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

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Date:   Wed, 6 Apr 2022 11:14:46 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Christoph Hellwig <hch@....de>
Cc:     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>,
        Robin Murphy <robin.murphy@....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 03:51:50PM +0200, Christoph Hellwig wrote:
> On Wed, Apr 06, 2022 at 09:07:30AM -0300, Jason Gunthorpe wrote:
> > Didn't see it
> > 
> > I'll move dev_is_dma_coherent to device.h along with
> > device_iommu_mapped() and others then
> 
> No.  It it is internal for a reason.  It also doesn't actually work
> outside of the dma core.  E.g. for non-swiotlb ARM configs it will
> not actually work.

Really? It is the only condition that dma_info_to_prot() tests to
decide of IOMMU_CACHE is used or not, so you are saying that there is
a condition where a device can be attached to an iommu_domain and
dev_is_dma_coherent() returns the wrong information? How does
dma-iommu.c safely use it then?

In any case I still need to do something about the places checking
IOMMU_CAP_CACHE_COHERENCY and thinking that means IOMMU_CACHE
works. Any idea?

Jason