Re: [RFC PATCH v1 0/6] Resolve unwanted DMA backing with IOMMU

From: Dmitry Osipenko
Date: Fri Jul 27 2018 - 10:10:29 EST

On Friday, 27 July 2018 12:03:28 MSK Will Deacon wrote:
> On Fri, Jul 27, 2018 at 10:25:13AM +0200, Joerg Roedel wrote:
> > On Fri, Jul 27, 2018 at 02:16:18AM +0300, Dmitry Osipenko wrote:
> > > The proposed solution adds a new option to the base device driver
> > > structure that allows device drivers to explicitly convey to the drivers
> > > core that the implicit IOMMU backing for devices must not happen.
> >
> > Why is IOMMU mapping a problem for the Tegra GPU driver?
> >
> > If we add something like this then it should not be the choice of the
> > device driver, but of the user and/or the firmware.
> Agreed, and it would still need somebody to configure an identity domain so
> that transactions aren't aborted immediately. We currently allow the
> identity domain to be used by default via a command-line option, so I guess
> we'd need a way for firmware to request that on a per-device basis.

The IOMMU mapping itself is not a problem, the problem is the management of
the IOMMU. For Tegra we don't want anything to intrude into the IOMMU
activities because:

1) GPU HW require additional configuration for the IOMMU usage and dumb
mapping of the allocations simply doesn't work.

2) Older Tegra generations have a limited resource and capabilities in regards
to IOMMU usage, allocating IOMMU domain per-device is just impossible for

3) HW performs context switches and so particular allocations have to be
assigned to a particular contexts IOMMU domain.

Some of the above is due to a SW driver model (and its work-in-progress
status), other is due to a HW model. So essentially we need a way for a driver
to tell the core not to mess with IOMMU stuff of drivers device behind the
drivers back.

I'm not sure what you guys are meaning by the "firmware", could you elaborate
please? Do you mean the Open Firmware and hence the devicetree or what?