linux-kernel - Re: [PATCH v3 03/33] iommu/io-pgtable-arm: Add quirk to quiet WARN

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Message-ID: <20250429140504.GD2260621@ziepe.ca>
Date: Tue, 29 Apr 2025 11:05:04 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Rob Clark <robdclark@...il.com>
Cc: dri-devel@...ts.freedesktop.org, freedreno@...ts.freedesktop.org,
	linux-arm-msm@...r.kernel.org, Connor Abbott <cwabbott0@...il.com>,
	Rob Clark <robdclark@...omium.org>, Will Deacon <will@...nel.org>,
	Robin Murphy <robin.murphy@....com>, Joerg Roedel <joro@...tes.org>,
	Nicolin Chen <nicolinc@...dia.com>,
	Kevin Tian <kevin.tian@...el.com>,
	Joao Martins <joao.m.martins@...cle.com>,
	"moderated list:ARM SMMU DRIVERS" <linux-arm-kernel@...ts.infradead.org>,
	"open list:IOMMU SUBSYSTEM" <iommu@...ts.linux.dev>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 03/33] iommu/io-pgtable-arm: Add quirk to quiet
 WARN_ON()

On Tue, Apr 29, 2025 at 06:58:32AM -0700, Rob Clark wrote:
> On Tue, Apr 29, 2025 at 5:28 AM Jason Gunthorpe <jgg@...pe.ca> wrote:
> >
> > On Mon, Apr 28, 2025 at 01:54:10PM -0700, Rob Clark wrote:
> > > From: Rob Clark <robdclark@...omium.org>
> > >
> > > In situations where mapping/unmapping squence can be controlled by
> > > userspace, attempting to map over a region that has not yet been
> > > unmapped is an error.  But not something that should spam dmesg.
> >
> > I think if you want to do something like that using the iommu API the
> > expectation is for the caller to do a iova_to_phys to check what is
> > mapped first? That seems kind of lame..
> >
> > Maybe page table driver should not not be doing these WARNs at all. If
> > we want to check for that the core iommu code should have the WARN_ON?
> >
> > eg iommufd already has a WARN_ON around iommu_unmap failures so having
> > one in the ARM page table is a double WARN.
> >
> > Don't really like using a quirk to change the API contract.
> 
> I'd also be ok to have the WARN_ON instead in the iommu code.  In the
> case where this quirk is needed, I'm using the io_pgtable helpers
> directly, not going via the iommu layer.

Yes, that was my thought

Then all the iommu_map/unmap() calls behave consistently here..

Jason