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
223
6
In a dual host topology, if any incoming packet, with an address mapped as to be forwarded to the outgoing
link and a srcid = 0, is received by a host it will be NXAed.
7
The ISA deadlock prevention mechanism, described in section C.1 of the HyperTransport Specification
Revision 0.17, is not supported. This mechanism has been removed in Revision 1.0 of the specification. As a
result, and as specified in Revision 1.0, no peer-to-peer transactions involving ISA are allowed.
8
The optional secondary bus reset functionality in the Capability command register is implemented.
9
The interface only supports an 8 bit CAD width. There is no support for the 2 or 4 bit widths that are described
in Revision 1.0.
10
The ordering between CPU (or any other internal device) writes to the HyperTransport fabric and DMA Read
Data Return to the HyperTransport fabric is imposed in the HyperTransport interface. This ordering is not part
of the ZBbus. In particular the I/O bridge and ZBbus behave as if the Response may pass posted Writes bit is
always set. If the PCI producer-consumer model is being used read responses should always pass posted
writes and this bit should always be clear. The result of not having the ordering rule internally is that if
HyperTransport device A polls a flag in memory that the CPU writes after writing HyperTransport device B
(which may or may not be the same device), the read response with the flag set may reach device A before
the write reaches B. The ordering can be enforced by the CPU doing a PCI or HyperTransport configuration
register read between the two writes. This forces the write to device B into the HyperTransport interface (where
the ordering rules apply) before the flag gets written in memory. (This also applies for the PCI interface).
11
Posted writes have the highest priority with regard to transmission to the HyperTransport link. Starvation of
non-posted transactions and read responses is prevented by implementing counters in all entries of the
interface transmit FIFOs. The outgoing entry counters will get incremented on each transaction issued. When
an entry’s counter overflows, its priority is elevated.
12
The correct ordering of interrupts with respect to writes from the HyperTransport into the BCM1250 is
maintained. An interrupt that follows a HyperTransport write will not be raised inside the BCM1250 until the
write is visible inside the ZBbus coherency boundary.
13
A non-posted write from the HyperTransport fabric to the ZBbus or PCI bus will be acknowledged when the
write leaves the HyperTransport interface, and will continue as a posted write. Error responses are never
generated for non-posted writes. Therefore, if the HyperTransport device needs to confirm the success of a
non-posted write, it must (1) wait for the write tgtDone response, then (2) issue a read to the write address for
confirmation. It cannot issue the read without waiting for tgtDone since non-posted transactions may bypass
each other.
14
No peer-to-peer I/O space traffic between the HyperTransport and PCI is supported.
15
No DMA master is allowed on an ISA bridge on the HyperTransport or PCI buses. It could result in a deadlock.
16
PCI devices designed for rev2.0 or earlier may not be bridged off the HyperTransport.
17
The LinkFreq register, described in Revision 1.0 of the Specification, has been added to the HyperTransport
Capability registers. This frequency set in this register sets both the transmitter frequency and the maximum
frequency the receiver can accept.
18
Support has been added for frequencies other than those specified in HyperTransport Revision 1.0 by using
the sipFrequencyDirect bit in the HyperTransport SRI Command register.
19
In a dual hosted system determination of the master and slave ends of the chain must be made before link
configuration. Neither the HyperTransport Specification nor the BCM1250 hardware specify the method for
determining this. System designers are free to implement it in whatever way matches their system. Three
possible methods on the BCM1250 are: use one of the software configuration bits latched into the System
Configuration Register from the IO_AD[31:26] during reset (see
); store the information in
the boot ROM; store the information in a configuration EEPROM on the SMBus.
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