Re: [RFC PATCH v2 1/2] pci: Create PCIe requester ID interface

[Bjorn Helgaas <bhelgaas-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>]

On Thu, Jul 25, 2013 at 02:25:04PM -0400, Don Dutile wrote:
> On 07/25/2013 01:19 PM, Bjorn Helgaas wrote:
> >On Wed, Jul 24, 2013 at 04:42:03PM -0400, Don Dutile wrote:
> >>On 07/23/2013 06:35 PM, Bjorn Helgaas wrote:
> >>>On Thu, Jul 11, 2013 at 03:03:27PM -0600, Alex Williamson wrote:
> >>>>+ * pcie_for_each_requester - Call callback @fn on each devices and DMA source
> >>>>+ *                           from @requestee to the PCIe requester ID visible
> >>>>+ *                           to @bridge.
> >>>
> >>>Transactions from a device may appear with one of several requester IDs,
> >>>but there's not necessarily an actual pci_dev for each ID, so I think the
> >>ditto above; have to have a pdev for each id....
> >
> >This *might* be true, but I don't think we should rely on it.  For
> >example:
> >
> >   00:1c.0 PCIe to PCI bridge to [bus 01]
> >   01:01.0 PCI endpoint
> >
> >The bridge will take ownership of DMA transactions from the 01:01.0
> >endpoint.  An IOMMU on bus 00 will see a bridge-assigned requester
> >ID of 01:00.0 (subordinate bus number, devfn zero), but there is no
> >01:00.0 device.
> >
> Clarification:
> I meant that each requester-id must have at least 1 PCI device associated
> with it.

I don't think that's true, as in the example above.  Requester ID
0x0100 has no pci_dev associated with it.  What am I missing?

Maybe you mean that requester ID 0x0100 is associated with pci_dev
01:01.0 in the sense that DMAs from 01:01.0 appear with that ID?
That's true, but I can't think of a reason why we would start with
ID 0x0100 and try to look up 01:01.0 from it.  And of course, if you
*did* try to look up the device, there could be several of them.

Bjorn