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. An expander mechanism comprising:
an elastic buffer for buffering a stream of data between a sender and a receiver;
a primitive decoder for identifying one or more control data-words from said stream of data as repeated primitives; and
a link connection controller for filling said elastic buffer with said stream of data followed, upon identification of a first known type of repeated primitive from said stream of data, with at least two repeated primitives of said first known type while discarding subsequent data-words identical to said at least two repeated primitives of said first known type,
wherein said link connection controller, upon identification of a second known type of repeated primitive from said stream of data, fills said elastic buffer with at least two repeated primitives of said second known type while discarding subsequent data-words identical to said at least two repeated primitives of said second known type so as to preserve within said elastic buffer a first primitive type change identified within said stream of data,
wherein said link connection controller, upon identification of a third known type of repeated primitive from said stream of data, fills said elastic buffer with at least two repeated primitives of said third known type while discarding subsequent data-words identical to said at least two repeated primitives of said third known type so as to preserve within said elastic buffer a second primitive type change identified within said stream of data, and
wherein said elastic buffer includes a reserve portion of said elastic buffer having storage capacity for at least two transitions in primitive type.
2. The expander mechanism as claimed in claim 1 , wherein said reserve portion 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 elastic 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.
3. A serial attached small computer system interface (SAS) expander for conditionally terminating repeated primitives at a data path buffer, said SAS expander comprising:
a receiver portion;
a transmitter portion;
a forward channel first-in-first-out buffer (FIFO), said forward channel FIFO located between said receiver portion and said transmitter portion and within a forward channel data path;
a backward channel first-in-first-out (FIFO) buffer, said backward channel FIFO located between said receiver portion and said transmitter portion and within a backward channel data path;
each said receiver portion and said transmitter portion including a primitive decoder and a link connection controller having a programmable threshold to indicate whether a repeated primitive identified by said primitive decoder is to be discarded or written to said forward channel FIFO during a forward path transmission or said backward channel FIFO during a backward path transmission,
wherein said repeated primitive is discarded only after at least two of said repeated primitives are entered into said forward channel FIFO during said forward path transmission, or said backward channel FIFO during said backward path transmission, so as to ensure said SAS expander transmits two or more of any repeated primitive, and
wherein said programmable threshold forms within each said forward channel FIFO and said backward channel FIFO a reserve portion having storage capacity for at least two transitions in primitive type.
4. The SAS expander as claimed in claim 3 , wherein said reserve portion within each said forward channel FIFO and said backward channel FIFO 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 FIFO buffers, (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.