Example Association Of Itds To Client Request Buffer - Intel IXP45X Developer's Manual

USB 2.0 Host Controller—Intel
Figure 60.

Example Association of iTDs to Client Request Buffer

Frame List
Frame i
Frame i+1
Frame I+2
Frame i+n
As noted above, the client request includes a pointer to the base of the buffer and
offsets into the buffer to annotate which buffer sections are to be used on each bus
transaction that occurs on this endpoint. System software must initialize each
transaction description in an iTD to ensure it uses the correct portion of the client
buffer. For example, for each transaction description, the PG field is set to index the
correct physical buffer page pointer and the Transaction Offset field is set relative to
the correct buffer pointer page (e.g. the same one referenced by the PG field). When
the host controller executes a transaction it selects a transaction description record
based on FRINDEX[2:0]. It then uses the current Page Buffer Pointer (as selected by
the PG field) and concatenates to the transaction offset field. The result is a starting
buffer address for the transaction. As the host controller moves data for the
transaction, it must watch for a page wrap condition and properly advance to the next
available Page Buffer Pointer. System software must not use the Page 6 buffer pointer
in a transaction description where the length of the transfer will wrap a page boundary.
Doing so will yield undefined behavior. The host controller hardware is not required to
'alias' the page selector to page zero. USB 2.0 isochronous endpoints can specify a
period greater than one. Software can achieve the appropriate scheduling by linking
iTDs into the appropriate frames (relative to the frame list) and by setting appropriate
transaction description elements active bits to a one.
9.14.7.2.1 Periodic Scheduling Threshold
The Isochronous Scheduling Threshold field in the HCCPARAMS capability register is an
indicator to system software as to how the host controller pre-fetches and effectively
caches schedule data structures. It is used by system software when adding
isochronous work items to the periodic schedule. The value of this field indicates to
system software the minimum distance it can update isochronous data (relative to the
current location of the host controller execution in the periodic list) and still have the
host controller process them.
August 2006
Order Number: 306262--, Revision: 004US
® ®
IXP45X and Intel IXP46X Product Line of Network Processors
iTD
iTD
iTD
Intel
0
1
n
® ®
IXP45X and Intel IXP46X Product Line of Network Processors
Client
Request
USB Xact
Information
B4503-01
Developer's Manual
429