linux-kernel - Re: [PATCH v4 03/13] PCI: pci_stub: Suppress kernel DMA ownership auto-claiming

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Date:   Thu, 30 Dec 2021 20:40:19 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joerg Roedel <joro@...tes.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Christoph Hellwig <hch@...radead.org>,
        Kevin Tian <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Dan Williams <dan.j.williams@...el.com>, rafael@...nel.org,
        Diana Craciun <diana.craciun@....nxp.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Eric Auger <eric.auger@...hat.com>,
        Liu Yi L <yi.l.liu@...el.com>,
        Jacob jun Pan <jacob.jun.pan@...el.com>,
        Chaitanya Kulkarni <kch@...dia.com>,
        Stuart Yoder <stuyoder@...il.com>,
        Laurentiu Tudor <laurentiu.tudor@....com>,
        Thierry Reding <thierry.reding@...il.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Jonathan Hunter <jonathanh@...dia.com>,
        Li Yang <leoyang.li@....com>,
        Dmitry Osipenko <digetx@...il.com>,
        iommu@...ts.linux-foundation.org, linux-pci@...r.kernel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 03/13] PCI: pci_stub: Suppress kernel DMA ownership
 auto-claiming

On Thu, Dec 30, 2021 at 04:24:14PM -0600, Bjorn Helgaas wrote:

> I was speculating that maybe the DMA ownership claiming must be done
> *before* the driver's .probe() method?  

This is correct.

> If DMA ownership could be claimed by the .probe() method, we
> wouldn't need the new flag in struct device_driver.

The other requirement is that every existing driver must claim
ownership, so pushing this into the device driver's probe op would
require revising almost every driver in Linux...

In effect the new flag indicates if the driver will do the DMA
ownership claim in it's probe, or should use the default claim the
core code does.

In almost every case a driver should do a claim. A driver like
pci-stub, or a bridge, that doesn't actually operate MMIO on the
device would be the exception.

Jason