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SubjectRe: [PATCH v4 03/13] PCI: pci_stub: Suppress kernel DMA ownership auto-claiming
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Hi Jason,

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.

>
> Jason
>

Best regards,
baolu

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