linux-kernel - Re: [PATCH v5 07/14] PCI: Add driver dma ownership management

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Date:   Mon, 14 Feb 2022 08:38:42 -0400
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Lu Baolu <baolu.lu@...ux.intel.com>,
        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 v5 07/14] PCI: Add driver dma ownership management

On Mon, Feb 14, 2022 at 11:03:42AM +0100, Greg Kroah-Hartman wrote:
> On Tue, Jan 04, 2022 at 09:56:37AM +0800, Lu Baolu wrote:
> > Multiple PCI devices may be placed in the same IOMMU group because
> > they cannot be isolated from each other. These devices must either be
> > entirely under kernel control or userspace control, never a mixture. This
> > checks and sets DMA ownership during driver binding, and release the
> > ownership during driver unbinding.
> > 
> > The device driver may set a new flag (no_kernel_api_dma) to skip calling
> > iommu_device_use_dma_api() during the binding process. For instance, the
> > userspace framework drivers (vfio etc.) which need to manually claim
> > their own dma ownership when assigning the device to userspace.
> > 
> > Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
> >  include/linux/pci.h      |  5 +++++
> >  drivers/pci/pci-driver.c | 21 +++++++++++++++++++++
> >  2 files changed, 26 insertions(+)
> > 
> > diff --git a/include/linux/pci.h b/include/linux/pci.h
> > index 18a75c8e615c..d29a990e3f02 100644
> > +++ b/include/linux/pci.h
> > @@ -882,6 +882,10 @@ struct module;
> >   *              created once it is bound to the driver.
> >   * @driver:	Driver model structure.
> >   * @dynids:	List of dynamically added device IDs.
> > + * @no_kernel_api_dma: Device driver doesn't use kernel DMA API for DMA.
> > + *		Drivers which don't require DMA or want to manually claim the
> > + *		owner type (e.g. userspace driver frameworks) could set this
> > + *		flag.
> 
> Again with the bikeshedding, but this name is a bit odd.  Of course it's
> in the kernel, this is all kernel code, so you can drop that.  And
> again, "negative" flags are rough.  So maybe just "prevent_dma"?

That is misleading too, it is not that DMA is prevented, but that the
kernel's dma_api has not been setup.

Though I agree the name as-is isn't great, I think the comment is good.

Jason