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

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Date:   Fri, 5 Jun 2020 18:19:09 -0500
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Zhangfei Gao <zhangfei.gao@...aro.org>
Cc:     Joerg Roedel <joro@...tes.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Arnd Bergmann <arnd@...db.de>,
        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-crypto@...r.kernel.org,
        iommu@...ts.linux-foundation.org, linux-acpi@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-pci@...r.kernel.org
Subject: Re: [PATCH 0/2] Introduce PCI_FIXUP_IOMMU

On Thu, Jun 04, 2020 at 09:33:07PM +0800, Zhangfei Gao wrote:
> On 2020/6/2 上午1:41, Bjorn Helgaas wrote:
> > On Thu, May 28, 2020 at 09:33:44AM +0200, Joerg Roedel wrote:
> > > On Wed, May 27, 2020 at 01:18:42PM -0500, Bjorn Helgaas wrote:
> > > > Is this slowdown significant?  We already iterate over every device
> > > > when applying PCI_FIXUP_FINAL quirks, so if we used the existing
> > > > PCI_FIXUP_FINAL, we wouldn't be adding a new loop.  We would only be
> > > > adding two more iterations to the loop in pci_do_fixups() that tries
> > > > to match quirks against the current device.  I doubt that would be a
> > > > measurable slowdown.
> > > I don't know how significant it is, but I remember people complaining
> > > about adding new PCI quirks because it takes too long for them to run
> > > them all. That was in the discussion about the quirk disabling ATS on
> > > AMD Stoney systems.
> > > 
> > > So it probably depends on how many PCI devices are in the system whether
> > > it causes any measureable slowdown.
> > I found this [1] from Paul Menzel, which was a slowdown caused by
> > quirk_usb_early_handoff().  I think the real problem is individual
> > quirks that take a long time.
> > 
> > The PCI_FIXUP_IOMMU things we're talking about should be fast, and of
> > course, they're only run for matching devices anyway.  So I'd rather
> > keep them as PCI_FIXUP_FINAL than add a whole new phase.
> > 
> Thanks Bjorn for taking time for this.
> If so, it would be much simpler.
> 
> +++ 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?

Bjorn