User`s guide

• I/O transfers from the CPU to the PCI or to 21071-DA CSRs are performed

in order. This policy guarantees a coherent view of PCI I/O space from the

CPU.

• The 21071-DA ﬂushes DMA write data to memory before acknowledging

a barrier command from the CPU. Because explicit ordering commands

are absent on the PCI, the MB instruction is used to order CPU and DMA

accesses.

• The 21071-DA also ﬂushes the I/O write buffer to the PCI before

acknowledging a barrier command. This preserves the order between

CPU I/O accesses and CPU memory accesses.

• The 21071-DA clears the system lock ﬂag on PCI exclusive read and write

transactions to system memory.

3.3.2.13 Deadlock Resolution

There are two major buses allocated for use during data transfers: the sysBus

and the PCI. Some data transfers require the use of both of these buses

to complete. In particular, CPU I/O transfers to or from the PCI require

ownership of the sysBus followed by ownership of the PCI. Similarly, PCI

DMA transfers to or from the memory subsystem require ownership of the PCI

followed by ownership of the sysBus.

Because of the nonpended nature of these buses, during read transfers (I/O or

DMA), both buses must be held at the same time for the transfer to complete.

Generally, because the 21071-DA features write-and-run style buffering, only

one bus must be held at a time during write transfers (I/O or DMA). However,

when a write buffer is full, both buses must be held at the same time so that

some data from the write buffer can be ﬂushed before new data is accepted.

The 21071-DA resolves deadlock by forcing the CPU to relinquish ownership of

the sysBus, thereby giving priority to the PCI agent. By giving priority to the

PCI agent, the 21071-DA provides the system designer more ﬂexibility in the

choice of PCI devices. That is, it supports devices that use the PCI disconnect

in handling deadlock situations. The 21071-DA forces the CPU to relinquish

the sysBus by using a preempt request while arbitrating for the sysBus.

3–18 Functional Description