Re: [PATCH v2 09/25] iommu/fsl_pamu: Implement an IDENTITY domain

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Jun 01, 2023 at 08:37:45PM +0100, Robin Murphy wrote:
> On 2023-05-16 01:00, Jason Gunthorpe wrote:
> > Robin was able to check the documentation and what fsl_pamu has
> > historically called detach_dev() is really putting the IOMMU into an
> > IDENTITY mode.
> 
> Unfortunately it was the other way around - it's the call to
> fsl_setup_liodns() from fsl_pamu_probe() which leaves everything in bypass
> by default (the PAACE_ATM_NO_XLATE part, IIRC), whereas the detach_device()
> call here ends up disabling the given device's LIODN altogether

Er, I see.. Let me think about it, you convinced me to change it from
PLATFORM, so maybe we should go back to that if it is all wonky.

> There doesn't appear to have ever been any code anywhere for putting
> things *back* into bypass after using a VFIO domain, so as-is these
> default domains would probably break all DMA :(

Sounds like it just never worked right.

ie going to VFIO mode was always a one way trip and you can't go back
to a kernel driver.

I don't think this patch makes it worse because we call the identity
attach_dev in all the same places we called detach_dev in the first
place.

We add an extra call at the start of time, but that call is NOP'd
by this:

> >	if (domain == platform_domain || !domain)
> > +		return 0;
> > +

(bah, and the variable name needs updating too)

Honestly, I don't really want to fix FSL since it seems abandoned, so
either this patch or going back to PLATFORM seems like the best option.

Jason