Compact PCI specifies a max of 8 slots (one of which is typically the
system board). Regular PCI doesn't have a hard and fast slot limit
(except for the logical limit of 32 devices per bus); the limits are
driven by electrical loading concerns. As I recall, a bus of typical
length can accommodate 10 "loads", where a load is either a device
pin or a slot connector (that is, an expansion card counts as two
loads, one for the device and one for the connector). (I take this to
be a rule of thumb, not a hard spec, based on the detailed electrical
requirements in the PCI spec.)
Still, the presence of bridges opens up the number of devices on a
root PCI bus to a very high number, logically. Certainly having three
or four quad Ethernet cards, so 12 or 16 devices, is a plausible
configuration. As for bandwidth, a 64x66 PCI bus has a nominal burst
bandwidth of 533 MB/second, which would be saturated by 20 full
duplex 100baseT ports that were themselves saturated in both
directions (all ignoring overhead). Full saturation is not reasonable
for either PCI or Ethernet; I'm just looking at order-of-magnitude
numbers here.
The bottom line is: don't make any hard and fast assumption about the
number of devices connected to a root PCI bus.
-- /Jonathan Lundell. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/