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SubjectRe: alpha iommu fixes
At 11:12 PM +1200 2001-05-22, Chris Wedgwood wrote:
>On Mon, May 21, 2001 at 03:19:54AM -0700, David S. Miller wrote:
>
> Electrically (someone correct me, I'm probably wrong) PCI is
> limited to 6 physical plug-in slots I believe, let's say it's 8
> to choose an arbitrary larger number to be safe.
>
>Minor nit... it can in fact be higher than this, but typically it is
>not. CompactPCI implementations may go higher (different electrical
>characteristics allow for this).

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.
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