Re: [PATCH 0/2] Introduce PCI_FIXUP_IOMMU

Bjorn Helgaas
2020-06-11T13:44:18Z

On Thu, Jun 11, 2020 at 10:54:45AM +0800, Zhangfei Gao wrote:
> On 2020/6/10 上午12:49, Bjorn Helgaas wrote:
> > On Tue, Jun 09, 2020 at 11:15:06AM +0200, Arnd Bergmann wrote:
> > > On Tue, Jun 9, 2020 at 6:02 AM Zhangfei Gao <zhangfei.gao@linaro.org> wrote:
> > > > On 2020/6/9 上午12:41, Bjorn Helgaas wrote:
> > > > > On Mon, Jun 08, 2020 at 10:54:15AM +0800, Zhangfei Gao wrote:
> > > > > > On 2020/6/6 上午7:19, 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.
> > > > > > However,
> > > > > > 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
0x100-0xfff).

> > > 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?

> 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.