Motorola MVME2400 Series Wartungshandbuch PDF herunterladen (Seite 103)

Functional Description

http://www.mcg.mot.com/literature 2-45

The PCI Master must make the determination to perform the resolution

function since it must make some decisions on possibly removing a

currently pending command from the PPC FIFO.

There are some performance issues related to bridge lock resolution. PHB

offers two mechanism that allow fine tuning of the bridge lock resolution

function.

Programmable Lock Resolution

Consider the scenario where the PPC Slave is hosting a read cycle and the

PCI Slave is hosting a posted write transaction. If both transactions happen

at roughly the same time, then the PPC Slave will hold off its transaction

until the PCI Slave can fill the PCI FIFO with write posted data. Once this

happens, both slaves will be stalled and a bridge lock resolution cycle will

happen. The effect of this was to make the PPC Slave waste PPC bus

bandwidth. In addition, a full PCI FIFO will cause the PCI Slave to start

issuing wait states to the PCI bus.

From the perspective of the PCI bus, a better solution would be to select a

PCI FIFO threshold that will allow the bridge lock resolution cycle to

happen early enough to keep the PCI FIFO from getting filled. A similar

case exists with regards to PCI read cycles. Having the bridge lock

resolution associated with a particular PCI FIFO threshold would allow the

PPC Master to get an early enough start at prefetching read data to keep the

PCI Slave from starving for read data.

From the perspective of the PPC bus, a selective FIFO threshold will make

the PPC Slave release the PPC bus at an earlier time thereby reducing

wasted PPC bus bandwidth. PHB offers an option to have the PPC Slave

remove a stalled transaction immediately upon detecting any PCI Slave

activity. This option would help in the case where distributing PPC60x bus

bandwidth between multiple masters is of utmost importance.

The PHB is tuned to provide the most efficient solution for bridge lock

resolution under normal operating conditions. If further fine tuning is

desired, the WLRT/RLRT (Write Lock Resolution Threshold/Read Lock

Resolution Threshold) fields within the HCSR can be adjusted

accordingly. Note that the FIFO full option exists mainly to remain

architecturally backwards compatible with previous bridge designs.

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Motorola MVME2400 Series Wartungshandbuch Seite 103