Abstract:
A data processing system is provided with a bus having separate write channels W and read channels R via which bus transactions are made. Bus transaction buffers  34  are provided within the bus structure to buffer write requests, particularly so as to alleviate problems associated with relatively slow bus slaves. The bus transaction buffers  34  are responsive to the memory addresses associated with write requests and read requests which pass through them to identify those to the same memory address, or memory addresses within a predetermined range, so as to either ensure a strict correct ordering of those transactions, read to follow write, or to satisfy a read following a write with a buffered write data value and then flushing the read request such that it does not reach its final destination.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to the field of data processing systems. More particularly, this invention relates to the management and control of bus transactions within data processing systems. 
   2. Description of the Prior Art 
   It is known to provide data processing systems having bus structures to connect different circuit elements and via which bus transactions, such as reads, writes and control functions take place. As data processing systems become more complex, such as increasingly complex system on-chip integrated circuits, the bus structures used to connect the various functional elements within the circuit have tended to become more complicated and critical in terms of overall system performance. One known type of bus structure uses a unified read and write channel along which serialized bus transactions are passed. Such a unified channel approach has the advantage that the order in which the bus transactions are issued will be the same as the order in which they are received since all of the bus transactions are serialized at issue on the unified channel. However, such an approach suffers from the disadvantage of a limitation in bus bandwidth and an increase in latency since all of the transactions must pass along a single channel. 
   An alternative approach is to provide the bus structure with separate read buses and write buses along which data can pass in parallel. Such an approach increases the bus bandwidth available and reduces latency. 
   SUMMARY OF THE INVENTION 
   Viewed from one aspect the present invention provides a bus transaction buffer comprising: 
   one or more write bus inputs and one or more read bus inputs coupled to respective write buses and read buses and operable respectively to receive write requests and read requests from one or more request sources; 
   one or more write bus outputs and one or more read bus outputs coupled to respective write buses and read buses and operable respectively to output write requests and read requests to one or more request destinations; and 
   a request buffering circuit operable to buffer write requests received from said request sources and to control of processing of write requests and read requests to said one or more request destinations such that one of: 
   (i) a read request to a given memory address received from a request source having a given request source identifier after a write request to said given memory address was received from said request source having said given request identifier is not output on a read bus output until said write request is acknowledged as completed at a write bus output; or 
   (ii) a read request to a given memory address received from a request source after a write request of a write data value to said given memory address whilst said write request is still buffered by said bus transaction buffer results in said write data value buffered within said bus transaction buffer being output to said read request source to satisfy said read request. 
   In the context of systems having parallel read and write channels within their bus structures, the present technique recognizes that there can be problems arising from the speeds with which different circuit elements connected to that bus structure can process bus transactions directed to them. As an example, if a system includes one or more particularly slow circuit elements, such as, for example, flash memory elements, then high performance circuit elements, such as, for example, high performance CPUs, may be held up in their processing waiting for bus transaction requests directed at the low performance circuit elements to be completed so as to release the high performance circuit elements to process further transactions. One way of addressing this problem is to provide buffering within the bus structure itself that can be used to buffer transaction requests to desired circuit elements so that other circuit elements can be released to continue processing prior to those buffered transactions actually being completed. In addition to this provision of buffering, the present technique recognizes a particular hazard that can then arise in that with separate read and write channels it is possible if buffering within the bus structure is provided that the order in which transactions are issued may not be the order in which they are completed when they have been subject to buffering in the different channels. The present technique recognizes this hazard and provides the solution of ensuring that the buffering circuits detect transactions to the same memory location from at least the same request source and either properly order these such that reads follow writes in the correct sequence or satisfy reads with the buffered write values as may be desired and appropriate in the particular system concerned. 
   Within such systems having separate read and write bus channels and within which buffering can take place performance may be increased if the request buffering circuit is arranged to output a request completed signal back to the requesting source prior to that request having been ultimately completed at its final destination. The buffering circuit can thus effectively take responsibility for that request ultimately being properly completed and completed within the correct relative ordering with respect to other bus requests. 
   In the above context the request buffering circuit is preferably operable to perform these devolved responsibilities for proper bus transaction completion by controlling the relative ordering or forwarding pending write data as desired or appropriate. 
   The performance of the system may be further improved in embodiments in which a request acknowledgement signal is issued by the request buffering circuit when it receives a write request even before this is notionally completed since this may permit resources such as the address bus to be released for other uses. 
   It will be appreciated that the request sources could take a variety of different forms including processor cores, direct memory access circuits or further bus transaction buffers within a hierarchical buffered bus structure. 
   In a similar way the request destinations can take a variety of different forms including memories, memory mapped peripheral circuits and further bus transaction buffers. 
   As one option requirement the relative ordering of reads to properly follow writes to the same memory address may be preserved when the request source identifier is identical. However, more generally, this principle may be applied when the requests are to the same memory address even if the requests originate from different request sources. 
   In a similar way, the satisfying of a read request by a buffered write data value may generally be used whenever the buffered write data value for the given memory address is available or alternatively may be more restrictively used when the request source for the data value from the given memory address is the same as the request source which initiated the write of the buffered write data value for that given memory address. 
   It is preferred within the overall system that non-bufferable write requests should also be supported as these may be appropriate for certain types of peripheral or other device within the overall system. Such non-bufferable requests are ones in which the memory request must be actually completed through to its final destination, irrespective of any intermediate buffering that may or may not occur, before a completed signal is passed back to the original request source. This may be important for situations in which absolute system wide ordering and relative timing is highly critical. 
   The matching of memory addresses of transactions which should be ordered or otherwise controlled may be a requirement for precise matching or alternatively may be a match determined by an address range rather than a specific memory address value. 
   Viewed from another aspect the present invention provides a method of control bus transactions, said method comprising the steps of: 
   receiving write requests and read requests from one or more request sources at one or more write bus inputs and one or more read bus inputs coupled to respective write buses and read buses; 
   outputting write requests and read requests to one or more request destinations at one or more write bus outputs and one or more read bus outputs coupled to respective write buses and read buses; and 
   buffering write requests received from said request sources and controlling processing of write requests and read requests to said one or more request destinations such that one of: 
   (i) a read request to a given memory address received from a request source having a given request source identifier after a write request to said given memory address was received from said request source having said given request identifier is not output on a read bus output until said write request is acknowledged as completed at a write bus output; or 
   (ii) a read request to a given memory address received from a request source after a write request of a write data value to said given memory address whilst said write request is still buffered by said bus transaction buffer results in said write data value buffered within said bus transaction buffer being output to said read request source to satisfy said read request. 
   The above, and other objects, features and advantages of this invention will be apparent from the following detailed description of illustrative embodiments which is to be read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  schematically illustrates a data processing system including a bus structure having separate read channels and write channels; 
       FIG. 2  schematically illustrates a bus transaction buffer; 
       FIG. 3  schematically illustrates a bus transaction buffer additionally providing multiplexing and demultiplexing functions; 
       FIG. 4  is a flow diagram schematically illustrating read request receipt processing within a bus transaction buffer; and 
       FIG. 5  is a flow diagram schematically illustrating read request issuing within a bus transaction buffer. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  schematically illustrates a data processing system including an integrated circuit  2  coupled to an external flash memory  4  and an external peripheral  6 . Within the integrated circuit  2 , there are provided multiple bus transaction sources  8 ,  10 ,  12  in the form of processor cores and direct memory access circuits. Other forms of bus transaction sources are also possible. Bus transaction destinations within the integrated circuit  2  include a random access memory  14 , a first peripheral  16  and a second peripheral  18 . As will be apparent from  FIG. 1 , separate read and write channels are provided between the various circuit elements as part of the bus structure. Multiplexers  20 ,  22  serve to multiplex together read and write channels received from multiple transaction sources down onto one read channel and one write channel. Demultiplexers  24 ,  26 ,  28  serve to demultiplex the bus structure back to separate read and write channels for separate transaction destinations. 
   Within the integrated circuit  2  there are provided bus transaction buffer circuits  30 ,  32  within the bus structure itself disposed between transaction sources and transaction destinations. More particularly, these bus transaction buffer circuits  30 ,  32  are provided upstream of relatively slow transaction destinations, such as the off-chip flash memory  4  and the peripheral devices  6 ,  16 ,  18 . In this way, bus transactions to these relatively slow devices may be buffered within the bus transaction buffer circuits  30 ,  32  and acknowledged as completed back to their transaction sources allowing those high performance transaction sources to continue processing and operation without having to wait for the relatively slow transaction destinations to actually complete the transactions directed to them. 
   It will be appreciated by those in this field that the illustration in  FIG. 1  shows the separate read and write channels in a highly schematic form and that each of these read and write channels may have its own data bus portion, address bus portion, control signal portion, hand shaking signal portion and the like. There are a wide variety of different possibilities for such systems having separate read channels and write channels. The present technique is also not limited to systems having only a single read channel and a single write channel as it is possible for the present techniques also to be used in systems having multiple read channels, multiple write channels or both multiple read and multiple write channels. 
     FIG. 2  schematically illustrates a bus transaction buffer  34  in more detail. The illustrated bus transaction buffer  34  includes a write bus input  36  and read bus input  38 . A write bus output  40  and a read bus output  42  are also provided. Within the bus transaction buffer  34 , a multiple entry write buffer  44  having the form of a FIFO is provided together with what can be considered to be a single entry read request buffer  46 . Between the read request buffer  46  and the write buffer  44 , request buffering control circuit  48  serves to compare the memory addresses at which read requests and write requests are directed to identify when these match, either precisely or within a predetermined range of address values, and then take predetermined action to preserve the transaction request ordering. 
   When a write request to a given memory address is received, this is buffered within the write buffer  44 . A read request to that same memory address may later be received by the bus transaction buffer  34  and this later received read request is identified by the request buffering control circuitry  48  such that the read request is held off (considered as buffered) within the read request buffer  46  until the matching write request within the write buffer  44  has been output from the bus transaction buffer  34  and acknowledged as completed by the transaction destination to which it is being sent. As an alternative, if the architecture and characteristics of the target device and overall systems so provide, then it is possible that the later received read request which matches a pending write request may be satisfied by returning the write data value which is buffered within the write buffer  44  in response to that read request rather than actually sending that read request onto its final transaction destination. Such an approach effectively also preserves the correct ordering of the transactions. 
   Whilst it is possible to match transactions together purely on the basis of the memory address to which they are directed, preferred embodiments may also match the transaction requesting source identifiers before taking action appropriate upon a match as described above. In this way only transactions issuing from the same logical source will be strictly ordered and different sources will have to deal with out-of-order issues as is already known within multi bus master systems. 
     FIG. 3  schematically illustrates a second example bus transaction buffer  50 . In this example, the bus transaction buffer  50  includes multiple write bus inputs, multiple write bus outputs, multiple read bus inputs and multiple read bus outputs. The bus transaction buffer  50  thus provides multiplexing, buffering, ordering and demultiplexing functions. 
     FIG. 4  is a flow diagram schematically illustrating read request receipt processing as may be performed by the bus transaction buffer  34  and more particularly control as performed by the request buffering control circuit  48 . At step  52 , the system waits for any read request to be received. When a read request is received, then step  54  checks the destination address for that read request against any already received write requests which are currently being buffered within the write buffer  44 . If a match is detected at step  56 , then processing proceeds to step  58  at which it is determined whether or not the address concerned is one which is able to be satisfied with respect to a read from a buffered write data value rather than actually reading the data value from the final transaction destination. If the read can be satisfied in this way, then step  60  returns the buffered write data value from the write buffer  44  as the response to the received read request and the received read request is flushed and not forwarded to its final destination. If such read request processing from a buffered write data value is not possible, then step  62  deals with the read request by marking it as one which should not be issued from the bus transaction buffer  34  until the matching write request has been issued and acknowledged as completed either by its final destination or by the next buffering element which is taking responsibility for the correct completion and ordering of that write request. If the read request is marked in this way, then it is then added to the read request buffer  46  at step  64 . Subsequent to either step  60  or step  64 , step  65  waits for the bus transaction buffer to receive the read response before proceeding to step  66  which outputs a request complete signal back to the original read request source to indicate that the bus transaction buffer  34  has now effectively taken responsibility for that read request, its proper completion and its in-order completion. If the match at step  56  was not detected, then processing proceeds to step  64  and step  66  directly. 
     FIG. 5  is a flow diagram schematically illustrating read request issue from the bus transaction buffer  34 . At step  68 , the system waits until there are buffered read requests to be issued. At step  70  the next to be serviced read request is examined to determine if it is marked as one which should be ordered to follow completion of a buffered write request. If the read request is so marked, then step  72  determines whether the matching write request in the other channel has yet been completed. If the matching write request has not been completed, then processing returns to step  68 . If the matching write request has been completed, or step  70  did not identify the read request as one marked as being ordered, then processing proceeds to step  74  at which the read request is output to the transaction destination. If the read request is non-bufferable, i.e. one which should not be acknowledged as completed until it is actually completed at the final destination, then step  76  diverts processing back to step  68  for a later request completed signal to be generated by another mechanism. If the read request is not non-bufferable, then step  78  outputs a read request completed signal once the read request has been output to the transaction destination, or at least the next bus transaction buffer, at step  74 . 
   Although illustrative embodiments of the invention have been described in detail 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 changes and modifications can be effected therein by one skilled in the art without departing from the scope and spirit of the invention as defined by the appended claims.