Re: [PATCH v2] iommu: iommu_group_claim_dma_owner() must always assign a domain

From: Jason Gunthorpe via iommu <iommu@lists.linux-foundation.org>
To: Robin Murphy <robin.murphy@arm.com>
Cc: Will Deacon <will@kernel.org>,
	"Tian, Kevin" <kevin.tian@intel.com>,
	"Rodel, Jorg" <jroedel@suse.de>,
	Qian Cai <quic_qiancai@quicinc.com>,
	"iommu@lists.linux-foundation.org"
	<iommu@lists.linux-foundation.org>
Subject: Re: [PATCH v2] iommu: iommu_group_claim_dma_owner() must always assign a domain
Date: Fri, 6 May 2022 10:10:53 -0300	[thread overview]
Message-ID: <20220506131053.GA522325@nvidia.com> (raw)
In-Reply-To: <9f6680ed-77b6-8440-078c-623406c823aa@arm.com>

On Fri, May 06, 2022 at 10:32:50AM +0100, Robin Murphy wrote:

> > > It is as discussed with Robin, NULL means to return the group back to
> > > some platform defined translation, and in some cases this means
> > > returning back to work with the platform's DMA ops - presumably if it
> > > isn't using the dma api.
> > 
> > This is clearer than the original text.
> 
> Perhaps we could just leave that sentence as "Otherwise the NULL domain
> represents platform-specific behaviour."

Sure, there are only three drivers that use this call path and who
knows what they do I guess..

> > > Soemtimes. This is a statement about the required
> > > atomicity. New/old/block are all valid translations during the
> > > operation. Identity is not.
> > 
> > but new/old/block are not the same type of classifications. A group
> > has an old domain and a new domain at this transition point, and
> > both old/new domains have a domain type (dma, unmanged, block,
> > identity, etc.). Mixing them together only increases confusion here.

Block refers to the global concept of blocking - not to a type.

> Good point - in particular I think the "DMA is always translated" part would
> be more accurately said as "DMA is always managed". 

So you want to say "DMA is always managed" when attaching a domain of
type IOMMU_DOMAIN_UNMANAGED? :)

Lets just clarify a bit that blocking isn't talking about a domain
type:

	/*
	 * Changing the domain is done by calling attach_dev() on the new
	 * domain. This switch does not have to be atomic and DMA can be
	 * discarded during the transition. DMA must always be translated by
	 * either new_domain or group->domain, never something else.
	 *
	 * Note that this is called in error unwind paths, attaching to a
	 * domain that has already been attached cannot fail.
	 */

(aside, I expect down the road we will have some domain capability
'attach is atomic' meaning if new and old have the same translation
then there is no disruption to DMA)

Thanks,
Jason
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