Subject: Re: FreeBSD Bus DMA
To: Jason Thorpe <email@example.com>
From: Justin T. Gibbs <firstname.lastname@example.org>
Date: 06/11/1998 17:32:46
> > one of the things Justin added was a driver-specific filter function.
> > Justin says that's a better way to detect mappings that oculd tickle
> > driver bugs. VLB buslogic boards were given as a specific example; it
> > sounds plausible to me. (I'd assume that if NetBSD added this, the
> > filter argument would be NULL in almost all cases).
>The only VLB BusLogic bug I'm aware of is the inability to properly
>do DMA using 32-bit addresses. Our driver currently handles that case.
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.
>Jason R. Thorpe email@example.com
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