Messages in this thread |  | | Date | Tue, 22 May 2001 10:51:43 -0700 | From | Jonathan Lundell <> | Subject | Re: 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. - 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/
|  |