User Manual
BCM1250/BCM1125/BCM1125H
10/21/02
B r o a d c o m C o r p o r a t i o n
Document
1250_1125-UM100CB-R
Section 8: PCI Bus and HyperTransport Fabric Page
219
P
EER
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TO
-P
EER
A
CCESSES
The interface supports peer-to-peer accesses from devices on the PCI bus to devices on the HyperTransport
fabric, and from devices on the HyperTransport fabric to devices on the PCI bus. Requests and data being
transferred in this way do not travel on the ZBbus, they are directly routed through special buffers in the I/O
Bridge 0. In the master bus interface of a peer-to-peer operation the queues are shared with DMA operations
(from I/O devices into the ZBbus) and on the slave side the queues are shared with PIO operations (from the
ZBbus to I/O devices).
Peer-to-peer operations in I/O space are not supported. Neither the PCI bus nor the HyperTransport bus will
accept an incoming I/O space request.
PCI B
US
T
O
H
YPER
T
RANSPORT
F
ABRIC
When the PCI interface is configured in Host Mode, requests on the PCI that fall into the secondary bus
memory range specified in the HyperTransport bridge header will be accepted from the PCI and forwarded to
the HyperTransport fabric. This forwarding is disabled by default, and must be enabled in the
Feature Control
register (offset h40 in the PCI configuration header). In addition requests that match BAR0 and have an
enabled mapping with the send_ldt bit set are forwarded to the HyperTransport (the send_ldt bit overrides any
destination implied by the address). The peer-to-peer bit in the
Feature Control
register must also be set to
enable forwarding through the map table.
If the PCI interface is configured in Device Mode and the peer-to-peer bit is set in the
Feature Control
register
the only requests that will be forwarded to the HyperTransport fabric are those that hit in BAR0 and have an
enabled mapping with the send_ldt bit set. This allows the part to act as a non-transparent bridge (i.e. one with
separate address spaces on each side and explicit routing between the sides) from the PCI host to the
HyperTransport fabric. Note that in device mode the Feature Control register is written by the host on the PCI
bus and cannot be accessed from the ZBbus, thus both the host (through Feature Control ptp_en) and device
(through BAR0 send_ldt) must agree for the PCI-HT access to be permitted.