Data processing system with bus access retraction
View Patent ↗A bus master may selectively retract a currently pending access based on one or more characteristics of the currently pending access. In this manner, bus master may better control its access requests. The one or more characteristics may include, for example, type of access (e.g. read/write, instruction/data, burst/non-burst, etc.), sequence or order of accesses, address being accessed (e.g. which address range is being accessed or which device is being accessed), the bus master requesting retraction (in an, e.g., multimaster system), or any combination thereof. A bus arbiter may also selectively retract currently pending access requests in favor of a subsequent access request based on one or more characteristics of the currently pending access request or the subsequent access request. These characteristics may include any of those listed above, priorities of the requesting masters (e.g. a priority delta between requesting masters), other attributes of the requesting masters, or any combination thereof.
1. A method for access retraction in a data processing system having bus arbitration between a plurality of master ports and at least one slave port, the method comprising:
storing access retraction control information which indicates at least one condition under which retraction is allowed or disallowed;
granting a first access request under control of an arbiter logic, the first access request corresponding to a first access address, to a first slave port while a second access request, corresponding to a second access address, to the first slave port is in progress;
after granting the first access request, receiving a third access request, corresponding to a third access address, to the first slave port; and
using retraction control logic coupled to the arbiter logic to request retraction of the first access request, the retraction control logic using the access retraction control information to selectively retract permanently the first access request based on at least one characteristic of at least one of the first access request or the third access request.
2. The method of claim 1 , wherein the at least one characteristic comprises a type of access.
3. The method of claim 2 , wherein the type of access is one of a read or write access.
4. The method of claim 2 , wherein the type of access is one of an instruction or data access.
5. The method of claim 2 , wherein the type of access is one of a burst or non-burst access.
6. The method of claim 1 , wherein selectively retracting the first access request is based on at least one characteristic of each of the first access request and the second access request.
7. The method of claim 1 , wherein the first access request is requested by a first master and the second access request is requested by a second master, and wherein the at least one characteristic comprises a priority difference between the first master and the second master.
8. The method of claim 1 , wherein the access retraction control information provides a delay value, wherein if retraction of the first access request is allowed, retraction of the first access request does not occur until after the delay value.
9. The method of claim 1 , wherein the access retraction control information provides a delay value, wherein if retraction of the first access request is allowed, retraction of the first access request only occurs if a comparison of the delay value with a time it takes for the second access request to complete results in a predetermined outcome.
10. The method of claim 1 , wherein the access retraction control information indicates when retraction by a parking master is allowed, wherein the at least one characteristic of the at least one of the first access request and the third access request comprises a requesting master of the at least one of the first access request and the third access.
11. The method of claim 10 , wherein using the access retraction control information to selectively retract the first access request comprises determining whether the requesting master is the parking master.
12. The method of claim 1 , wherein the access retraction control information provides at least one condition under which retraction is allowed or disallowed for a corresponding address range, and wherein the at least one characteristic of the at least one of the first access or the third access comprises at least one of an address range into which the first address falls or an address range into which the third address falls.
13. The method of claim 1 , wherein if the access retraction control information indicates that retraction is allowed based on the at least one characteristic, selectively retracting comprises:
retracting the first access request in favor of the third access request.
14. The method of claim 1 , wherein if the access retraction control information indicates that retraction is allowed based on the at least one characteristic, selectively retracting comprises:
reordering the first access request and the third access request.
15. Interconnect circuitry comprising:
a first master port;
a second master port;
a slave port;
arbiter logic coupled to the first master port, the second master port, and the slave port, the arbiter logic arbitrating access requests from the first and second master ports to the slave port;
retraction control storage circuitry for storing retraction control information indicating at least one condition under which retraction of an access request is allowed; and
retraction control logic coupled to the arbiter logic and retraction control storage circuitry, the retraction control logic, in response to receiving a retraction request from the arbiter logic, uses the retraction control information to selectively allow permanent retraction of a pending access request previously granted by the arbiter logic based on at least one characteristic of at least one of a pending access request or a subsequently received access request.
16. The interconnect circuitry of claim 15 , further comprising:
parking control register circuitry, coupled to the arbiter logic and retraction control logic, for indicating which master coupled to the first or second master ports is a parking master,
wherein the access retraction control register indicates when retraction of an access request by the parking master is allowed, and wherein the at least one characteristic comprises whether a requesting master of the at least one of the pending access request or the subsequently received access request is the parking master.
17. The interconnect circuitry of claim 15 , further comprising:
master priority storage circuitry, coupled to the arbiter logic and retraction control logic, for storing a priority corresponding to each of a first master coupled to the first master port and of a second master coupled to the second master port, wherein the at least one characteristic of the at least one of the pending access request or the subsequently received access request comprises a priority of a requesting master.
18. The interconnect circuitry of claim 17 , wherein the retraction control logic selectively retracts the pending access request based on at least one characteristic of each of the pending access request and the subsequently received access request, and wherein the at least one characteristic comprises a priority difference between a requesting master of the pending access request and a requesting master of the subsequently received access request.
19. The interconnect circuitry of claim 17 , wherein the access retraction control information indicates when retraction by a highest priority master is allowed, wherein the retraction control logic determines whether a requesting master of the at least one characteristic of the at least one of the pending access request and the subsequently received access request is the highest priority master.
20. The interconnect circuitry of claim 15 , wherein the access retraction control information provides a delay value, wherein if retraction of the currently pending access request is allowed by the retraction control logic, retraction of the currently pending access request occurs after a delay value amount of clocks.
21. The interconnect circuitry of claim 15 , wherein if retraction of the currently pending access request is allowed by the retraction control logic, the interconnect circuitry retracting the currently pending access request in favor of the subsequently received access request.
22. The interconnect circuitry of claim 15 , wherein if retraction of the currently pending access request is allowed by the retraction control logic, the interconnect circuitry reordering the currently pending access request and the subsequently received access request.
23. The interconnect circuitry of claim 15 , wherein the at least one characteristic comprises a type of access.
24. The interconnect circuitry of claim 23 , wherein the type of access is one of a read or write access.
25. The interconnect circuitry of claim 23 , wherein the type of access is one of an instruction or data access.
26. The interconnect circuitry of claim 15 , wherein the at least one characteristic comprises an address range into which at least one of a currently pending access request or a subsequently requested access request falls.
27. A method for access retraction in a data processing system having bus arbitration between a plurality of master ports and at least one slave port, the method comprising:
storing access retraction control information which indicates at least one condition under which retraction is allowed or disallowed;
using an arbiter to grant a first access request from a first master, corresponding to a first access address, to a first slave port while a second access request, corresponding to a second access address, to the first slave port is in progress;
after granting the first access request, receiving a third access request from a second master, corresponding to a third access address, to the first slave port;
using retraction control logic coupled to the arbiter, requesting retraction of the first access request; and
using the access retraction control information to selectively retract permanently the first access request based on a priority of at least one of the first master and the second master.
28. The method of claim 27 , wherein the access retraction control information indicates when retraction by a highest priority master is allowed, wherein using the access retraction control information to selectively retract the first access request comprises determining whether the second master is the highest priority master.
29. The method of claim 27 , wherein the access retraction control information indicates a minimum priority difference between the first and second master for which retraction is allowed.
30. The method of claim 29 , wherein at least one characteristic comprises a priority difference between the first master and the second master.