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

[Jason Gunthorpe]

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.

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

Jason