From: Geoff Steckel <gwes@oat.com>
Subject: 32 bit PCI device DMA - low memory or IOMMU?
To: tech <tech@openbsd.org>
Date: Mon, 14 Jul 2025 23:39:02 -0400

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.

If not it's IOMMU time and that doesn't look simple.

thanks very much,
Geoff Steckel