[fcorella@gslxsrv2.rose.hp.com: address aliasing]

[Francisco Corella <fcorella@gslxsrv2.rose.hp.com>]

Return-Path: <fcorella@gslxsrv2.rose.hp.com>
Date: Mon, 4 Nov 1996 20:03:53 -0800
From: Francisco Corella <fcorella@gslxsrv2.rose.hp.com>
To: alan@znyx.com
Cc: pci-sig-request@znyx.com, larry_shintaku@hp-santaclara-om3.om.hp.com,
        roberts@rosemail.rose.hp.com, zhangc@ecs.ecs.csus.edu,
        ac@rosemail.rose.hp.com, linn@rosemail.rose.hp.com,
        rb@rosemail.rose.hp.com, fcorella@rosemail.rose.hp.com
Subject: address aliasing

Alan,

We've had an internal email discussion here at HP about your solution
for the ordering problem by address aliasing.  Here are some comments
extracted from that discussion.  

We recognize that your solution has the important advantage that it
does not require the use of reserved connector pins, nor additional
pins on the chips.  It does not seem to work for DMA, but that could
be fixed by combining it with part of our solution, as discussed
earlier.  However, aliasing has additional drawbacks:

1. The card manufacturer would have to anticipate whether the card is
going to be used by multiple masters, and if so by how many.  A card
accessible by N masters will do for less than N, but how do you choose
N?  In practice, manufacturers will choose the simplest and cheapest
option, N = 1, i.e. no aliasing, and the problem will remain.

2. If a card with N-way aliasing must be addressed by N+1 masters,
then, to avoid the problem it will be necessary to ensure mutual
exclusion of accesses by masters that share an alias, and this will by
complex and will impact performance.

3. Software is required to assume the burden of allocating the alias
images among the masters.

4. If something goes wrong in all this and two masters end up
accessing the same alias at the same time, you will have the worst
kind of intermittent error.  The system will usually work fine, and
perhaps even pass a rigorous test.  But then there could be rare and
mysterious problems in the field to deal with.

Francisco

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