netdev - Re: [PATCH 1/5] iommu: Replace uses of IOMMU_CAP_CACHE_COHERENCY with dev_is_dma

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Message-ID: <20220406160623.GI2120790@nvidia.com>
Date:   Wed, 6 Apr 2022 13:06:23 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Christoph Hellwig <hch@....de>
Cc:     Robin Murphy <robin.murphy@....com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Christian Benvenuti <benve@...co.com>,
        Cornelia Huck <cohuck@...hat.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Gerald Schaefer <gerald.schaefer@...ux.ibm.com>,
        iommu@...ts.linux-foundation.org, Jason Wang <jasowang@...hat.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-arm-msm@...r.kernel.org, linux-rdma@...r.kernel.org,
        linux-s390@...r.kernel.org,
        Matthew Rosato <mjrosato@...ux.ibm.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Nelson Escobar <neescoba@...co.com>, netdev@...r.kernel.org,
        Rob Clark <robdclark@...il.com>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        virtualization@...ts.linux-foundation.org,
        Will Deacon <will@...nel.org>,
        "Tian, Kevin" <kevin.tian@...el.com>
Subject: Re: [PATCH 1/5] iommu: Replace uses of IOMMU_CAP_CACHE_COHERENCY
 with dev_is_dma_coherent()

On Wed, Apr 06, 2022 at 05:50:56PM +0200, Christoph Hellwig wrote:
> On Wed, Apr 06, 2022 at 12:18:23PM -0300, Jason Gunthorpe wrote:
> > > Oh, I didn't know about device_get_dma_attr()..
> 
> Which is completely broken for any non-OF, non-ACPI plaform.

I saw that, but I spent some time searching and could not find an
iommu driver that would load independently of OF or ACPI. ie no IOMMU
platform drivers are created by board files. Things like Intel/AMD
discover only from ACPI, etc.

So it might be OK in the constrained case of having an iommu_domain
attached to the device being queried?

Jason