BCM1250/BCM1125/BCM1125H
User Manual
10/21/02
B r o a d c o m C o r p o r a t i o n
Page
212
Section 8: PCI Bus and HyperTransport Fabric
Document
1250_1125-UM100CB-R
Force Isochronous Mode Address Range
Allowing the isochronous bit to set the L2 cacheable bit on the ZBbus is useful for new devices that support
setting the Isochronous bit during requests, but cannot be used by devices that do not allow control of the
Isochronous bit or that are bridged from other buses by a bridge that will not translate requests appropriately.
Therefore for revisions RevId 3 or greater of the HT interface there is an additional BAR (the IsocBAR) and
mask (the IsocIgnMask) that is used to specify a range of addresses that will be forced to behave as if the
Isochronous bit is set. The function is enabled when bit [0] of the IsocBAR is set. The IsocIgnMask is used to
indicate which bits should be ignored when the address from a HT request is compared with the IsocBAR. If
the other bits match between the received address and the IsocBAR then the isochronous bit in the request is
forced to be set and the L2CA flag will be set on the ZBbus. The isochronous bit is forced if:
(htAddr[35:5] | IsocIgnMask[31:1]) == (IsocBAR[31:1] | IsocIgnMask[31:1)
In addition to allowing the PCI BAR style power-of-two windows this allows for more complicated situations.
For example consider the configuration:
IsocBAR =
32'h08120001
IsocIgnMask = 32'h0200FF02
This defines a 1MByte region with base 00_8120_0000 in which the Isochronous bit will be forced for the first
64 bytes in every 4K (since addr[11:6] must be zero and addr[5] is ignored). Bit 25 of the IsocIgnMask is set
to ignore address[29] since that is only used to set the endian policy. Assuming buffer alignment this could be
used to allow packet headers to be cached and bodies to be written to memory.
Note that bits [39:36] are always ignored in the comparison so that the
Ex_xxxx_xxxx
address will have the
same L2CA behaviour as the
0x_xxxx_xxxx
address that it maps to.
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The interface provides limited support for double-hosted HyperTransport chains. This is primarily intended for
interconnection of two BCM1250s, two BCM1125Hs or a BCM1250 to a BCM1125H. The HT configuration
registers are accessible from the HyperTransport fabric as device=0 function=0. At system startup one part
must be designated the master, and the other the slave (this could be done by using the reset configuration
resistors that are allocated for software use, or by having different bootstrap code on the two devices). The
master will configure the chain as described in the HyperTransport specification.
The two parts can communicate over the fabric. Since they have the same address map, a mapping must be
provided to allow each access to the other's memory space. This is done in a simple manner using a direct
map. Accesses from the ZBbus to the region of the address space
E0_0000_0000
-
EF_FFFF_FFFF
will be
sent to the HyperTransport fabric (since it is in region N in
). This range will be accepted
by the host bridge on the part on the other end of the fabric, and direct mapped into the low address space
(
00_0000_0000
-
0F_FFFF_FFFF
) and a second level decode performed to determine if the request should
be forwarded to the ZBbus, the local PCI or sent back out on the HyperTransport. Accesses to these addresses
pass over the I/O fabric and therefore outside the coherency domain. Software will need to manage the
coherency of any data between the two parts. The space should be mapped either uncacheable or cacheable
non-coherent in the CPU, and will not be cached in the L2 cache.
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