2. Interface Operation
43
Tsi308 User Manual
80D4000_MA001_02
Tundra Semiconductor Corporation
www.tundra.com
Similar to in PCI mode, Tsi308 can handle up to four PCI-X read requests and they are assigned
to Delayed Request Buffers as usual, however a single PCI-X read request can request up to 4K
bytes. The Tsi308 implements a maximum of 512 byte buffering per request. For reads
requesting greater than 512 bytes, Tsi308 throttles the requests issued on HyperTransport by
issuing a subrequest on HyperTransport if at least 64 bytes of buffer space is available in
Outbound Data Buffer allotted for that particular request until the byte count is satisfied. As the
data arrives from HyperTransport into Outbound Data Buffer, PCI-X Master behaving as Split
Completer connects on PCI-X bus and transfers the data to the original requester in chunks of
128 bytes disconnecting at naturally aligned 128-byte boundary or Allowable Disconnect
Boundary (ADB).
2.4.6
SrcTags
The SrcTag for each HyperTransport read request is formed by concatenating the delayed
request buffer number with the number of the HyperTransport subrequest being issued by that
buffer.
2.4.7
Sequences
HyperTransport subrequests that are part of the same PCI-X request must be tagged with a
matching nonzero SeqID to guarantee ordering at the target. This 4-bit SeqID is formed by
concatenating a leading 1 (guaranteeing a nonzero result) with the 2-bit delayed request buffer
number and one bit that toggles for each occupation of the delayed request buffer. The
concatenation prevents consecutive PCI-X reads from being issued with the same SeqID and
appearing to have HyperTransport ordering requirements.
2.4.8
Read Responses
As the read responses return from HyperTransport, the data is stored in the Outbound Data
Buffers. Even though sequenced requests are guaranteed to reach the target in order, responses
may be received from the target out of order. When all the data from the first HyperTransport
requests is received (the amount required is controlled by the InitCount fields of the Read
Control CSRs), the PCI interface ceases retrying the request. Read data is supplied from the
buffers when the request is next reissued. Data streams to the PCI bus until the transaction is
disconnected by the PCI master or until the next data required is not present in the Outbound
Data Buffers.
The difference in operation while operating in PCI-X mode is that:
•
Original read request is split first time while latching request and requestor information.
•
Tsi308 connects itself with original requester on PCI-X as Split Completer as opposed to
waiting for the request to be reissued.
•
Data is transferred in ADBs as PCI-X master is not allowed to disconnect arbitrarily.
Содержание TSI308
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