linux-kernel - Re: [PATCH v2] iommu/arm-smmu-qcom: Only enable stall on smmu-v2

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Message-ID: <CAF6AEGtZSOMi-=AOmjoXSVkwfyvKOymSuRRMZ7jOcM2wyhu5qg@mail.gmail.com>
Date: Tue, 7 Jan 2025 07:26:44 -0800
From: Rob Clark <robdclark@...il.com>
To: Will Deacon <will@...nel.org>
Cc: iommu@...ts.linux.dev, linux-arm-msm@...r.kernel.org, 
	freedreno@...ts.freedesktop.org, Robin Murphy <robin.murphy@....com>, 
	Rob Clark <robdclark@...omium.org>, Joerg Roedel <joro@...tes.org>, 
	"moderated list:ARM SMMU DRIVERS" <linux-arm-kernel@...ts.infradead.org>, 
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] iommu/arm-smmu-qcom: Only enable stall on smmu-v2

On Tue, Jan 7, 2025 at 4:57 AM Will Deacon <will@...nel.org> wrote:
>
> On Thu, Jan 02, 2025 at 10:32:31AM -0800, Rob Clark wrote:
> > From: Rob Clark <robdclark@...omium.org>
> >
> > On mmu-500, stall-on-fault seems to stall all context banks, causing the
> > GMU to misbehave.  So limit this feature to smmu-v2 for now.
> >
> > This fixes an issue with an older mesa bug taking outo the system
> > because of GMU going off into the weeds.
> >
> > What we _think_ is happening is that, if the GPU generates 1000's of
> > faults at ~once (which is something that GPUs can be good at), it can
> > result in a sufficient number of stalled translations preventing other
> > transactions from entering the same TBU.
>
> MMU-500 is an implementation of the SMMUv2 architecture, so this feels
> upside-down to me. That is, it should always be valid to probe with
> the less specific "SMMUv2" compatible string (modulo hardware errata)
> and be limited to the architectural behaviour.

I should have been more specific and referred to qcom,smmu-v2

> So what is about MMU-500 that means stalling doesn't work when compared
> to any other SMMUv2 implementation?

Well, I have a limited # of data points, in the sense that there
aren't too many a6xx devices prior to the switch to qcom,smmu-500..
but I have access to crash metrics for a lot of sc7180 devices
(qcom,smmu-v2), and I've been unable to find any signs of this sort of
stall related issue.

So maybe I can't 100% say this is qcom,smmu-500 vs qcom,smmu-v2, vs
some other change in later gens that used qcom,smmu-500 or some other
factor, I'm not sure what other conclusion to draw.

BR,
-R