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

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Date:   Thu, 4 Jun 2020 21:33:07 +0800
From:   Zhangfei Gao <zhangfei.gao@...aro.org>
To:     Bjorn Helgaas <helgaas@...nel.org>, Joerg Roedel <joro@...tes.org>
Cc:     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 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.

Thanks