmissions  202  as well as the granted normal link command  258 . Each of the second-level arbiters  204  selects one transmission from the granted normal link command  258  and the pool of scheduled transmissions  202  for transmission on the USB link  122  according to the priority rule associated with the respective second-level arbiter  204 . According to one embodiment: second-level arbiter  204 - 1  employs Priority Rule 1 of Table 1 and second-level arbiter  204 - 2  employs Priority Rule 2 of Table 2. Each second-level arbiter  204  outputs the selected transmission on its respective second-level output  224 - 1  and  224 - 2 , referred to collectively as  224 . 
         [0024]    The USB port transmitter  104  also includes a 2-to-1 second-level multiplexer  206 . The second-level multiplexer  206  receives on its two inputs the two outputs  224 - 1  and  224 - 2 . A second-level control register  208  provides a second-level control signal  226  to a control input of the second-level multiplexer  206  to control selection of which input  224 - 1  or  224 - 2  is provided on the second-level multiplexer  206  output as the final granted transmission  212 , i.e., as the transmission that will be transmitted on the USB link  122 . In one embodiment, the second-level control register  208  is a 1-bit register capable of holding a value that selects one of the second-level arbiter  204  outputs  224 . In one embodiment, the second-level 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 USB device  100 . 
         [0025]    Tables 1 through 4 shown below specify the four priority rules used by the four priority rule arbiters  204  of  FIG. 2  according to one embodiment. With reference to Tables 1 through 4 below, priority 1 is the highest priority and priority 4 is the lowest priority. SKP ordered sets are described in section 6.3.5 on page 6-8, Table 6-1 of the USB 3.0 Specification; header/data packets are described in the Terms and Abbreviation section on pages 2-1 to 2-7 of the USB 3.0 Specification; normal link commands (LGOOD_n, LBAD, LCRD_x, LGO_Ux, LAU, LXU, LPMA, LTRY) and special link commands (LUP, LDN) are described in section 7.2.2.2 on pages 7-11 to 7.14 of the USB 3.0 Specification. 
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Priority Rule 1. 
               
             
          
           
               
                 Priority 
                 Transmission Type 
               
               
                   
               
               
                 1 
                 SKP ordered sets 
               
               
                 2 
                 Normal link commands (flow control [LGOOD_n, LBAD, 
               
               
                   
                 LCRD_x] and power management [LGO_Ux, 
               
               
                   
                 LAU, LXU, LPMA]) 
               
               
                 3 
                 Header/Data packets 
               
               
                 4 
                 Special link commands (LUP or LDN) 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Priority Rule 2. 
               
             
          
           
               
                 Priority 
                 Transmission Type 
               
               
                   
               
               
                 1 
                 SKP ordered sets 
               
               
                 2 
                 Header/Data packets 
               
               
                 3 
                 Normal link commands (flow control [LGOOD_n, LBAD, 
               
               
                   
                 LCRD_x] and power management [LGO_Ux, LAU, 
               
               
                   
                 LXU, LPMA]) 
               
               
                 4 
                 Special link commands (LUP or LDN) 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Priority Rule 3. 
               
             
          
           
               
                 Priority 
                 Normal Link Command Type 
               
               
                   
               
               
                 1 
                 LGOOD_n 
               
               
                 2 
                 LBAD 
               
               
                 3 
                 LCRD_x 
               
               
                 4 
                 LGO_Ux 
               
               
                 5 
                 LAU/LXU 
               
               
                 6 
                 LPMA 
               
               
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Priority Rule 4. 
               
             
          
           
               
                 Priority 
                 Normal Link Command Type 
               
               
                   
               
               
                 1 
                 LGOOD_n 
               
               
                 2 
                 LBAD 
               
               
                 3 
                 LCRD_x 
               
               
                 4 
                 LAU/LXU 
               
               
                 5 
                 LGO_Ux 
               
               
                 6 
                 LPMA 
               
               
                   
               
             
          
         
       
     
         [0026]    In one embodiment, Priority Rules 1 and 3 are the default priority rules. 
         [0027]    Referring now to  FIG. 3 , a flowchart illustrating operation of the USB port transmitter  104  of  FIG. 2  according to the present invention is shown. Flow begins at block  302 . 
         [0028]    At block  302 , the USB port transmitter  104  is ready to transmit a transmission from the pool of scheduled SKP ordered set, header/data packet, and special link command transmissions  202  and from the pool of normal link command transmissions  232  on the USB USB link  122  of  FIG. 1 . Generally, the USB port transmitter  104  is permitted to transmit at will. However, certain restraints, such as initialization, flow control, or low power state restraints, may require the USB port transmitter  104  to wait to transmit a transmission. Flow proceeds to block  304 . 
         [0029]    At block  304 , each of the first-level arbiters  234  of  FIG. 2  examines the normal link commands scheduled for transmission from the pool  232  of  FIG. 2  and selects one of the normal link commands for transmission. Each first-level arbiter  234  uses its respective priority rule (i.e., one of the priority rules from Tables 3 and 4 above) to select its normal link command. Each first-level arbiter  234  outputs the selected normal link command on its respective output  254 - 3  and  254 - 4  of  FIG. 2 , each of which is provided to the first-level multiplexer  236  of  FIG. 2 . Flow proceeds to block  306 . 
         [0030]    At block  306 , the first-level multiplexer  236  selects one of the inputs  254 - 3  or  254 - 4  as the granted normal link command  258 . Specifically, the first-level multiplexer  236  selects as the granted normal link command  258  the one of the first-level arbiters  234 - 3  or  234 - 4  specified by the output  256  of the first-level control register  238 . The granted normal link command transmission  258  is provided to each of the second-level arbiters  204 . Flow proceeds to block  308 . 
         [0031]    At block  308 , each of the second-level arbiters  204  of  FIG. 2  examines the granted normal link command transmission  258  and the SKP ordered set, header/data packet, and special link command transmissions scheduled for transmission from the pool  202  of  FIG. 2  and selects one of the transmissions for transmission. Each second-level arbiter  204  uses its respective priority rule (i.e., one of the priority rules from Tables 1 and 2 above) to select the transmission. Each second-level arbiter  204  outputs the selected transmission on its respective output  224 - 1  and  224 - 2  of  FIG. 2 , each of which is provided to a respective input of the second-level multiplexer  206  of  FIG. 2 . Flow proceeds to block  312 . 
         [0032]    At block  312 , the second-level multiplexer  206  selects one of the inputs  224 - 1  or  224 - 2  as the final granted transmission  212 . Specifically, the second-level multiplexer  206  selects as the final granted transmission  212  the one of the second-level arbiters  204  specified by the output  226  of the second-level control register  208 . Flow ends at block  312 . 
         [0033]    Referring now to  FIG. 4 , a flowchart illustrating the programmability of the first-level control register  238  and the second-level control register  208  of  FIG. 2  according to an embodiment of the present invention is shown. Flow begins at block  402 . 
         [0034]    At block  402 , an entity predicts that a particular combination of one of the Priority Rule of Table 1 and Table 2 and one of the Priority Rule of Table 3 and Table 4 will yield the best performance for a mix, or stream, of transmission types that the USB port transmitter  104  of  FIG. 2  will be transmitting on the USB link  122  in the future. The predicting entity may be system software, such as a device driver executing on the system CPU. The predicting entity may be another USB device in the USB network that includes the USB device  100  of  FIG. 1 ; or, the predicting entity may be the software controlling the other USB device. The USB device  100  of  FIG. 1  may be a USB host, USB hub, or USB peripheral device. Flow proceeds to block  404 . 
         [0035]    At block  404 , the predicting entity programs the first-level control register  238  and the second-level control register  208  of  FIG. 2  with the specific values to select the appropriate Priority Rules that are likely to provide the best performance for the mix of transmissions that the USB port transmitter  104  will be transmitting in the future. Flow ends at block  404 . 
         [0036]    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 USB transmission to transmit from among multiple USB transmissions of different types may result in better performance than a USB port transmitter that always employs the same single priority rule. 
         [0037]    Although various embodiments have been described in which the port transmitter may employ a particular number of levels (two according to one embodiment) and a particular number of priority rules in each level (two according to one embodiment), other numbers of levels and 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. For example, although an embodiment has been described in which LGOOD_n normal link commands are prioritized ahead of LCRD_x normal link commands, embodiments are contemplated in which LCRD_x normal link commands are prioritized ahead of LGOOD_n normal link commands. For another example, although an embodiment has been described in which the special link command type transmissions (LUP link commands) are included in the pool of transmissions prioritized by the second-level arbiters, embodiments are contemplated in which the special link command type transmissions are included in the pool of normal link command type transmissions. 
         [0038]    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.