linux-kernel - Re: [Freedreno] [PATCH v9 4/7] iommu/arm-smmu: Add a pointer to the attached device to smmu

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Message-ID: <20200716085053.GB6771@willie-the-truck>
Date:   Thu, 16 Jul 2020 09:50:53 +0100
From:   Will Deacon <will@...nel.org>
To:     Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>,
        linux-arm-msm@...r.kernel.org, Joerg Roedel <joro@...tes.org>,
        Robin Murphy <robin.murphy@....com>,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        John Stultz <john.stultz@...aro.org>,
        freedreno@...ts.freedesktop.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [Freedreno] [PATCH v9 4/7] iommu/arm-smmu: Add a pointer to the
 attached device to smmu_domain

On Mon, Jul 13, 2020 at 11:19:17AM -0600, Jordan Crouse wrote:
> On Mon, Jul 13, 2020 at 04:09:02PM +0100, Will Deacon wrote:
> > On Fri, Jun 26, 2020 at 02:00:38PM -0600, Jordan Crouse wrote:
> > > diff --git a/drivers/iommu/arm-smmu.h b/drivers/iommu/arm-smmu.h
> > > index 5f2de20e883b..d33cfe26b2f5 100644
> > > --- a/drivers/iommu/arm-smmu.h
> > > +++ b/drivers/iommu/arm-smmu.h
> > > @@ -345,6 +345,7 @@ struct arm_smmu_domain {
> > >  	struct mutex			init_mutex; /* Protects smmu pointer */
> > >  	spinlock_t			cb_lock; /* Serialises ATS1* ops and TLB syncs */
> > >  	struct iommu_domain		domain;
> > > +	struct device			*dev;	/* Device attached to this domain */
> > 
> > This really doesn't feel right to me -- you can generally have multiple
> > devices attached to a domain and they can come and go without the domain
> > being destroyed. Perhaps you could instead identify the GPU during
> > cfg_probe() and squirrel that information away somewhere?
> 
> I need some help here. The SMMU device (qcom,adreno-smmu) will have at least two
> stream ids from two different platform devices (GPU and GMU) and I need to
> configure split-pagetable and stall/terminate differently on the two domains.

Hmm. How does the GPU driver know which context bank is assigned to the GPU
and which one is assigned to the GMU? I assume it needs this information so
that it can play its nasty tricks with the TTBR registers?

I ask because if we need to guarantee stability of the context-bank
assignment, then you could match on that in the ->init_context() callback,
but now I worry that it currently works by luck :/

Do we need to add an extra callback to allocate the context bank?

Will