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0feda94c86
On many systems that have an AMD IOMMU the following sequence of
warnings is observed during bootup.
```
pci 0000:00:00.2 can't derive routing for PCI INT A
pci 0000:00:00.2: PCI INT A: not connected
```
This series of events happens because of the IOMMU initialization
sequence order and the lack of _PRT entries for the IOMMU.
During initialization the IOMMU driver first enables the PCI device
using pci_enable_device(). This will call acpi_pci_irq_enable()
which will check if the interrupt is declared in a PCI routing table
(_PRT) entry. According to the PCI spec [1] these routing entries
are only required under PCI root bridges:
The _PRT object is required under all PCI root bridges
The IOMMU is directly connected to the root complex, so there is no
parent bridge to look for a _PRT entry. The first warning is emitted
since no entry could be found in the hierarchy. The second warning is
then emitted because the interrupt hasn't yet been configured to any
value. The pin was configured in pci_read_irq() but the byte in
PCI_INTERRUPT_LINE return 0xff which means "Unknown".
After that sequence of events pci_enable_msi() is called and this
will allocate an interrupt.
That is both of these warnings are totally harmless because the IOMMU
uses MSI for interrupts. To avoid even trying to probe for a _PRT
entry mark the IOMMU as IRQ managed. This avoids both warnings.
Link: https://uefi.org/htmlspecs/ACPI_Spec_6_4_html/06_Device_Configuration/Device_Configuration.html?highlight=_prt#prt-pci-routing-table [1]
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Fixes:
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.