linux-kernel - Taming "BIOS assigned incorrect VT-d unit" warning?

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Date:	Tue, 14 Jun 2016 15:56:07 +0200
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Taming "BIOS assigned incorrect VT-d unit" warning?

I'm are getting some user reports with this warning:

static void quirk_ioat_snb_local_iommu(struct pci_dev *pdev)
{
...
        rc = pci_bus_read_config_dword(pdev->bus, PCI_DEVFN(0, 0),
0xb0, &vtbar);
        vtbar &= 0xffff0000;
        /* we know that the this iommu should be at offset 0xa000 from vtbar */
        drhd = dmar_find_matched_drhd_unit(pdev);
        if (WARN_TAINT_ONCE(!drhd || drhd->reg_base_addr - vtbar != 0xa000,
                            TAINT_FIRMWARE_WORKAROUND,
                            "BIOS assigned incorrect VT-d unit for
Intel(R) QuickData Technology device\n"))
                pdev->dev.archdata.iommu = DUMMY_DEVICE_DOMAIN_INFO;
}

I experimented a bit and found out that on their machine,
dmar_find_matched_drhd_unit(pdev) returns NULL.

That is, in their case it's not known that
drhd->reg_base_addr - vtbar is wrong.
The warning is actually not true.

I tried a patch

        drhd = dmar_find_matched_drhd_unit(pdev);
+       if (!drhd)
+               return;

and the affected machine works after it.

I'm not too familiar with this code.
Is dmar_find_matched_drhd_unit(pdev) == NULL normal?