On Fri, Jun 19, 2020 at 10:26:54AM +0800, Zhangfei Gao wrote:
> Have studied _DSM method, two issues we met comparing using quirk.
> 1. Need change definition of either pci_host_bridge or pci_dev, like adding
> member can_stall,
> while pci system does not know stall now.
> a, pci devices do not have uuid: uuid need be described in dsdt, while pci
> devices are not defined in dsdt.
>     so we have to use host bridge.

PCI devices *can* be described in the DSDT.  IIUC these particular
devices are hardwired (not plug-in cards), so platform firmware can
know about them and could describe them in the DSDT.

> b,  Parsing dsdt is in in pci subsystem.
> Like drivers/acpi/pci_root.c:
>        obj = acpi_evaluate_dsm(ACPI_HANDLE(bus->bridge), &pci_acpi_dsm_guid,
> 1,
>                                 IGNORE_PCI_BOOT_CONFIG_DSM, NULL);
> After parsing DSM in pci, we need record this info.
> Currently, can_stall info is recorded in iommu_fwspec,
> which is allocated in iommu_fwspec_init and called by iort_iommu_configure
> for uefi.

You can look for a _DSM wherever it is convenient for you.  It could
be in an AMBA shim layer.

> 2. Guest kernel also need support sva.
> Using quirk, the guest can boot with sva enabled, since quirk is
> self-contained by kernel.
> If using  _DSM, a specific uefi or dtb has to be provided,
> currently we can useQEMU_EFI.fd from apt install qemu-efi

I don't quite understand what this means, but as I mentioned before, a
quirk for a *limited* number of devices is OK, as long as there is a
plan that removes the need for a quirk for future devices.

E.g., if the next platform version ships with a DTB or firmware with a
_DSM or other mechanism that enables the kernel to discover this
information without a kernel change, it's fine to use a quirk to cover
the early platform.

The principles are:

  - I don't want to have to update a quirk for every new Device ID
    that needs this.

  - I don't really want to have to manage non-PCI information in the
    struct pci_dev.  If this is AMBA- or IOMMU-related, it should be
    stored in a structure related to AMBA or the IOMMU.
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