Abstract:
Preferred embodiments of the present invention increase the efficiency of access to a constructed list of memory locations, or DMA list by a host processor. In order to circumvent the problem of latency between an I/O device and the host processor, preferred embodiments of the present invention store a copy of the DMA head pointer in the system memory, as well as on the I/O device. When the head pointer data is changed on the I/O device, the I/O device will use DMA to write the updated head pointer back to system memory.

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. § 119(e) from U.S. Provisional Patent Application Ser. No. 60/574,402, filed May 25, 2004, from U.S. Provisional Patent Application Ser. No. 60/599,565, filed Aug. 5, 2004, and from U.S. Provisional Patent Application No. 60/599,605, filed Aug. 5, 2004. The entirety of each of these provisional patent applications is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to data transmission and reception and specifically to data transmission and reception in an Interconnect Chip. 
     2. Description of Background Art 
     The speed of data read and write within an integrated circuit is affected in part by the cost (in time) of accessing the status of entries in a direct memory access (DMA) list. Each read of the DMA list over a slow connection by a host processor adds a delay in processing, yet to work correctly the host processor needs to know the current status of the DMA list. 
     DMA occurs when a hardware device transfers data from an input/output (I/O) port of the hardware device to and/or from system memory without direct host processor interaction. DMA lists tell the I/O device which memory locations and ranges are to be transferred, and in which direction. The DMA list is accessed via a DMA head pointer. In a conventional DMA system, head pointer information is read from the I/O device by the host processor, which introduces latency. 
     In a conventional DMA system, the host processor accesses the DMA head pointer to determine which location in the DMA list is to be processed next. Once the processing of the next item in the DMA list is complete, the conventional approach is to have the host processor read the DMA head pointer from the I/O device to determine if the head pointer has changed. Either the host processor polls the I/O device, or the I/O device interrupts the host processor to be read. In either case, access to the I/O device is slow, and the host processor is stalled until the read of the head pointer is complete. 
     The typical process for determining memory locations and memory status begins when a host processor constructs a DMA list. The DMA list is then written either directly into the I/O device using programmed input/output (PIO) instructions or into system memory accessed via a head pointer that is written into the I/O device. The I/O device then directly accesses the information in the list by fetching DMA through the DMA head pointer. At this point, the I/O device registers containing the DMA head pointer can be read by the host processor, pointing it to the next item to be processed. 
     Alternatively, the host processor can access the entries for the DMA list by reading status words directly from system memory. This method has an overhead cost for accessing the status words and determining whether the I/O device or the host processor changed a memory location last. 
     What is needed are methods and systems for eliminating the delay in conventional DMA. 
     SUMMARY OF THE INVENTION 
     The features and advantages described in this summary and the following detailed description are not all-inclusive, and particularly, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims hereof. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and may not have been selected to delineate or circumscribe the inventive subject matter, resort to the claims being necessary to determine such inventive subject matter. 
     Preferred embodiments of the present invention increase the efficiency of access to a constructed list of memory locations, or DMA list by a host processor. In order to circumvent the problem of latency between an I/O device and the host processor, preferred embodiments of the present invention store a copy of the DMA head pointer in the system memory, as well as on the I/O device. Accessing system memory is faster than accessing the I/O device, and the host processor can continue working during its memory access process. When the head pointer data is changed on the I/O device, the I/O device will use DMA to write the updated head pointer back to system memory. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a system in accordance with a preferred embodiment of the present invention. 
         FIG. 2  is a flow diagram illustrating a method in accordance with a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A preferred embodiment of the present invention is now described with reference to the figures where like reference numbers indicate identical or functionally similar elements. Also in the figures, the leftmost digit(s) of each reference number correspond(s) to the figure in which the reference number is first used. 
     The described embodiment of the present invention increases an access rate of a host processor to a DMA list of a host system by storing a copy of a DMA head pointer in system memory of the host, as well as in an I/O device that connects the host system to a network. Accessing system memory is faster than accessing the I/O device, and the host processor can continue working during the memory access process. 
     The system of  FIG. 1  represents a preferred embodiment of the present invention. Within the context of this preferred embodiment, a host system  105  is in communication with an input/output (I/O) device  114  by way of a bus connection  103 . Bus  103  is preferably a packetized bidirectional bus such as HyperTransport protocol or via a non-packetized bus such as PCI Express, although any appropriate bus or protocol may be used. Initially, the host processor  110  constructs a DMA list  115  within the system memory  112  containing various DMA operations. These operations can be, for example, read operations, write operations, or a combination or read and write operations. The host system  105  then writes the DMA list  115  into the I/O device  114  and, through a DMA operation, updates the DMA list tail pointer within the I/O device  114  (not shown) to indicate the entries are ready to process. After the head pointer data  111  is changed on the I/O device  114  (due to completion of a DMA operation), the I/O device  114  uses DMA to write the DMA list head pointer  111  back to the system memory  112 , thus invalidating the host processor cache in the process. 
     Because the DMA head pointer  111 ′ is stored in system memory  112 , the host processor  110  can access the DMA list  115  information at any time without incurring the overhead of read from the I/O device  114 . This is especially important for polling-based systems, but is also beneficial for interrupt-based systems. 
     The head pointer  111 ′ in system memory  112  provides a better way to poll for changes in DMA list  115  status. The host processor  110  only needs to look in one low latency location (head pointer  111 ′) rather than reading the location from an I/O device  114 , which is slow and stalls the host processor  110  until the read is complete, or checking multiple status word locations in main memory (one per DMA list entry), which needs to be verified for last read/write access. 
     It is to be understood that the head pointer  111 ′ is typically updated by the I/O device  114  and ready by the host processor  110 , not the reverse. This ensures that the I/O device  114  was the last one to write to the memory location. There is a matching tail pointer (not shown) in the I/O device  114  that is written only by the host processor  110  and read by the I/O device  114 . 
       FIG. 2  is a flow diagram that illustrates a method for carrying out DMA list retrieval in accordance with a preferred embodiment of the present invention. The method begins when a host processor  110  constructs  210  a DMA list  115  and the host processor then writes  212  the DMA list into the I/O device  114 . In a data read, for example, the I/O device receives a data packet and eventually the received data is DMA&#39;d to, e.g., a buffer in system memory  112 . When the initiation of the DMA transfer operation is complete  213 , the I/O device updates  214  the DMA list head pointer  111 . The updating  214  of the DMA head pointer  111  is the triggering mechanism for the I/O device to then use DMA to duplicate  216  the updated DMA list head pointer  111  to a copy of the head pointer  111 ′ in system memory  112 . 
     The host processor  110  needs to look only in one low latency location (head pointer  111 ′ in system memory) to determine when data is present in system memory and ready to process rather than reading the head pointer  111  from the I/O device  114 . Because the host processor  110  has more direct access to data within system memory  112 , and because the host processor does not have to wait for a slow I/O read of the I/O device  114 , the entire DMA process is considerably more efficient. 
     As shown in  FIG. 2  updating the head pointer  111 ′ also causes invalidation  218  of the host processor cache (not shown) in system memory  112 . Because a read has occurred, the cached data is no longer guaranteed to be reliable. Once host processor  110  determines that the head pointer has changed in value, host processor  110  accesses  220  the data from a buffer in system memory  112 . 
     The present invention may also be used in a system where the I/O device uses DMA to receive data from system memory (i.e., for system writes). In this case, I/O device  114  updates the head pointer when a packet or packets containing the data is sent onto the network from the I/O device. 
     As will be understood by those familiar with the art, the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Likewise, the particular naming and division of the modules, managers, features, attributes, methodologies and other aspects are not mandatory or significant, and the mechanisms that implement the invention or its features may have different names, divisions and/or formats. Furthermore, as will be apparent to one of ordinary skill in the relevant art, the modules, managers, features, attributes, methodologies and other aspects of the invention can be implemented as software, hardware, firmware or any combination of the three. Of course, wherever a component of the present invention is implemented as software, the component can be implemented as a standalone program, as part of a larger program, as a plurality of separate programs, as a statically or dynamically linked library, as a kernel loadable module, as a device driver, and/or in every and any other way known now or in the future to those of skill in the art of computer programming. 
     Additionally, the present invention is in no way limited to implementation in any specific programming language, or for any specific operating system or environment. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.