Software Operational Model For Itds - Intel IXP45X Developer's Manual

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• The endpoint is an OUT and Transaction X Length goes to zero before all the Mult
transactions have executed (ran out of data), or
• The endpoint is an IN and the endpoint delivers a short packet, or an error occurs
on a transaction before Mult transactions have been executed. The end of micro-
frame may occur before all of the transaction opportunities have been executed.
When this happens, the transfer state of the transfer description is advanced to
reflect the progress that was made, the result written back to the iTD and the host
controller proceeds to processing the next micro-frame. Refer to Appendix D for a
table summary of the host controller required behavior for all the high-bandwidth
transaction cases.
9.14.7.2

Software Operational Model for iTDs

A client buffer request to an isochronous endpoint may span 1 to N micro-frames.
When N is larger than one, system software may have to use multiple iTDs to read or
write data with the buffer (if N is larger than eight, it must use more than one iTD).
Figure 60, "Example Association of iTDs to Client Request Buffer" on page 429
illustrates the simple model of how a client buffer is mapped by system software to the
periodic schedule (i.e. the periodic frame list and a set of iTDs). On the right is the
client description of its request. The description includes a buffer base address plus
additional annotations to identify which portions of the buffer should be used with each
bus transaction. In the middle is the iTD data structures used by the system software
to service the client request. Each iTD can be initialized to service up to 24
transactions, organized into eight groups of up to three transactions each. Each group
maps to one micro-frame's worth of transactions. The EHCI controller does not provide
per-transaction results within a micro-frame. It treats the per-micro-frame transactions
as a single logical transfer. On the left is the host controller's frame list. System
software establishes references from the appropriate locations in the frame list to each
of the appropriate iTDs. If the buffer is large, then system software can use a small set
of iTDs to service the entire buffer. System software can activate the transaction
description records (contained in each iTD) in any pattern required for the particular
data stream.
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Intel IXP45X and Intel IXP46X Product Line of Network Processors
Developer's Manual
428
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IXP45X and Intel IXP46X Product Line of Network Processors—USB 2.0 Host Controller
August 2006
Order Number: 306262-004US