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

[Lu Baolu]

On 12/31/21 9:10 AM, Lu Baolu wrote:
> On 12/31/21 8:40 AM, Jason Gunthorpe wrote:
> > 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.
>
> We still need to call iommu_device_use_dma_api() in bus dma_configure()
> callback. But we can call iommu_device_unuse_dma_api() in the .probe()
> of vfio (and vfio-approved) drivers, so that we don't need the new flag
> anymore.

Oh, wait. I didn't think about the hot-plug case. If we call
iommu_device_use_dma_api() in bus dma_configure() anyway, we can't bind
any (no matter vfio or none-vfio) driver to a device if it's group has
already been assigned to user space. It seems that we can't omit this
flag.

Best regards,
baolu