linux-kernel - Re: [PATCH 0/2] Introduce PCI_FIXUP

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Date:   Tue, 23 Jun 2020 10:04:27 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Zhangfei Gao <zhangfei.gao@...aro.org>
Cc:     Arnd Bergmann <arnd@...db.de>, Joerg Roedel <joro@...tes.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Hanjun Guo <guohanjun@...wei.com>,
        Sudeep Holla <sudeep.holla@....com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>,
        jean-philippe <jean-philippe@...aro.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        kenneth-lee-2012@...mail.com, Wangzhou <wangzhou1@...ilicon.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "open list:HARDWARE RANDOM NUMBER GENERATOR CORE" 
        <linux-crypto@...r.kernel.org>,
        "open list:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-pci <linux-pci@...r.kernel.org>,
        Thanu Rangarajan <Thanu.Rangarajan@....com>,
        Souvik Chakravarty <Souvik.Chakravarty@....com>,
        wanghuiqiang <wanghuiqiang@...wei.com>
Subject: Re: [PATCH 0/2] Introduce PCI_FIXUP_IOMMU

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.