Texas Instruments TMS320TCI648x manual RX Cppi Mode Explanation

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SRIO Functional Description

Although the switch fabric delivers the segments of multi-packet messages in the order they were sent, buffer resources at the receiving endpoint may only become available after the initial segment(s) of a message have had to be retried. The peripheral can accept out-of-order segments and track completion of the overall message. Scenario A in Figure 20 shows this concept.

For applications that are set up for specific message flows between a single source and destination, it may require in-order delivery of messages. This is described in scenario B of Figure 20. This scenario is similar to scenario A, although one message may be retried due to a lack of receiver resources, subsequent pipelined messages may arrive just as resources are freed up. This is a problem for systems requiring in-order message delivery. In this case, the peripheral needs to record the Src_id/mailbox/letter of the first retried message and retry all subsequent new requests until resources are available and a segment for that Src_id/mailbox/letter is received. As long as all messages are from the same source and that source sends (and retries) packets in order, then all messages will be received in order. Note that this solution is less effective when multiple sources share an RX queue. The RX CPPI Control register (Address offset 0744h) sets this mode of operation on all receive queues. Once this mode is set and a retry is issued, the queue will continue to wait for an incoming message that matches the Src_id/mailbox/letter combination. If no such packet arrives, the RX queue is unusable in a locked state. To reenable the queue, the in-order bit in the RX CPPI Control register must be disabled by software for that queue, after which it may be enabled again if desired. The in-order mode of operation is only valid on multi-segment queues because single-segment messages will never generate RETRY responses.

Figure 20. RX CPPI Mode Explanation

Scenario A -￿Default

Switch

Scenario￿B￿-￿In￿order￿mode

Open

C0

Retry

Open

B2

Retry

Data￿flow￿destined￿for￿the

same￿RX￿queue

RX￿queue￿status￿when

packet￿arrives

OpenOpen

B1 B0

RetryRetry

Action

RX￿queue￿status￿when

packet￿arrives

Open

A1

Accept

Full

A0

Retry

Endpoint

Switch

Open

C0

Retry

Open

B2

Retry

Open

B1

Retry

Open

B0

Retry

Action

Full

A1

Retry

Full

A0

Retry

Endpoint

Records￿SourceID/letter￿of

first￿retry￿packet

SPRUE13A –September 2006 Serial RapidIO (SRIO) 49

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