From: Mark Kettenis <mark.kettenis@xs4all.nl>
Subject: Re: 32 bit PCI device DMA - low memory or IOMMU?
To: Geoff Steckel <gwes@oat.com>
Cc: tech@openbsd.org
Date: Tue, 15 Jul 2025 10:27:19 +0200

> Date: Mon, 14 Jul 2025 23:39:02 -0400
> From: Geoff Steckel <gwes@oat.com>
> 
> Hi,
> 
> I hope this is a simple question.
> If the answer is "maybe" or "there are gotchas" I'll experiment.
> 
> I have a 2000-era PCI frame grabber (itifg).
> It can only address 32 bits. It needs huge contiguous buffers.
> Other than that, porting the ancient Linux driver is pretty straightforward.
> 
> Given a modern amd64 architecture and 16G,
> is it likely that 100M under 4G would be available at attach time?
> There are no other devices needing low memory.

That depends.  If you only have 256M of memory in the machine, there
is a good chance that a single contiguous memory block isn't available
anymore by the time the device attaches.  If you have 2GB of memory
your chances are much better.

> If not it's IOMMU time and that doesn't look simple.
> 
> thanks very much,
> Geoff Steckel
> 
> 
>