Patent Publication Number: US-6223238-B1

Title: Method of peer-to-peer mastering over a computer bus

Description:
RELATED APPLICATIONS 
     The subject matter of U.S. Patent entitled SYSTEM FOR PEER-TO-PEER MASTERING OVER A COMPUTER BUS, filed on even date herewith, issued as U.S. Pat. No. 6,073,198 is related to this application. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to information processing systems, such as a personal computer (PC). More particularly, the invention relates to processing transactions in a computer system having multiple bus architecture. 
     2. Background of the Related Art 
     Modern computer systems, such as personal computers (PCs), process an enormous amount of information in a relatively short time. To perform its sophisticated functions, a computer system typically includes a main processor, memory modules, various system and bus control units, and a wide variety of data input/output (I/O) devices. Typically, these computer devices communicate control and data signals in accordance with a predetermined signal protocol. However, with the employment of a multiple bus architecture, these devices often communicate across a plurality of bus protocols and bridging devices. A bridging device performs protocol conversion between two buses, thereby allowing each device involved in a transaction to know how and when the other device is going to perform a particular task. 
     A transaction over a particular bus normally involves a requesting device (the “requester”) and a responding device (the “target”). The requester requests the transfer of data or a completion signal from a target in the system. The request typically includes a number of control bits indicating the type of request and an address of the desired data or device. The target, in turn, responds to the transaction by sending a completion signal along with any data if necessary. With the presence of various devices acting as requesters and targets in the system, bus protocols often include the ability to handle multiple transactions among multiple devices concurrently. One example of such a bus is the pipelined bus which allows requests by various requesters to be pending (i.e., unfulfilled) over the bus at the same time. The incorporation of separate data and address buses makes this possible. In a pipelined transaction, a requester sends a request on the address bus, and a target returns a reply on the data bus. Multiple requesters may send multiple requests over the address bus, and multiple targets may respond in the same order of the requests over the data bus. In a special pipelined bus, commonly referred to as the split transaction bus, the order of the responses does not have to occur in the same order as their corresponding requests. Each transaction is tagged so that the requester and the target keep track of the status of the transaction. This characteristic permits maximum utilization of the pipelined bus by effectively increasing bus bandwidth. This advantage, however, is obtained at a cost of experiencing a higher latency than when a request is held during the pendency of a transaction. 
     The Pentium II or Pentium Pro processor is an example of a processor which supports a pipelined bus, commonly referred to as the P6 bus. The P6 bus includes a 64-bit external data bus and a 32- or 36-bit address bus. The speed of the P6 bus may be 66 or 100 MHz, and the processor clock rate may be two, three, or four times the speed of the bus. The P6 bus employs “packet” transmission to transmit data the same way that a network transmits packets. A data packet is known as a chunk which may be up to 64 bits. The P6 bus supports split transactions. Accordingly, a P6 processor sends an address and then releases the bus for use by other bus requesters while waiting for the target device (e.g., a main memory) to respond. When the target is ready to respond, the target returns the requested data over the data bus in 64-bit packets. 
     The maximum data transfer that is supported by the P6 processor bus is four 64-bit wide transfers, commonly referred to as a “cache line” transfer. As noted above, the P6 processor supports split transactions. This feature is often characterized as a “deferred response” whereby a target device defers its response to a request by a requester. A deferred response allows the P6 bus to be freed to execute other requests while waiting for the response from a device with relatively long latency. A single P6 processor may have up to four transactions outstanding at the same time. 
     FIG. 1 is a functional block diagram of an exemplary computer hardware layout having a multiple bus architecture. As shown in FIG. 1, a main processor CPU  110  is connected to a Host bus  120 . A Host bridge  130  connects the Host bus  120  to a secondary bus PCI1 bus  140 . One or more input/output devices IOD1  142  is connected to the PCI1 bus  140 . The Host bridge  130  supports communication between PCI devices, e.g., IOD1  142 , and devices present on the Host bus  120  or elsewhere in the system. Another Host bridge  150  is often employed to connect another PCI2 bus  160  to the Host bus  120 . Moreover, other I/O devices, e.g., IOD2  162 , are connected to the PCI2 bus  160 . Similarly, the Host bridge  150  supports communication between PCI devices, e.g., IOD2  162 , and devices present on the Host bus  120  or elsewhere in the system. 
     A bus transaction over the Host bus  120  is often in the form of a read request issued by a requester device. For example, in single chunk requests, the IOD1  142  on the PCI1 bus  140  may issue a read request to the IOD2  162  on the PCI2 bus  160 . The purpose of the read request may be to obtain data being processed by or available at the IOD2  162 . The Host bridge  130  receives the read request from the IOD1  142 , decodes the address from the read request, and issues a single chunk (i.e., 64 bits) read request over the Host bus  120  to the PCI2 bus  160 . A cache line read may not be issued to the PCI2 bus  160  because a PCI bus does not support a cache line read (i.e., four 64 bits). A cache line read on a PCI bus triggers a retry request after the target transfers one or more words to the requester. The retry request is often triggered because a PCI device may need to execute other requests before servicing the read request. A retry request is problematic over a PCI bus because speculative reads are not permitted. A speculative read is a read operation (usually in response to a retry request) of an already read data. Hence, in response to the single chunk read request by the Host bridge  130 , the Host bridge  150  detects the request and issues a single chunk read request to the IOD2  162  on the PCI2 bus  160 . 
     There are several inherent inefficiencies with single chunk requests. A single chunk request does not utilize a host bus efficiently. In view of its limited bandwidth occupancy, a single chunk request slows down the computer system. Moreover, unless retried, a single chunk request ties up the host bus until fulfilled. A defer transaction is not an option for PCI devices because, as a bus requester, a host-PCI bridge does not support deferred transactions. 
     Therefore, there is a need in the technology to make a more efficient use of a host bus, e.g., the P6 bus. The utilization of full bus bandwidth on the host bus should be accommodated on secondary buses, e.g., the PCI bus. More particularly, cache line requests, which are accommodated by the host bus, should be supported by secondary buses. 
     SUMMARY OF THE INVENTION 
     To overcome the above-mentioned problems, the invention provides a system for executing peer-to-peer mastering over a host bus in a computer system. The system includes a host bridge which performs deferred bus transactions in a computer system having a multiple bus architecture. The host bridge supports communication among multiple input/output devices (IODs) without interrupting or involving the main processor. 
     In one embodiment of the invention, a method of communication between a requester and a target device is provided. The method comprises the act of establishing a handshake between an IDE Controller and a host master, and issuing a request by the host master over a host bus. The method further comprises the act of acknowledging the request, and transmitting a deferred response to the requester. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other aspects, features and advantages of the invention will be better understood by referring to the following detailed description, which should be read in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a functional block diagram of an exemplary computer hardware layout. 
     FIG. 2 is a functional block diagram of a computer system employing one embodiment of the invention. 
     FIG. 3 is a functional block diagram of the Host bridge as used in the computer system of FIG.  2 . 
     FIG. 4 is a flow chart describing the execution of a bus transaction in the computer system of FIG.  2 . 
     FIG. 5 is a timing diagram for a deferred transaction as executed by the internal control bus of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A detailed description of the system for peer-to-peer mastering over a computer bus is provided below. In describing particular embodiments of the invention, the disclosure is not intended to limit the enumerated claims, but to serve as a particular example of the invention. 
     The invention provides a system for peer-to-peer mastering over a computer bus, such as the Pentium II or Pentium Pro (“P6”) bus. Other processors which support pipelined transactions may also be used. FIG. 2 is a functional block diagram of a computer system employing one embodiment of the invention. As shown in FIG. 2, one or more processors CPU 1   204  and CPU 2   208  are connected to a Host bus  210 . The CPU 1   204  and CPU 2   208  may be the Pentium II or Pentium Pro (P6) processors manufactured by Intel Corporation. Using the P6 processors, the Host bus  210  is commonly referred to as the P6 bus. The P6 bus supports multiple, pipelined transactions having data transfers of four 64-bit wide words (commonly referred to as a “cache line” transfer). A host bridge (HB 1 )  214  is connected to the Host bus  210  to support communication between the Host bus  210  and other buses in the system, such as a PCI1 bus  218  and an input/output device (IOD) bus  220 . One or more I/O devices may be connected to the PCI1 bus  218 , such as IOD 1   222  and IOD 2   226 . The devices IOD 1   222  and IOD 2   226  may be any PCI compatible devices, such as a LAN interface, a SCSI card, an audio card, or a graphics card. One or more I/O devices may be connected to the IOD bus  220 , e.g., Bus Mastering IOD (BMIOD)  230 . The BMIOD  230  may be any type of I/O device such as a hard disk, a CD-ROM, or similar. 
     Another Host bridge HB 2   232  may also be connected to the Host bus  210  to support communication between the Host bus  210  and other buses in the system, such as a PCI2 bus  236 , and a memory bus  240 . The HB 2   232  may be identical to the HB 2   214 , or may be a 82454KX/GX PCI Bridge manufactured by Intel Corporation. The memory bus  240  supports communication between the HB 2   232  and a main memory  244 . The PCI 2  Bus  236  supports communication between the HB 2   232  and multiple I/O devices, e.g., IOD 3   248  and IOD 4   252 . The devices IOD 3   248  and IOD 4   252  may be any PCI compatible devices, such as a LAN interface, a SCSI card, an audio card, or a graphics card. In this embodiment, the PCI 1  bus  218  and PCI 2  bus  236  are standard peripheral component interconnect (PCI) buses which conform to the PCI Local Bus Specification (Revision 2.1 or later). 
     In this embodiment, any one of the devices CPU 1   204 , CPU 2   208 , and HB 1   214  may issue “deferred” transactions on the Host bus  210 . The deferred transaction capability allows the Host bus to be freed to execute other requests while waiting for the response from a request to a device with relatively long latency. Each one of the devices CPU 1   204 , CPU 2   208 , HB 1   214 , and HB 2   232  may have up to four transactions outstanding at the same time. The number of transactions that may target a particular bus device is configured separately from the total number of transactions allowed on the bus. Each of the HB 1   214  and HB 2   232  may accept up to four transactions into its in-order queue (IOQ) (not shown in this figure) that target its associated buses. 
     Hence, for example, the HB 1   214  may have a read request directed to IOD 3   248  and a read request directed to the main memory  244  pending at the same time on the Host bus  210 . A bus mastering IOD-type device, e.g., BMIOD  230 , issues a read request to the HB 1   214  over the IOD bus  220 , having the main memory  244  as its destination. Another bus mastering IOD-type device may issue another read request to the HB 1   214  over the IOD bus  220 , having the IOD 3   248  as its target. The HB 1   214 , in turn, may issue both read requests with a deferred capability over the Host bus  210 . The HB 2   232  detects and delivers each read request to its intended destination, i.e., to the main memory  244  via the memory bus  240 , or IOD 3   248  via the PCI 2  bus  236 . In view of the deferred response feature, the Host bus  210  is freed up to execute other transactions, and each read request may remain pending at the same time, until the main memory  244  and IOD 3   248  are ready to respond to the BMIOD  230 . 
     In addition to its bridging capability, the HB 2   232  includes a memory controller (not shown) which comprises a DRAM Controller, a Data Path, and one or more Memory Interface Components. The combined memory controller uses one physical load on the memory bus. The memory controller includes two sets of registers (I/O space registers and configuration registers). Examples of the memory controller components include the 82453KX/GX DRAM Controller, 82452KX/GX Memory Data Path, and 82451KX/GX Memory Interface Component manufactured by Intel Corporation. 
     FIG. 3 is a functional block diagram of the Host bridge HB 1   214  as used in the computer system of FIG.  2 . As noted above, the HB 2   232  of FIG. 2 may be designed and implemented as the HB 1   214  FIG.  3 . The HB 1   214  comprises a Host Master (HM)  310 , an integrated driver electronics (IDE) Controller  320 , and an Internal Control Bus (“IC bus”)  330  which supports internal communication between the IDE Controller  320  and the HM  310 . The HM  310  performs several bridging functions including conversion of signal protocols between secondary buses and the Host bus  210 . Additionally, the HM  310  supports deferred transactions over the Host bus  210 . The IDE Controller  320  conforms to the disk interface standard based on the IBM industry standard architecture (ISA) 16-bit bus. The IDE Controller  320  controls the transfer of signal traffic between bus mastering IODs, such as the BMIOD  230 , and the main processor, e.g., CPU1  204  or CPU2  208  (FIG.  2 ). Additionally, the IDE Controller  320  controls the transfer of signal traffic between the BMIOD  230  and the main memory  244  or other IODs in the system. 
     The HM  310  accepts and arbitrates requests received from PCI targets, PCI requesters, and IDE controllers. The HM  310  issues these requests to the Host bus  210 . The HM  310  receives these requests from requesters via the IC bus  330 . The HM  310  comprises one or more host bus interface modules (HIMs), a host master requester (HMR), a host master arbiter (HMA), an in-order queue (IOQ), a host master snooper (HMS), and a host master responder (HMP) (not shown in this figure). During each clock cycle, the HIMs register all input signals received by the HM  310  from the Host bus  210 . All signals are first registered before the HM  310  makes any logic decision. The HMR accepts requests from and arbitrates access among up to four different requesters. The HMR generates one or more flag signals to the HMA to initiate arbitration. The HMR buffers the first request received from a requester, and holds off any subsequent requests from that requester until the first request is executed. The HMR further formats and provides signals to the Host bus  210 . 
     The HMA arbitrates access to the Host bus  210  among several requesters. The HMA further maintains and tracks the state of the Host bus  210 . The state of the Host bus  210  may be free, throttled, or stalled. In the free state, the HM  310  may issue requests freely over the Host bus  210 . In the throttled state, the HM  310  may issue only one request over the Host bus  210 . In the stalled state, the HM  310  may not issue any requests over the Host bus  210 . The IOQ is a register which stores information about (up to eight) transactions which may be outstanding on the Host bus  210 . The IOQ stores request codes (each identifying the requester which issued the request, i.e., an IDE or PCI device), byte enables, length of transaction, command codes (type of transaction), and a snoop done bit. The snoop done bit indicates that the snoop phase is complete. As specified in Intel&#39;s Pentium II or Pentium Pro processor standard, the snoop phase occurs during a bus transaction and is controlled by the HIT# (hit), HITM# (hit modified), and DEFER# (defer) host bus interface signals. The IOQ head indicates which transaction to be completed next on the Host bus  210 . The IOQ tail indicates the next position in the IOQ to be filled. The HMS samples one or more snoop signals, and tracks the “snoop stall” state to determine when the snoop phase is complete. The HMP tracks the response phase on, provides the write data to, and accepts the read data from the Host bus  210 . 
     The IC bus  330  comprises a 32- or 36-bit address bus and a 64-bit data bus for supporting communication between the IDE controller  320  and the HM  310 . There are several IC bus interface signals which characterize the operation of the IC bus  330 . The interface signals include: REQ, REQ_BUSY, ADDR, BE, COUNT, STATUS, REQ_RETRY, REQ_DEFER, DEFER_ENABLE, DEFER_ID, DATA_DEFER, DATA_RETRY, WRITE_STROBE, WRITE_DATA, WRITE_BUSY, WRTIE_COMPLETE, READ_STROBE, READ_DATA, AND READ_BUSY. The REQ signal is asserted by a requester device to request the transfer of certain data from a target device. In one embodiment, if the requester is provided with capability for multiple outstanding requests, then multiple REQ signals may be asserted by the same requester on the IC bus  330 . The requester and target device are responsible for keeping track of their respective outstanding transactions. 
     The REQ_BUSY signal represents the commonly known handshake signal between devices. The REQ_BUSY signal is typically issued by the target to the requester to indicate that the target is ready to receive the request command. By asserting the REQ_BUSY signal, the target further indicates that the target has read the address, byte enables, count and status bits. The ADDR signal is a 32- or 36-bit address signal which may be issued by any device, i.e., the requester or target, to indicate the device&#39;s address. The BE signal represents an 8-bit field for a byte enable asserted by the requester during a request transaction. The COUNT signal is a 2-bit field representing the type of transfer being requested. For example, a 00 represents one 64-bit transfer, 01 represents two 64-bit transfers, 10 represents three 64-bit transfer, and 11 represents four 64-bit transfers, between the requester and target. The STATUS signal is a multi-bit field representing the type of request. For instance, a 0000 is an interrupt acknowledge, 0001 is a special cycle, 0010 is an I/O read, 0011 is an I/O write, 0110 is a memory read, 0111 is a memory write, 1010 is a config read, 1011 is a config write, and 1110 is a defer enable. 
     The REQ_RETRY signal is asserted by the target to the requester indicating that the preceding request should be retransmitted by the requester. Typically, a target asserts the REQ_RETRY signal when the target has lost and is unable to recover the request, or the target is not ready to process the request at the time it was received. The REQ_DEFER signal is asserted by the target to the requester indicating that the request has been deferred for execution at a later time. By asserting the REQ_DEFER signal, the target advises the requester to remove the request from the requester&#39;s IOQ and release the bus. A deferred response is possible when the defer enable of the STATUS signal is asserted. The DEFER_ENABLE signal is asserted by a requester to indicate that the requester supports deferred transactions if the target device is not available to execute the request promptly. The DEFER_ID signal is asserted by both the requester and target devices to keep track of the identity and order of the defer transaction in the request queue. The DATA_DEFER signal is asserted by the target to the requester indicating that the data is now being sent in response to the deferred request. 
     The WRITE_STROBE signal indicates that the write data is valid from the requester for data transfer to the target. The WRITE_DATA signal indicates the 64-bit on the data bus to be written by the requester into the target. The WRITE_COMPLETE signal is a response issued by the target to the requester indicating that the target has completed servicing the write request at the top of the IOQ. The READ_STROBE signal is a command issued by the target to the requester indicating that the read data is valid for data transfer to the requester. The READ_DATA signal indicates the 64-bit on the data bus to be read by the target into the requester. The READ_BUSY signal is a response issued by the requester to the target indicating that the data path in the requester is presently busy and, thus, not available to receive the read data. 
     The IDE controller  320  and the HM  310  manipulate the above signals in conformance with the protocol of the IC bus  330  to communicate requests between IODs and the Host bus  210 . FIG. 4 is a flow chart describing the execution of a bus transaction in the computer system of FIG.  2 . As shown in FIG. 4, the process typically begins at step  400  when the main processor, e.g., CPU 1   204  or CPU 2   208 , configures the IDE Controller  320  and I/O devices, e.g., the BMIOD  230 . The BMIOD  230  issues a read request to the IDE Controller  320  via the IOD bus  220  indicating its intent to read data from a target device in the system. The read request includes, among other things, the address of the target device, location of the data in the memory space of the target device, and other control and identification information. At step  410 , the IDE Controller  320  acknowledges the read request and issues a cache line read (i.e., 4×64 bits) over the IC bus  330  (FIG.  3 ). To initiate a cache line read request, the IDE Controller  320  asserts the REQ signal on the IC bus  330  to the HM  310 . At step  420 , the HM  310  acknowledges the cache line read request and, when ready to execute the request, accepts the request by asserting a REQ_BUSY signal and REQ_DEFER signal on the IC bus  330 . As noted above, by asserting the REQ_BUSY signal, the HM  310  establishes a handshake with the IDE Controller  320  to indicate that the HM  210  is ready to accept the request information. In establishing a handshake, the active or asserted REQ_BUSY signal occurs when the signal is at a low voltage level. By asserting the REQ_DEFER signal, the HM  310  indicates to the IDE Controller  320  that the request may be deferred by the target, in the event that the target is not ready to execute the request promptly. 
     As noted above, in this embodiment, the Host bus  210  may be a P6 bus as specified by the Pentium II or Pentium Pro processor&#39;s standard of Intel Corporation. Hence, the Host bus  210  conforms to the host bus interface signals specified by the Pentium II or Pentium Pro processor bus. The Pentium II or Pentium Pro processor bus standard designates the EXF#1 as the extended function signal for Defer Enable to support deferred transactions. Pursuant to the Pentium II or Pentium Pro specifications, the “#” symbol at the end of the signal name indicates that the active, i.e., asserted state occurs when the signal is at a low voltage level. At step  430 , the HM  310  issues a cache line read request over the Host bus  210  with the Defer Enable signal asserted. In addition to asserting the Defer Enable signal, the HM  310  includes a unique Defer ID X to identify or tag the defer transaction so that the requester recognizes the defer response upon receipt. At step  440 , the target HB 2   232  detects and accepts the cache line read request based on the target address information contained therein. Hence, for example, if the read request is destined to the main memory  244 , then the read request contains address information indicating the main memory  244  as its destination, i.e., target. Thus, the HB 2   232  recognizes that the request on the Host bus  210  belongs to the main memory  244  (which is connected to the HB 2   232 ) based on the address information of the main memory  244 . 
     At step  450 , the HB 2   232  issues the cache line read request on the bus (e.g., memory bus  240 ) connected to the target device, e.g., the main memory  244 . The target, in turn, receives the cache line read request and, if not busy, returns the desired data in response to the read request having a destination address of the requester device, i.e., BMIOD  230 . If the target is busy, or data is not available, the target defers its response and instructs the HB 2   232  to release the Host bus  210  to execute other transactions. When the target is ready, the target returns the desired data to the requester via the HB 2   232 . At step  460 , the HM  310  detects the data incoming from the target and returns the data to the IDE Controller  320  via the IC bus  330 . In conformance with the IC bus signal interface, the HM  310  returns the data with the DATA_DEFER signal asserted to indicate that this is the data response from the previous deferred request. The IDE Controller  320  forwards the data to the requester BMIOD  230 . At step  470 , the IDE Controller  320  determines if there are any more cache line read requests to be performed. More read requests may be needed if the requester desires additional data which were not contained in the received cache line read. If the IDE Controller  320  determines that there are more cache line reads desired, then the process starts over at step  410 . If no more cache line reads are desired, then the process terminates at step  480 . 
     FIG. 5 is a timing diagram for a deferred transaction as executed by the IC bus of FIG.  3 . The timing diagram  500  shows the logical states of the various interface signals of the IC bus  330  during a deferred read transaction as a function of time. The IDE Controller  320  and the HM  310  manipulate interface signals in the timing diagram  500  during a deferred read transaction. As shown in FIG. 5, the HCLK  504  is a clock signal controlling the timing of the various interface signal transitions during a transaction on the IC bus  330 . A timescale  530  in nanoseconds (nsec.) is shown to indicate the time separation between various timing events. As shown in FIG. 5, at about 4 nsec., the IDE Controller  320  asserts a REQ signal  508  to request a read operation from a target device. At about 18 nsec., the HM  310  responds to the REQ signal  508  by asserting a low REQ_BUSY signal  512  to indicate that it is ready to accept the request commands (i.e., data) from the IDE Controller  320 . During the duration of the low REQ_BUSY signal  512 , the HM  310  captures the flow of data from the IDE Controller  320  including the ADDR signal  516 , the BE signal  520 , COUNT signal  524 , and STATUS signal  528 . Moreover, at substantially the same time as the transition of the REQ_BUSY signal to low, the HM  310  asserts the REQ_DEFER signal thereby indicating to the IDE Controller  320  that the request may be deferred by the target device. Finally, the RETRY signal  536  is kept low during the entire duration of the transaction on the IC bus  330  thereby indicating that no retry transaction will occur. 
     In view of the foregoing, it will be appreciated that the invention overcomes the long-standing need for a method of peer-to-peer mastering over a computer bus, e.g., the P6 bus. The method provides the ability for master devices, e.g., host bridges, to defer bus transactions without the disadvantages of tying up the host bus or issuing retry transactions. The invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiment is to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.