# Patent

## Title
Enforcing strongly-ordered requests in a weakly-ordered processing

## Abstract

The disclosure is directed to a weakly-ordered processing system and method for enforcing strongly-ordered memory access requests in a weakly-ordered processing system. The processing system includes a plurality of memory devices and a plurality of processors. Each of the processors are configured to generate memory access requests to one or more of the memory devices, with each of the memory access requests having an attribute that can be asserted to indicate a strongly-ordered request. The processing system further includes a bus interconnect configured to interface the processors to the memory devices, the bus interconnect being further configured to enforce ordering constraints on the memory access requests based on the attributes.

## Background
The present Application for patent claims priority to Provisional Application No. 60/664,749 entitled “Method and Apparatus for Efficient Strongly Ordered Transactions” filed Mar. 23, 2005, and assigned to the assignee hereof and hereby expressly incorporated by reference herein.


BACKGROUND
1. Field
The present disclosure relates generally to processing systems, and more particularly, to a method and apparatus for enforcing strongly-ordered requests in a weakly-ordered processing system.
2. Background
Computers and other modern processing systems have revolutionized the electronics industry by enabling sophisticated tasks to be performed with just a few strokes of a keypad. These sophisticated tasks often involve a number of devices that communicate with one another in a fast and efficient manner using a bus. The bus provides a shared communication link between devices in a processing system.
The types of devices connected to a bus in a processing system may vary depending on the particular application. Typically, the sending devices on the bus may be processors, and the receiving devices on the bus may be memory devices or memory mapped devices. In these systems, the processors often achieve performance benefits by allowing memory operations to be performed out-of-order. For example, a sequence of memory operations could be reordered to allow all operations to the same page in memory to be executed before a new page is opened. Processing systems that are allowed to reorder memory operations are generally referred to as “weakly-ordered” processing systems.
In certain instances, the reordering of memory operations may unpredictably affect program behavior. For instance, an application may require a processor to write data to memory before the processor reads from that memory location. In a weakly-ordered processing system, there is no guarantee that this will occur. This result may be unacceptable.
Various techniques have been employed for executing ordered memory operations in a weakly-ordered processing system. One technique is simply to delay certain memory operations until all memory operations before it are executed. In the previous example, the processor may delay issuing a read request until after it writes to the memory location. Another technique is to use a bus command referred to as a memory barrier when an ordered memory operation is required. A “memory barrier” may be used to ensure that all memory access requests issued by a processor before the memory barrier are executed before all memory access requests issued by the processor after the memory barrier. Again, in the previous example, a memory barrier could be sent to the memory by the processor before issuing a read request. This would ensure that the processor writes to memory before it reads from the same memory location.
Both techniques are effective, but inefficient from a system performance perspective. The memory barrier may be particularly inefficient in processing systems with multiple memory devices. In these processing systems, a memory barrier would need to be issued by the processor to every memory device it can access to enforce an ordering constraint on memory operations. Thus, there is a continuing need for more efficient methods to perform ordered memory operations in a weakly-ordered processing system.
SUMMARY
One aspect of a weakly-ordered processing system is disclosed. The processing system includes a plurality of memory devices and a plurality of processors. Each of the processors are configured to generate memory access requests to one or more of the memory devices, with each of the memory access requests having an attribute that can be asserted to indicate a strongly-ordered request. The processing system further includes a bus interconnect configured to interface the processors to the memory devices, the bus interconnect being further configured to enforce ordering constraints on the memory access requests based on the attributes.
Another aspect of a weakly-ordered processing system is disclosed. The processing system includes a plurality of memory devices, and a plurality of processors. Each of the processors are configured to generate memory access requests to one or more of the memory devices, with each of the memory access requests having an attribute that can be asserted to indicate a strongly-ordered request. The processing system further includes a bus interconnect having means for interfacing the processors to the memory devices, and means for enforcing ordering constraints on the memory access requests based on the attributes.
One aspect of a bus interconnect is disclosed. The bus interconnect includes a bus switch configured to interface a plurality of processors to a plurality of memory devices in a weakly-ordered processing system. Each of the processors are configured to generate memory access requests to one or more of the memory devices, with each of the memory access requests having an attribute that can be asserted to indicate a strongly-ordered request. The bus interconnect further includes a controller configured to enforce ordering constraints on the memory access requests based on the attributes.
One aspect of a method for enforcing strongly-ordered memory access requests in a weakly-ordered processing system is disclosed. The method includes receiving, from a plurality of processors, memory access requests for a plurality of memory devices, one of the memory access requests having a memory attribute indicating a strongly-ordered memory access request. The method further includes enforcing an ordering constraint for the strongly-ordered memory access request in relation to the other memory access requests in response to the attribute.
It is understood that other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein it is shown and described only various embodiments of the invention by way of illustration. As will be realized, the invention is capable of other and different embodiments and its several details are capable of modification in various other respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not as restrictive.



BRIEF DESCRIPTION OF THE DRAWINGS
Various aspects of the present invention are illustrated by way of example, and not by way of limitation, in the accompanying drawings, wherein:
FIG. 1 is a conceptual block diagram illustrating an example of a weakly-ordered processing system;
FIG. 2 is a functional block diagram illustrating an example of a bus interconnect in a weakly-ordered processing system;
FIG. 3 is a functional block diagram illustrating an example of a controller in a bus interconnect for a weakly-ordered processing system; and
FIG. 4 is a functional block diagram illustrating another example of the controller in a bus interconnect for a weakly-ordered processing system.



DETAILED DESCRIPTION
The detailed description set forth below in connection with the appended drawings is intended as a description of various embodiments of the invention and is not intended to represent the only embodiments in which the invention may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of the invention. However, it will be apparent to those skilled in the art that the invention may be practiced without these specific details. In some instances, well known structures and components are shown in block diagram form in order to avoid obscuring the concepts of the invention.
FIG. 1 is a conceptual block diagram illustrating an example of a weakly-ordered processing system. The processing system 100 may be a computer, resident in a computer, or any other system capable of processing, retrieving and storing information. The processing system 100 may be a stand-alone system, or alternatively, embedded in a device, such as a wireless or wired telephone, a personal digital assistant (PDA), a desktop computer, a laptop computer, a game console, a pager, a modem, a camera, automotive equipment, industrial equipment, video equipment, audio equipment, or any other suitable device requiring processing capability. The processing system 100 may be implemented as integrated circuit, part of an integrated circuit, or distributed across multiple integrated circuits. Alternatively, the processing system 100 may be implemented with discrete components, or any combinations of discrete circuits and integrated circuit. Those skilled in the art will recognize how best to implement the processing system 100 for each particular application.
The processing system 100 is shown with multiple processors 102a-102c in communication with multiple memory devices 104a-104c over a bus 106. The actual number of processors and memory devices required for any particular application may vary depending on the computational power required and the overall design constraints. A bus interconnect 108 may be used to manage bus transactions between the processors 102a-102c and memory devices 104a-104c using point-to-point switching connections. In at least one embodiment of the bus interconnect 108, multiple direct links may be provide to allow several bus transactions to occur simultaneously. Alternatively, the bus interconnect 108 may be configured to support a shared bus arrangement.
Each processor 102a-102c may be implemented as any type of bus mastering device including, by way of example, a general purpose processor, a digital signal processor (DSP), application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic, discrete gate or transistor logic, discrete hardware components, or any other processing entity or arrangement. One or more of the processors 102a-102c may be configured to execute instructions under control of an operating system or other software. The instructions may reside in one or more of the memory devices 104a-104c. Data may also be stored in the memory devices 104a-104c, and retrieved by the processors 102a-102c to execute certain instructions. The new data resulting from the execution of these instructions may be written back into the memory devices 104a-104c. Each memory device 104a-104c may include a memory controller (not shown) and a storage medium (not shown). The storage medium may include RAM memory, DRAM memory, SDRAM memory, flash memory, ROM memory, PROM memory, EPROM memory, EEPROM memory, CD-ROM, DVD, registers, hard disk drive, a removable disk, or any other suitable storage medium.
Each processor 102a-102c may be provided with a dedicated channel 106a-106c on the bus 106 for communicating with the bus interconnect 108. Similarly, the bus interconnect 108 may use a dedicated channel 106d-106f on the bus to communicate with each memory device 104a-104c. By way of example, a first processor 102a can access a target memory device 104b by sending a memory access request over its dedicated channel 106a on the bus 106. The bus interconnect 108 determines the target memory device 104b from the address of the memory access request and forwards the request to the target memory device 104b over the appropriate channel 106e on the bus 106. A “memory access request” may be a write request, a read request, or any other bus related request. An originating processor 102a-102c may issue a write request to a target memory device 104a-104c by placing the appropriate address with the payload on the bus 106 and asserting a write enable signal. An originating processor 102a-102c may issue a read request to a target memory device 104a-104c by placing the appropriate address on the bus 106 and asserting a read enable signal. In response to the read request, the target memory device 104a-104c will send the payload back to the originating processor 102a-102c. 
In at least one embodiment of the processing system 100, the processors 102a-102c may transmit a “memory attribute” with each memory access request. The “memory attribute” may be any parameter that describes the nature of the memory access request. The memory attribute may be transmitted with the address over the address channel. Alternatively, the memory attribute may be transmitted using sideband signaling or some other methodology. The memory attribute may be used to indicate whether or not the memory access request is strongly-ordered. A “strongly-ordered” request refers to a memory access request that cannot be executed out of order.
The bus interconnect 108 may be used to monitor the memory attribute for each memory access request from the processors 102a-102c. If a memory attribute indicates a strongly-ordered memory access request, the bus interconnect 108 may enforce an ordering constraint on that request. By way of example, a memory access request from a first processor 102a to a target memory device 104a may include a memory attribute. The bus interconnect 108 may be used to determine from the memory attribute whether the request is strongly-ordered. If the bus interconnect 108 determines that the request is strongly-ordered, it sends a memory barrier to every memory device 104b and 104c that the first processor 102a is capable of accessing, other than the target memory device 104a. The bus interconnect 108 also sends the memory access request to the target memory 104a without a memory barrier because the target memory device 104a will implicitly handle it as a strongly-ordered request due to the memory attribute associated with the memory access request.
FIG. 2 is a functional block diagram illustrating an example of a bus interconnect in a weakly-ordered processing system. The manner in which the bus interconnect is actually implemented will depend on the particular application and the design constraints imposed on the overall system. Those skilled in the art will recognize the interchangeability of various designs, and how best to implement the functionality described herein for each particular application.
Referring to FIG. 2, a bus register 202 may be used to receive and store information from the bus 106. The bus register 202 may be any type of storage device such as a first-in-first-out (FIFO) memory, or other suitable storage device. The information received and stored by the bus register 202 may be any bus related information, but typically includes the address and memory attribute for each memory access request, and in the case of a write request, the payload. The address for each memory access request is also provided to a decoder 204. The decoder 204 may be used to determine the target memory device for each memory access request in the bus register 202. This determination is used to generate a signal 205 which controls a bus switch 206. The bus switch 206 is used to demultiplex each memory access request in the bus register 202 to the appropriate channel of the bus 106 for its target memory device. A controller 208 may be used to control the timing of memory access requests released from the bus register 202.
FIG. 3 is a functional block diagram illustrating an example of a controller in a bus interconnect for a weakly-ordered processing system. The controller 208 is primarily responsible for enforcing ordering constraints on memory operations based on a signal 209 it receives from the decoder 204. The signal 209 includes information relating to the memory attribute for each memory access request, which may be stored in a first register 302. The signal 209 also includes information that identifies each memory device, other than the target memory device, that the originating processor is capable of accessing. The particular memory devices accessible by each processor are preconfigured during the design stage, and therefore, can be programmed or hard wired into the decoder. In any event, a second register 304 may be used to store this data. The first and second registers 302, 304 may be separate registers as shown in FIG. 3, or alternatively a single register. In some embodiments of the controller 208, the information from the decoder 204 may be stored in registers shared with other bus interconnect functions. Each register may be a FIFO or any other suitable storage medium.
The controller 208 enforces ordering constraints on memory operations by controlling the timing of memory access requests released from the bus register 202. The process will first be described in connection with a memory attribute which indicates a that strongly-ordered memory access request is ready to be released from the bus register 202. In this case, the memory attribute is provided by the first register 302 to a memory barrier generator 306 as an enabling signal. At the same time, the data stored in the second register 304 is provided to the input of the memory barrier generator 306. As indicated above, the data stored in the second register 304 includes data that identifies each memory device, other than the target memory device, that the originating processor is capable of accessing. When the memory barrier generator 306 is enabled by the memory attribute, this information is used to generate a memory barrier for each memory device identified by the data. Each memory barrier may be provided to the appropriate memory device by issuing a bus command with an attribute identifying the originating processor which initiated the strongly-ordered request. Alternatively, the memory barriers may be provided to the appropriate memory devices using sideband signaling, or by other suitable means.
Logic 308 in the controller 208 may be used to monitor feedback from the memory devices for memory barrier acknowledgements. A “memory barrier acknowledgement” is a signal from a memory device indicating that every memory access request received by that memory device from the originating processor issuing the strongly-ordered request that precedes the memory barrier will be computed before the completion of any subsequent memory access request from the originating processor. The data from the second register 304 is used by the logic 308 to determine which memory devices need to be monitored for memory barrier acknowledgements. When the logic 308 determines that all memory barrier acknowledgements have been received, it generates a trigger that is used to release the corresponding memory access request from the bus register 202 via a first connection 211. More specifically, the memory attribute from the first register 302 is provided to the select input of a multiplexer 310. The multiplexer 310 is used to couple the trigger generated by the logic 308 to the bus register 202 when the memory attribute indicates that the memory access request is strongly-ordered. The trigger output from the multiplexer 310 is also coupled to the decoder to synchronize the timing of the bus switch 206 via a second connection 210 (see FIG. 2).
Once the memory access request is released from the bus register, it is routed to the target memory device through the bus switch 206 (see FIG. 2). A second multiplexer 312 in the controller 208 may be used to delay the release of data from the first and second registers 302, 304 until a memory access acknowledgement is received from the target memory device when a memory attribute indicating a strongly-ordered request is applied to the select input. As discussed earlier, the memory attribute included in the memory access request enforces an ordering constraint on the target memory device. Namely, the target memory device executes all outstanding memory access requests issued by the originating processor before executing the strongly-ordered memory access request. A memory access acknowledgement is generated by the target memory device following the execution of the strongly-ordered request. The memory access acknowledgement is fed back to the multiplexer 312 in the controller 208, where it is used generate a trigger to release new data from the first and second register 302, 304 corresponding to the next memory access request in the bus register 202. If the new data includes a memory attribute indicating that the corresponding memory access request in the bus register 202 is strongly-ordered, then the same process is repeated. Otherwise, the memory access request can be released immediately from the bus register 202.
The controller 208 is configured to immediately release a memory access request from the bus register 202 when the corresponding memory attribute in the first register 302 indicates that the request is weakly-ordered. In that case, the memory attribute is used to disable the memory barrier generator 306. In addition, the memory attribute forces the multiplexer 310 into a state which couples an internally generated trigger to the bus register 202 to release the memory access request via the first connection 211. The memory access request is released from the bus register 202 and coupled to the target memory device through the bus switch 206 (see FIG. 2). The data corresponding to the next memory access request is then released from the first and second registers 302, 304 by an internally generated trigger output from the second multiplexer 312 in the controller 208.
FIG. 4 is a functional block diagram illustrating another example of a controller in a bus interconnect for a weakly-ordered processing system. In this example, a strongly-ordered memory access request is released from the bus register 202 by the controller 208 at the same time the memory barriers are provided to the appropriate memory devices. More specifically, a memory attribute for a memory access request in the bus register 202 is provided by the first register 302 to the memory barrier generator 306. If the memory attribute indicates that the corresponding memory access request is strongly-ordered, then the memory barrier generator 306 is enabled. When the memory barrier generator 306 is enabled, the data in the second register 304 is used to generate a memory barrier for each memory device accessible by the originating processor, other than the target memory device.
With the memory barrier generator 306 enabled, logic 314 in the controller 208 may be used to prevent subsequent memory access requests from being released from the bus register 202 until the strongly-ordered request is executed by the target memory device. A delay 316 may be used to allow an internally generated trigger to release the strongly-ordered memory request from the bus register 202 before the trigger is gated off by the memory attribute. In this way, the memory access request can be provided to the target memory device concurrently with the memory barriers for the other memory devices accessible by the originating processor.
Logic 318 may be used to monitor feedback from the target memory device for the memory access acknowledgement, and the other memory devices accessible by the originating processor for the memory barrier acknowledgements. The data from the second register 304 is used by the logic 318 to determine which memory devices need to be monitored for memory barrier acknowledgements. When the logic 318 determines that the various acknowledgements have been received, it generates a trigger to release new data from the first and second registers 302, 304 corresponding to the next memory access request in the bus register 202. The trigger is coupled through a multiplexer 320 which is forced into the appropriate state by the memory attribute from the first register 202. If the new data includes a memory attribute indicating that the corresponding memory access in the bus register 202 is strongly-ordered, then the same process is repeated. Otherwise, the memory access request can be released immediately from the bus register 202 with an internally generated trigger via the logic 314. An internally generated trigger may also be coupled through the multiplexer 320 to release the data from the first and second registers 302, 304 for the next memory access request in the bus register 202.
The previous description is provided to enable any person skilled in the art to practice the various embodiments described herein. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments. Thus, the claims are not intended to be limited to the embodiments shown herein, but is to be accorded the full scope consistent with the language claims, wherein reference to an element in the singular is not intended to mean “one and only one” unless specifically so stated, but rather “one or more.” All structural and functional equivalents to the elements of the various embodiments described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed under the provisions of 35 U.S.C. §112, sixth paragraph, unless the element is expressly recited using the phrase “means for” or, in the case of a method claim, the element is recited using the phrase “step for.”

## Claims

1. A weakly-ordered processing system, comprising:
a plurality of memory devices;
a plurality of processors, each of the processors configured to generate memory access requests to one or more of the memory devices, each of the memory access requests having an attribute that can be asserted to indicate a strongly-ordered request; and
a bus interconnect configured to interface the processors to the memory devices, the bus interconnect being further configured to enforce ordering constraints on the memory access requests based on the attributes,
wherein the bus interconnect is further configured to enforce ordering constraints for a strongly-ordered memory access request from an originating processor to a target memory device by sending a memory barrier to each of the other memory devices accessible to the originating processor.

2. The weakly-ordered processing system of claim 1 wherein the bus interconnect is further configured to enforce ordering constraints for said strongly-ordered memory access request by delaying the transmission of subsequent memory access requests until a memory barrier acknowledgement is received from each of the memory devices receiving a memory barrier.

3. The weakly-ordered processing system of claim 1 wherein the bus interconnect is further configured to enforce ordering constraints for a strongly-ordered memory access request from an originating processor to a target memory device by sending said strongly ordered memory access request with its memory attribute to the target memory device.

4. The weakly-ordered processing system of claim 3 wherein the bus interconnect is further configured to enforce ordering constraints for said strongly-ordered memory access request by delaying the transmission of subsequent memory access requests until a memory access acknowledgement is received from the target memory device.

5. The weakly-ordered processing system of claim 3 wherein the bus interconnect is further configured to enforce ordering constraints for said strongly-ordered memory access request by sending a memory barrier to each of the other memory devices accessible to the originating processor.

6. The weakly-ordered processing system of claim 5 wherein the bus interconnect is further configured to enforce ordering constraints for said strongly-ordered memory access request by delaying the transmission of subsequent memory access requests until a memory barrier acknowledgement is received from each of the memory devices receiving a memory barrier.

7. The weakly-ordered processing system of claim 5 wherein the bus interconnect is further configured to enforce ordering constraints for said strongly-ordered memory access request by delaying the transmission of subsequent memory access requests until a memory access acknowledgement is received from the target memory device.

8. A weakly-ordered processing system, comprising:
a plurality of memory devices;
a plurality of processors, each of the processors configured to generate memory access requests to one or more of the memory devices, each of the memory access requests having an attribute that can be asserted to indicate a strongly-ordered request; and
a bus interconnect having means for interfacing the processors to the memory devices, and means for enforcing ordering constraints on the memory access requests based on the attributes,
the bus interconnect having further means for enforcing ordering constraints for a strongly-ordered memory access request from an originating processor to a target memory device by sending a memory barrier to each of the other memory devices accessible to the originating processor.

9. A bus interconnect, comprising:
a bus switch configured to interface a plurality of processors to a plurality of memory devices in a weakly-ordered processing system, each of the processors being configured to generate memory access requests to one or more of the memory devices, and each of the memory access requests having an attribute that can be asserted to indicate a strongly-ordered request; and
a controller configured to enforce ordering constraints on the memory access requests based on the attributes,
wherein the controller is further configured to enforce ordering constraints for a strongly-ordered memory access request from an originating processor to a target memory device by sending a memory barrier to each of the other memory devices accessible to the originating processor.

10. The bus interconnect of claim 9 wherein the controller is further configured to enforce ordering constraints for said strongly-ordered memory access request by delaying the transmission of subsequent memory access requests until a memory barrier acknowledgement is received from each of the memory devices receiving a memory barrier.

11. The bus interconnect of claim 9 wherein the controller is further configured to enforce ordering constraints for a strongly-ordered memory access request from an originating processor to a target memory device by sending said strongly-ordered memory access request with its memory attribute to the target memory device.

12. The bus interconnect of claim 11 wherein the controller is further configured to enforce ordering constraints for said strongly-ordered memory access request by delaying the transmission of subsequent memory access requests until a memory access acknowledgement is received from the target memory device.

13. A method of enforcing strongly-ordered memory access requests in a weakly-ordered processing system, comprising:
receiving, from a plurality of processors, memory access requests for a plurality of memory devices, one of the memory access requests having a memory attribute indicating a strongly-ordered memory access request; and
enforcing an ordering constraint on the strongly-ordered memory access request III relation to the other memory access requests in response to the attribute,
wherein the controller is further configured to enforce ordering constraints for a strongly-ordered memory access request from an originating processor to a target memory device by sending a memory barrier to each of the other memory devices accessible to the originating processor.

14. The method of claim 13 wherein the ordering constraint is enforced by delaying the transmission of subsequent memory access requests until a memory barrier acknowledgement is received from each of the memory devices receiving a memory barrier.

15. The method of claim 13 wherein the strongly-ordered request is from an originating processor requesting access to a target memory device, and wherein the ordering constraint is enforced by sending the strongly-ordered memory access request with its memory attribute to the target memory device.

16. The method of claim 15 wherein the ordering constraint is enforced by delaying the transmission of subsequent memory access requests until a memory access acknowledgement is received from the target memory device.

17. The method of claim 16 wherein the ordering constraint is enforced by sending a memory barrier to each of the other memory devices accessible to the originating processor.