Abstract:
A method and arrangement for the implementation of a simple algorithm to store an N-bit checksum into any unaligned position within a larger NxP-bit word, which avoids the use of a logic-intensive implementation that employs a bank of demultiplexers, or a latency-increasing approach of “read-modify-write”.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to methods and arrangements for effecting partial word stores in networking adapters, and particularly to the disposition of such a task with less delay and which is capable of working at faster frequencies. 
     BACKGROUND OF THE INVENTION 
     Typically, in designs for networking adapters, challenges are encountered where a partial word (e.g., 16 bit of IP checksum) has to be inserted into packets in buffers that are typically aligned to bus widths (e.g., 64 bit as in the case of 8× PCI Express interface; an example of such an interface can be found at www.pcisig.com, PCI Express Base spec. Rev 1.0a). In fact, this is frequently required in hardware logic that implements a “checksum offload” feature. In many conventional designs (e.g., U.S. Pat. No. 5,898,713, “IP Checksum Offload”, Melzer et al., wherein IP checksum computation is offloaded to a control unit to reduce processor cycles consumed by the host, thereby improving the performance of the host computer and network), the hardware logic is required to insert the partial word into any specified offset into the packet; this insert position in the buffers could be odd or even. 
     A conventional method for undertaking such a partial word write involves using a shifter that employs 16 1:8 demultiplexers with lower order 3 bits of the offset (i.e., the least significant 3 bits of the specified checksum position within the packet, e.g., chksum_pos(2:0)) acting as the “select” lines which determine the amount of shift. The rest of the higher order bits of the offset act as an address into the buffer being written with byte enables. One problem with this method is that it is highly logic-intensive and also reduces the frequency of operation, since demultiplexers are inserted right in the critical data path. Further, in the absence of byte enable at the buffer interface, the design will require a read-modify-store approach; this will further increase the latency. 
     Another key drawback of the above-noted conventional approach is that in case the checksum has to be inserted at the buffer word boundary (e.g. checksum Position=7), the word to be written in the packet buffer has to be computed separately (i.e., the lower checksum byte is written at byte  7  at a word address, and in the next cycle the upper checksum byte is written at byte  0  of the next word address). Accordingly, the logic needs a separate multiplexer to select data for these two write cycles. 
     High Performance Computing (HPC) networking adapters which currently exist tend to require hardware to perform operations with low latency, and with less consumption of logic cells at high frequency. Such optimization is particularly important for FPGA implementation, where a frequency of operation of 250 MHz is typical for supporting support high throughput requirements of a GigaEthernet interface. Simply, conventional arrangements in the mold discussed above are not adequate to respond to such demands. Accordingly, a strong and compelling need has been recognized in connection with improving upon the performance of conventional arrangements and implementing a system that can meet demands of the type just described. 
     SUMMARY OF THE INVENTION 
     Broadly contemplated herein, in accordance with at least one presently preferred embodiment of the invention, is the implementation of a simple algorithm to store 2 Bit IP checksum into any unaligned position within an 8 Bit word. This avoids the use of a logic-intensive implementation that employs 16 1:8 demultiplexers, or a latency-increasing approach of “read-modify-write”. 
     In summary, one aspect of the invention provides a method of effecting a partial word store in a networking adapter, said method comprising the steps of: accepting a main data packet; conveying the main data packet towards a packet buffer and a checksum adder; conveying a checksum packet from the checksum adder to a single multiplexer; and conveying a checksum word from the single multiplexer towards the packet buffer. 
     A further aspect of the invention provides a system for effecting a partial word store in a networking adapter, said system comprising: a checksum adder for accepting a main data packet; a single multiplexer; and a packet buffer; said checksum adder acting to convey a checksum packet to said single multiplexer; said single multiplexer acting to convey a checksum word towards said packet buffer. 
     Furthermore, an additional aspect of the invention provides a program storage device readable by machine, tangibly embodying a program of instructions executable by the machine to perform a method comprising the steps of: accepting a main data packet; conveying the main data packet towards a packet buffer and a checksum adder; conveying a checksum packet from the checksum adder to a single multiplexer; and conveying a checksum word from the single multiplexer towards the packet buffer. 
     For a better understanding of the present invention, together with other and further features and advantages thereof, reference is made to the following description, taken in conjunction with the accompanying drawings, and the scope of the invention will be pointed out in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  schematically illustrates a conventional arrangement for effecting a partial word store. 
         FIG. 2  schematically illustrates an arrangement in accordance with a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For a better understanding of the present invention, together with other and further features and advantages thereof, reference is made to the following description, taken in conjunction with the accompanying drawings, and the scope of the invention will be pointed out in the appended claims. 
     It will be readily understood that the components of the present invention, as generally described and illustrated in the Figures herein, may be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the apparatus, system, and method of the present invention, as represented in  FIGS. 1 through 2 , is not intended to limit the scope of the invention, as claimed, but is merely representative of selected embodiments of the invention. 
     Reference throughout this specification to “one embodiment” or “an embodiment” (or the like) means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. 
     The illustrated embodiments of the invention will be best understood by reference to the drawings, wherein like parts are designated by like numerals or other labels throughout. The following description is intended only by way of example, and simply illustrates certain selected embodiments of devices, systems, and processes that are consistent with the invention as claimed herein. 
       FIG. 1  illustrates a conventional arrangement of the type described further above. Shown is checksum adder  102  and, indicated at  104 , the aforementioned shifter employing 16 1:8 demultiplexers. 
     As is conventionally known, inputs into a packet buffer  116  originate from two gates  108 / 112 , and a decoder  114 , now to be more fully appreciated. By way of an illustrative and non-restrictive example (which provides a point of comparison with the inventive arrangement shown in  FIG. 2 ), a main data packet “pkt_data [ 63 : 0 ]”, or 63 bits of a 64-bit packet, and a single bit “pkt_valid” (for validation purposes as known) are fed to two destinations each, the former to adder  102  and a gate  110 , and the latter to adder  102  and gate  108 . Gates  110  and  112  are shown here as 2:1 Multiplexers, while gates  106  and  108  are shown here as OR gates. When the main data packet is received, adder  102  generates chksum_wr, and, if the checksum position (chksum_pos) is on a word boundary, chksum_wr_again. As shown, chksum_wr is fed both to gate  110  and another gate  106 , while chksum_wr_again is fed to gate  106  and gate  112 . 
     As shown, shifter  104  receives chksum [ 15 : 0 ] from adder  102  as well as the packet chksum_pos [ 2 : 0 ], an input indicating the checksum position. Outputs from shifter  104  include chksum_word [ 63 : 0 ] and chksum_word_again [ 63 : 0 ]; these are fed, respectively, to gates  110  and  112 . The full combined output of gate  110  proceeds to gate  112 , and the full combined output of gate  112  proceeds to packet buffer  116 . 
     As further shown, decoder  114  receives the packet chksum_pos [ 2 : 0 ] and generates wr_be[ 7 : 0 ] to be input into packet buffer  116 . It should be noted that when chksum_wr_again is asserted, the wr_be [ 7 : 0 ] generated by the decoder will be x“01”. The address of chksum_write in this case is chksum_pos [n- 1 : 3 ]+1. i.e. the next word in the packet. It should further be noted that the chksum_word_again [ 63 : 0 ] will have chksum [ 15 : 8 ] as its least significant byte, that is:
         chksum_word_again [ 7 : 0 ]=chksum [ 15 : 8 ]   chksum_word_again [ 63 : 8 ]=don&#39;t care (i.e., the byte enables of these bytes will be zeroes; accordingly, they are not written into the packet buffer, and their values are of no practical significance)       

     Further, it will be noted that chksum_word [ 63 : 0 ] is formed by shifting left chksum [ 15 : 0 ]. Thus, for example if (as shown) chksum_pos [ 2 : 0 ] is 0, then:
         chksum_word [ 63 : 0 ]=x“000000000000” &amp; chksum[ 15 : 0 ],   . . . 1; chksum_word [ 63 : 0 ]=x“0000000000” &amp; chksum[ 15 : 0 ] &amp; x“00”, . . .   . . . 6; chksum_word [ 63 : 0 ]=chksum[ 15 : 0 ] &amp; x“000000000000”   . . . 7; chksum_word [ 63 : 0 ]=chksum[ 7 : 0 ] &amp; “00000000000000”       

     In contrast,  FIG. 2  schematically illustrates an arrangement in accordance with a preferred embodiment of the present invention. For the purposes of clear comparison and illustration, mostly those aspects which differ with respect to  FIG. 1  will be discussed. Components in  FIG. 2  that are similar or analogous to those presented in  FIG. 1  bear reference numerals advanced by  100 . A fundamental difference is the lack of a shifter of the type discussed above (and indicated at  104  in  FIG. 1 ). 
     Accordingly, an “adapter” implementation in accordance with a preferred embodiment of the present invention involves IP/TCP/UDP packets streaming into the packet buffer  216  directly from the host 8x PCI Express bus (where pkt_data (63:0) is shown). In this way, 8 B bytes of packet can be written into the packet buffer  216  in every cycle. Preferably, the hardware will implement “checksum offload” upon adding and accumulating every 2 bytes of adjacent packet data. Preferably, an “end around carry” is employed. In an “end around carry”, checksum is accumulated over the entire packet length by following action: checksum( 17 : 0 )=checksum( 15 : 0 )+checksum( 17 : 16 )+pkt_data( 15 : 0 )+pkt_data( 31 : 16 )+pkt_data( 47 : 32 )+pkt_data( 63 : 48 ); the resultant checksum is obtained from checksum( 15 : 0 ), which will have “carry out” (i.e. checksum( 17 : 16 )) added to it. Preferably, once the entire packet is received by the adapter (at the pkt_data( 63 : 0 ), qualified by pkt_valid) and stored in the packet buffer  216 , it inserts the computed 2 Byte IP checksum into the packet buffer  216  at an offset specified as chksum_pos[n- 1 : 0 ]. 
     Preferably, an approach in accordance with at least one embodiment of the present invention will use the least significant bit of the checksum position (chksum_pos[ 0 ]) to which the checksum is to be stored to select between the computed checksum or the byte swapped value of the checksum, i.e., implement a multiplexer  218  which has checksum (e.g., x1234) as one of its inputs and byte-swapped checksum (e.g., x3412) as its other input. Preferably, the output of this multiplexer  218  is replicated 4 times to get an 8 Byte (8 B) word. Thus, the checksum word (8 B) to be stored is either x1234123412341234 or x3412341234123412. This resultant 8 B word is stored in the packet buffer  216  at the address specified higher order bits of the checksum position. i.e. chksum_pos[n- 1 : 3 ]. The byte-enables wr_be[ 7 : 0 ] to the packet buffer  216  are decoded (via decoder  214 ) from the chksum_pos[ 2 : 0 ]. For instance, if chksum_pos[ 2 : 0 ]=“101”, then the checksum is stored at bytes  5  &amp;  6  of the word, i.e., the byte-enable (wr_be[ 7 : 0 ]) is “01100000”. 
     By way of further clarification, checksum adder  202  preferably generates chksum_wr (and chksum_wr_again if the chksum_pos is on word boundary) after an entire packet has been received. Further, the wr_be[ 7 : 0 ] generated by the decoder  214  is x“01” when chksum wr again is asserted. The address of chksum write in this case is chksum_pos[n- 1 : 3 ]+1. i.e. the next word in the packet. 
     By way of advantages, an implementation in accordance with at least one preferred embodiment of the present invention will avert the need to have 16 8:1 demultiplexers for shifting and an additional 64 2:1 multiplexers for selecting between the checksum data when it has two be written into two adjacent locations in buffers when the checksum position specified fall on a word boundary just 16 2:1 multiplexers. This reduction in logic invariably reduces the timing delays in the critical data path. The same write data word can be used to for both write cycles in case the specified checksum position straddles two words. i.e. chksum_pos=7 or 15, etc. 
     It is to be understood that the present invention, in accordance with at least one presently preferred embodiment, includes elements that may be implemented on at least one general-purpose computer running suitable software programs. These may also be implemented on at least one Integrated Circuit or part of at least one Integrated Circuit. Thus, it is to be understood that the invention may be implemented in hardware, software, or a combination of both. 
     If not otherwise stated herein, it is to be assumed that all patents, patent applications, patent publications and other publications (including web-based publications) mentioned and cited herein are hereby fully incorporated by reference herein as if set forth in their entirety herein. 
     Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be affected therein by one skilled in the art without departing from the scope or spirit of the invention.