Re: [PATCH v2 1/6] ARM/dma-mapping: Remove iommu_detach_device()

[Jason Gunthorpe <jgg@nvidia.com>]

On Sat, Feb 18, 2023 at 02:59:16PM +0800, Baolu Lu wrote:

> > arm_iommu doesn't work that way, it allocates and assigns domains so
> > when the platform DMA ops are in control the group->domain is not NULL
> 
> This is what the iommu core assumes, right? Any iommu group should
> always has a domain attached, default domain, blocking domain or driver-
> owned unmanaged domain. The iommu core just switches between different
> domains.

That would be nice, but we still have NULL domains in some cases right
now.
 
> > - which is the opposite of S390's design. Further when arm_iommu asks
> > for a NULL domain it doesn't mean "put it back to platform DMA ops" it
> > really means "park the IOMMU it is not being used anymore"
> 
> This is what identity domain and blocking domains were designed to do,
> right?
>
> If my understanding is right, ARM presumably could implement the
> identity default domain and blocking domain. With that implemented,
> iommu_attach/detac_device() could be removed from drivers and everything
> then could go through the iommu core.

Yes, ideally, but I have no idea what the few reamining drivers do
with their code to properly classify it. So what I showed in the
little sketch was to just mark it DOMAIN_PRIVATE and if someone knows
the right answer they can change it to blocking/identity someday.

In the mean time we can get rid of the NULL domain situation. The core
code would immediately attach either the default or 'idle' domain on
probe and a device will always have a domain attached until release.

I still don't entirely understand how exynox works. In ARM64 mode it
should have default domains, but since it doesn't supply a
def_domain_type() op it only gets the default set of default domains
eg PCI devices.

So on ARM64 some devices don't get default domains, and on ARM32 those
devices use arm_iommu but crashed because of a lack of default domain?

It is confusing. It is harmelss to create a DMA default domain on
ARM32, at worst it wastes a bit of memory. So maybe the fact it
crashes on ARM32 indicates a bug on ARM64 where some devices are not
properly using default domains??

Jason