Serial tunneling protocol (STP) flow control in SAS expanders without SATA link state machine
View Patent ↗A method and apparatus are disclosed for implementing STP flow control in SAS expander devices. SAS expanders are commonly used within a SAS network topology to allow multiple disk drives to connect to multiple host devices. Connections to a SATA HDD are supported using SATA Tunnelling Protocol (STP), which allows SATA traffic to be carried over a SAS network topology. Flow control in a STP connection is applied through a set of special SATA primitives, both for forward and backward flow control. A method is described herein in which STP flow control is supported without the use of a SATA link layer state machine. This allows STP flow control to be terminated on a hop-by-hop basis without knowing the data channel direction or maintaining a SATA link state machine, and while minimizing gate count.
1. A method of serial advanced technology attachment tunneling protocol (STP) flow control in a serial attached small computer system interface (SAS) expander, said method comprising:
when primitives of a first type are received by said SAS expander, forwarding said primitives of said first type through an ingress buffer and sending primitives of a second type on an egress buffer;
when said primitives of said first type are not received by said SAS expander, but said primitives of said second type are received, discarding a data word incoming to said SAS expander;
when neither said primitives of said first type nor said primitives of said second type are received by said SAS expander and a fill level of said ingress buffer is not greater than a threshold, writing said data word to said ingress buffer;
when neither said primitives of said first type nor said primitives of said second type are received by said SAS expander and said fill level of said ingress buffer is greater than said threshold and said data word is a repeatable data word, discarding said data word incoming to said SAS expander; and
when neither said primitives of said first type nor said primitives of said second type are received by said SAS expander and said fill level of said ingress buffer is greater than said threshold and said data word is not a repeatable data word, writing said data word to said ingress buffer;
wherein said primitives of said first type and said primitives of said second type are interlocking relative to one another.
2. The method as claimed in claim 1 , wherein said primitives of said first type are HOLD primitives and said primitives of said second type are HOLDA primitives.
3. The method as claimed in claim 1 , further comprising a step of:
when neither said primitives of said first type nor said primitives of said second type are received by said SAS expander and said fill level of said ingress buffer is greater than said threshold and said data word is a further primitive type not identical to either said primitives of said first type nor said primitives of said second type, writing said data word to said ingress buffer.
4. The method as claimed in claim 3 , wherein said threshold is programmable and provides said ingress buffer with a reserve portion having storage capacity for at least two transitions in primitive type.
5. The method as claimed in claim 3 , wherein said threshold within said ingress buffer is determined by constraints of discard threshold, (thr DISCARD ), flow control threshold, (thr FLOW ), maximum flow control loop time, (T LOOP ), and size of said ingress buffer, (FIFO SIZE ), where:
thr FLOW >T LOOP ,
FIFO SIZE >2*thr FLOW , and
thr DISCARD is set to a value that is less than thr FLOW but greater than 0.
6. The method as claimed in claim 5 , wherein:
when data flow is in a backward channel direction and said primitives of said first type are received by said SAS expander, forwarding said primitives of said first type through said ingress buffer and sending said primitives of said second type on said egress buffer;
when data flow is in said backward channel direction and said primitives of said first type are not received by said SAS expander and said fill level of said ingress buffer is not greater than said flow control threshold, sourcing data words from said egress buffer;
when data flow is in said backward channel direction and said primitives of said first type are not received by said SAS expander and said fill level of said ingress buffer is greater than said flow control threshold, generating data words identical to said primitives of said first type.
7. The method as claimed in claim 6 , wherein said primitives of said first type are HOLD primitives and said primitives of said second type are HOLDA primitives.