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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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