On 2020/6/11 下午9:44, Bjorn Helgaas wrote:
+++ b/drivers/iommu/iommu.c
@@ -2418,6 +2418,10 @@ int iommu_fwspec_init(struct device *dev, struct
fwnode_handle *iommu_fwnode,
            fwspec->iommu_fwnode = iommu_fwnode;
            fwspec->ops = ops;
            dev_iommu_fwspec_set(dev, fwspec);
+       if (dev_is_pci(dev))
+               pci_fixup_device(pci_fixup_final, to_pci_dev(dev));

Then pci_fixup_final will be called twice, the first in pci_bus_add_device.
Here in iommu_fwspec_init is the second time, specifically for iommu_fwspec.
Will send this when 5.8-rc1 is open.
Wait, this whole fixup approach seems wrong to me.  No matter how you
do the fixup, it's still a fixup, which means it requires ongoing
maintenance.  Surely we don't want to have to add the Vendor/Device ID
for every new AMBA device that comes along, do we?

Here the fake pci device has standard PCI cfg space, but physical
implementation is base on AMBA
They can provide pasid feature.
1, does not support tlp since they are not real pci devices.
2. does not support pri, instead support stall (provided by smmu)
And stall is not a pci feature, so it is not described in struct pci_dev,
but in struct iommu_fwspec.
So we use this fixup to tell pci system that the devices can support stall,
and hereby support pasid.
This did not answer my question.  Are you proposing that we update a
quirk every time a new AMBA device is released?  I don't think that
would be a good model.
Yes, you are right, but we do not have any better idea yet.
Currently we have three fake pci devices, which support stall and pasid.
We have to let pci system know the device can support pasid, because of
stall feature, though not support pri.
Do you have any other ideas?
It sounds like the best way would be to allocate a PCI capability for it, so
detection can be done through config space, at least in future devices,
or possibly after a firmware update if the config space in your system
is controlled by firmware somewhere.  Once there is a proper mechanism
to do this, using fixups to detect the early devices that don't use that
should be uncontroversial. I have no idea what the process or timeline
is to add new capabilities into the PCIe specification, or if this one
would be acceptable to the PCI SIG at all.
That sounds like a possibility.  The spec already defines a
Vendor-Specific Extended Capability (PCIe r5.0, sec 7.9.5) that might
be a candidate.
Will investigate this, thanks Bjorn
FWIW, there's also a Vendor-Specific Capability that can appear in the
first 256 bytes of config space (the Vendor-Specific Extended
Capability must appear in the "Extended Configuration Space" from
Unfortunately our silicon does not have either Vendor-Specific Capability or
Vendor-Specific Extended Capability.

Studied commit 8531e283bee66050734fb0e89d53e85fd5ce24a4
Looks this method requires adding member (like can_stall) to struct pci_dev, looks difficult.

If detection cannot be done through PCI config space, the next best
alternative is to pass auxiliary data through firmware. On DT based
machines, you can list non-hotpluggable PCIe devices and add custom
properties that could be read during device enumeration. I assume
ACPI has something similar, but I have not done that.
Yes, thanks Arnd
ACPI has _DSM (ACPI v6.3, sec 9.1.1), which might be a candidate.  I
like this better than a PCI capability because the property you need
to expose is not a PCI property.
_DSM may not workable, since it is working in runtime.
We need stall information in init stage, neither too early (after allocation
of iommu_fwspec)
nor too late (before arm_smmu_add_device ).
I'm not aware of a restriction on when _DSM can be evaluated.  I'm
looking at ACPI v6.3, sec 9.1.1.  Are you seeing something different?
DSM method seems requires vendor specific guid, and code would be vendor specific.
Except adding uuid to some spec like pci_acpi_dsm_guid.
obj = acpi_evaluate_dsm(ACPI_HANDLE(bus->bridge), &pci_acpi_dsm_guid, 1,

By the way, It would be a long time if we need modify either pcie
spec or acpi spec.  Can we use pci_fixup_device in iommu_fwspec_init
first, it is relatively simple and meet the requirement of platform
device using pasid, and they are already in product.
Neither the PCI Vendor-Specific Capability nor the ACPI _DSM requires
a spec change.  Both can be completely vendor-defined.
Adding vendor-specific code to common files looks a bit ugly.


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