Texas Instruments TMS320TCI648x manual RX Buffer Descriptor Field Descriptions

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SRIO Functional Description

If a RX message’s length is greater than that of the targeted buffer descriptor, an ERROR response is sent back to the source device. In addition, the DSP is notified with the use of the CC field of the RX CPPI buffer descriptor, described as follows. This situation can result from a DSP software error (misallocating a buffer for the queue), or as a result of sender error (sending to a wrong mailbox).

An RX transaction timeout is used by all multi-segment queues, in order to not hang receive mailbox resources in the event that a message segment is lost in the fabric. This response-to-request timer controls the time-out period for sending a response packet and receiving the next request packet of a given multi-segment message. It has the same value and is analogous to the request-to-response timer discussed in the TX CPPI and LSU sections, which is defined by the 24-bit value in the port response time-out CSR (See Section 2.3.3.3). The RapidIO Interconnect Specification states that the maximum time interval (all 1s) is between 3 and 6 seconds. Each multi-segment receive timer requires a 4-bit register. The register is loaded with the current timecode when the response is sent. Each time the timecode changes, a 4-bit compare is done to the register. If the register becomes equal to the timecode again, without the next message segment being seen, then the transaction has timed out. If this happens, the RX buffer resources can be released.

The buffer descriptor points to the corresponding data buffer in memory and also points to the next buffer descriptor in the queue. As segments of a received message arrive, the msgseg field of each segment is monitored to detect the completion of the received message. Once a full message is received, the OWNERSHIP bit is cleared in the packet’s buffer descriptor to give control to the host. At this point, a host interrupt is issued. This interface works with programmable interrupt rate control. There is an ICSR bit for each supported queue. On interrupt, the CPU processes the RX buffer queue, detecting received packets by the status of the OWNERSHIP bit in each buffer descriptor. The host processes the RX queue until it reaches a buffer descriptor with a set OWNERSHIP bit, or set EOQ bit. Once processing is complete, the host updates the RX DMA State Completion Pointer, allowing the peripheral to reuse the buffer.

Figure 19 shows the RX buffer descriptor fields and Table 18 describes them. A RX buffer descriptor is a contiguous block of four 32-bit data words aligned on a 32-bit boundary. Accesses to these registers are restricted to 32-bit boundaries.

Figure 19. RX Buffer Descriptor Fields

Word															Bit￿Fields
Offset	31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16	15	14	13	12	11	10	9	8	7	6	5	4	3	2	1	0
0													next_descriptor_pointer
1															buffer_pointer
2								src_id									pri		tt			reserved							mailbox

o

wt

ne

	s	e	e	e	a
					r
3	o	o	r	o		reserved	cc	message_length
					d
	p	p	s	q

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	Table 18. RX Buffer Descriptor Field Descriptions
Field	Description
next_descriptor_pointer	Next Descriptor Pointer: The 32-bit word aligned memory address of the next buffer
	descriptor in the RX queue. This references the next buffer descriptor from the current
	buffer descriptor. If the value of this pointer is zero, then the current buffer is the last
	buffer in the queue. The DSP core sets the next_descriptor_pointer.
buffer_pointer	Buffer Pointer: The byte aligned memory address of the buffer associated with the
	buffer descriptor. The DSP core sets the buffer_pointer.
sop = 1	Start of Message: Indicates that the descriptor buffer is the first buffer in the message.
	∙ This bit will always be set, as this device only supports one buffer per message.
eop = 1	End of Message: Indicates that the descriptor buffer is the last buffer in the message.
	∙ This bit will always be set, as this device only supports one buffer per message.

SPRUE13A –September 2006	Serial RapidIO (SRIO)	47
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