How to reduce PCI initialization from 5 s (1.5 s adding them to IOMMU groups)

[Paul Menzel]

Dear Linux folks,


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?


Kind regards,

Paul