[Asterisk-Users] PCI front mount chassis?

[Steven Critchfield]

On Fri, 2004-03-12 at 05:26, Rich Adamson wrote:
> > > I too am running 6 cards in my system, although not in a "high traffic 
> > > capacity" load environment.
> > > 
> > > So far my (limited) high-load simulations have shown no problems.
> > 
> > 
> > So - is it apocryphal that the Digium cards (drivers) won't share
> > interrupts?
> > 
> > If there is a real issue with sharing interrupts then it seems to me
> > to be a bug that needs fixing.  PCI bus supports shared interrupts,
> > why doesn't the hardware/driver?
> 
> In most cases, sharing an interrupt is not a problem at all. There have
> been a few cases where _some_ issue was resolved by moving cards around,
> however the majority of those seem to be: a) abrupt system changes with
> no effort to seriously identify the root-cause, b) newbie installations
> where the condition of the underlying system infrastructure is totally
> unknown, or, c) wild recommendations that might have had some basis a
> long time ago but no longer apply.

What I was referring to in this case about sharing an IRQ was the actual
wire trace in the PCI bus. As I understand the PCI spec, there are 4
interrupt lines called A,B,C, and D. In slot 1, They appear in that
order. In slot 2 they shift, in slot 3 they shift and again in slot 4.
By the time you get to slot 5, all interrupts have been in each of the 4
spots and now they have the choice of staying in the same as slot 4 or
shifting again. Most cards use interrupt line A as if they are in slot
1, and therefore if you move them from one slot to another, they would
most likely get their own access to the interrupt line.

This is part of the reason why moving a card around in the chassis
helps. This is also why I would be cautious of trying to run more than 4
high interrupt cards on the PCI bus. 

My next point would be that of, if you need more than 4 cards, you need
more than 1 computer controlling it. That is way too many eggs for one
basket. 
-- 
Steven Critchfield  <critch at basesys.com>