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

[Jason Gunthorpe]

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@xxxxxxxxxxxxxxx>
> > > >  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.

> 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?

Jason