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

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Message-ID: <YgpbhlPOZsLFm4It@kroah.com>
Date:   Mon, 14 Feb 2022 14:39:18 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Jason Gunthorpe <jgg@...dia.com>
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 09:11:17AM -0400, Jason Gunthorpe wrote:
> On Mon, Feb 14, 2022 at 01:51:06PM +0100, Greg Kroah-Hartman wrote:
> > On Mon, Feb 14, 2022 at 08:38:42AM -0400, Jason Gunthorpe wrote:
> > > 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.
> > 
> > "has not been" or "will not be"?
> 
> "has not been" as that action was supposed to happen before probe(),
> but the flag skips it.
> 
> A driver that sets this flag can still decide to enable the dma API on
> its own. eg tegra drivers do this.

So you are just forcing the driver to manage this all on their own, so
how about, "driver_managed_dma", or even shorter "managed_dma"?

> > What you want to prevent is the iommu core claiming the device
> > automatically, right?  So how about "prevent_iommu_dma"?
> 
> "claim" is not a good description. iommu always "claims" the device -
> eg sets a domain, sets the dev and bus parameters, etc.
> 
> This really is only about setting up the in-kernel dma api, eg
> allowing dma_map_sg()/etc to work.
> 
> dma api is just one way to operate the iommu, there are others too.
> 
> Think of this flag as 
>   false = the driver is going to use the dma api (most common)
>   true = the driver will decide how to use the iommu by itself
> 
> Does it help think of a clearer name?

See above, you want a driver author to know instantly what this is and
not have to look anything up.
"I_will_manage_the_dma_myself_as_I_really_know_what_I_am_doing" might be
good, but a bit too long :)

thanks,

greg k-h