Magma PCIe Expansion System EB2R - Manual (Page 65)

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2. Most computers allow multiple logical bridges (and thus PCIe
buses) to be correctly configured. Some computers,
unfortunately, place an arbitrary limit on the number of bridge
levels that can be traveled during power-on configuration. Your
computer “power-on-software” should be compliant with the
PCIe bus specification 1.1 in order to effectively use a Magma
PCIe Expansion System. For "fan-out" and "daisy chained"
system configurations, where more than one PCIe expansion
system is being used, you should count all of the logical bridge
levels to your most deeply nested PCI bus to determine the
maximum number of bridge levels that must be traveled.
Exceeding this number may cause software configuration
conflicts even though hardware-wise the PCIe system is
properly configured.
3. For easier understanding, a given PCI system should be
viewed as a “Resource Toll road” that runs from the computer
CPU (resource home), through the “Toll Booths” (the PCIe
switches and logical PCI bridges), to the “Resource Users”
(PCIe Cards). As in the case of a real Toll Road, the more Toll
Booths you have to cross (even if it takes a nanosecond), the
longer it will take you (your data) to get from the PCIe Card to
the CPU and back again. Under most circumstances the effect
cannot even be detected. However, under other conditions, it
can be important to know about.