[PATCH v2 0/3] iommu: Enable non-strict DMA on QCom SD/MMC

[Rajat Jain]

Hi Robin, Doug,

On Wed, Jul 14, 2021 at 8:14 AM Doug Anderson <dianders at chromium.org> wrote:
>
> Hi,
>
> On Tue, Jul 13, 2021 at 11:07 AM Robin Murphy <robin.murphy at arm.com> wrote:
> >
> > On 2021-07-08 15:36, Doug Anderson wrote:
> > [...]
> > >> Or document for the users that want performance how to
> > >> change the setting, so that they can decide.
> > >
> > > Pushing this to the users can make sense for a Linux distribution but
> > > probably less sense for an embedded platform. So I'm happy to make
> > > some way for a user to override this (like via kernel command line),
> > > but I also strongly believe there should be a default that users don't
> > > have to futz with that we think is correct.
> >
> > FYI I did make progress on the "punt it to userspace" approach. I'm not
> > posting it even as an RFC yet because I still need to set up a machine
> > to try actually testing any of it (it's almost certainly broken
> > somewhere), but in the end it comes out looking surprisingly not too bad
> > overall. If you're curious to take a look in the meantime I put it here:
> >
> > https://gitlab.arm.com/linux-arm/linux-rm/-/commits/iommu/fq

I was wondering if you got any closer to testing / sending it out? I
looked at the patches and am trying to understand, would they also
make it possible to convert at runtime, an existing "non-strict"
domain (for a particular device) into a "strict" domain leaving the
other devices/domains as-is? Please let me know when you think your
patches are good to be tested, and I'd also be interested in trying
them out.

>
> Being able to change this at runtime through sysfs sounds great and it
> fills all the needs I'm aware of, thanks! In Chrome OS we can just use
> this with some udev rules and get everything we need.

I still have another (inverse) use case where this does not work:
We have an Intel chromebook with the default domain type being
non-strict. There is an LTE modem (an internal PCI device which cannot
be marked external), which we'd like to be treated as a "Strict" DMA
domain.

Do I understand it right that using Rob's patches, I could potentially
switch the domain to "strict" *after* booting (since we don't use
initramfs), but by that time, the driver might have already attached
to the modem device (using "non-strict" domain), and thus the damage
may have already been done? So perhaps we still need a device property
that the firmware could use to indicate "strictness" for certain
devices at boot?

Thanks,
Rajat