Bug 214953

Summary: How to reduce PCI initialization from 5 s (1.5 s adding them to IOMMU groups)?
Product: Drivers Reporter: Paul Menzel (pmenzel+bugzilla.kernel.org)
Component: PCIAssignee: drivers_pci (drivers_pci)
Status: NEW ---    
Severity: normal    
Priority: P1    
Hardware: All   
OS: Linux   
Kernel Version: 5.10.70 Subsystem:
Regression: No Bisected commit-id:
Attachments: Linux 5.10.70 messages (`dmesg`)

Description Paul Menzel 2021-11-05 12:01:34 UTC 
Created attachment 299457 [details]
Linux 5.10.70 messages (`dmesg`)

[Sent also to linux-pci and iommu lists. [1]]

On a PowerEdge T440/021KCD, BIOS 2.11.2 04/22/2021, Linux 5.10.70 takes almost five seconds to initialize PCI. According to the timestamps, 1.5 s are from assigning the PCI devices to the 142 IOMMU groups.

```
$ lspci | wc -l
281
$ dmesg
[…]
[    2.918411] PCI: Using host bridge windows from ACPI; if necessary, use "pci=nocrs" and report a bug
[    2.933841] ACPI: Enabled 5 GPEs in block 00 to 7F
[    2.973739] ACPI: PCI Root Bridge [PC00] (domain 0000 [bus 00-16])
[    2.980398] acpi PNP0A08:00: _OSC: OS supports [ExtendedConfig ASPM ClockPM Segments MSI HPX-Type3]
[    2.989457] acpi PNP0A08:00: _OSC: platform does not support [LTR]
[    2.995451] acpi PNP0A08:00: _OSC: OS now controls [PME PCIeCapability]
[    3.001394] acpi PNP0A08:00: FADT indicates ASPM is unsupported, using BIOS configuration
[    3.010511] PCI host bridge to bus 0000:00
[…]
[    6.233508] system 00:05: [io  0x1000-0x10fe] has been reserved
[    6.239420] system 00:05: Plug and Play ACPI device, IDs PNP0c02 (active)
[    6.239906] pnp: PnP ACPI: found 6 devices
[…]
[    6.989016] pci 0000:d7:05.0: disabled boot interrupts on device [8086:2034]
[    6.996063] PCI: CLS 0 bytes, default 64
[    7.000008] Trying to unpack rootfs image as initramfs...
[    7.065281] Freeing initrd memory: 5136K
[…]
[    7.079098] DMAR: dmar7: Using Queued invalidation
[    7.083983] pci 0000:00:00.0: Adding to iommu group 0
[…]
[    8.537808] pci 0000:d7:17.1: Adding to iommu group 141
[    8.571191] DMAR: Intel(R) Virtualization Technology for Directed I/O
[    8.577618] PCI-DMA: Using software bounce buffering for IO (SWIOTLB)
[…]
```

Is there anything that could be done to reduce the time?

[1]: https://lore.kernel.org/linux-pci/de6706b2-4ea5-ce68-6b72-02090b98630f@molgen.mpg.de/T/#u