Subject: Re: FreeBSD Bus DMA
To: Paul A Vixie <>
From: Justin T. Gibbs <>
List: tech-kern
Date: 06/11/1998 23:13:49
>> That is an oversimplification.  There are two problems with the VLB cards.
>> DMA to pages above 16MB that fall into a "mirror" of the BIOS window fail
>> on some revisions.  The other is that transfers that wrap a 16MB boundary
>> additionally fail on other early versions.  Bouncing all memory above 16MB
>> is a "sledgehammer" approach to fixing these problems.
>and how many of these machines are there?

You'd be surprised, but I won't argue that the number is significant.

>and are any new ones being built?

This depends on whether other bogus hardware could benefit from having
a filter function or not.  The question is, is it generically useful?

>but to change an API design in a way that impacts the rest of the drivers,
>one would have to show that the change is going to help the system grow to
>be more useful to more NEW users.  i've got to say that VLB only *deserves*
>a sledgehammer at this point.  let's support it, but let's not change every
>other part of the system to make it possible to eke out the last erg of
>performance from it.

The difference is the addition of two NULL arguments to all clients.  It's
not rocket science, but I'm willing to concede this change if it brings the
two implementations closer together.

>arguments of the form "but this generalization, however painful on a few
>architectures, means that drivers are more portable to other architectures"
>hold a lot more weight, but for the same reason.

I don't have the experience to say whether this "feature" would be useful
for supporting other hardware or systems.