[BUG] Apple Mac mini 2018 + Thunderbolt 3 eGPU: PCI bridge window / BAR allocation failure prevents NVIDIA and AMD GPUs from initializing

["Michal Babička" <michal.babka@gmail.com>]

Subject: Apple Mac mini 2018 + Thunderbolt 3 eGPU: PCI bridge window / BAR allocation failure prevents NVIDIA and AMD GPUs from initializing

Hardware / platform:
- Host: Apple Mac mini 2018
- Connection: Thunderbolt 3
- eGPU enclosure: Razer Core X Chroma
- Tested GPUs:
  - NVIDIA Quadro P400
  - NVIDIA Quadro P4000
  - AMD Radeon Vega 64

Linux distributions / kernels tested:
- Ubuntu-based systems
- Zorin OS 18 Pro
- Multiple kernels tested, including 6.12.x and 6.17.x

Problem summary:
On Apple Mac mini 2018, an external GPU connected through Thunderbolt 3 in a Razer Core X Chroma enclosure is detected and enumerates on the PCI bus, but GPU initialization fails because PCI bridge windows / BAR resources are not assigned correctly.

This is reproducible across different Linux installations and with GPUs from different vendors, which strongly suggests a platform/topology-level PCIe resource allocation problem rather than a bug in one specific GPU driver.

Observed behavior:
- The Thunderbolt enclosure is detected correctly.
- boltctl shows the enclosure as connected / authorized.
- The GPU appears in lspci.
- The kernel loads the vendor driver module.
- GPU initialization then fails.

For NVIDIA, nvidia-smi reports:
  "NVIDIA-SMI has failed because it couldn't communicate with the NVIDIA driver."

Relevant NVIDIA kernel messages include:
- "BAR 1 [mem size 0x10000000 64bit pref]: can't assign; no space"
- "BAR 3 [mem size 0x02000000 64bit pref]: can't assign; no space"
- "This PCI I/O region assigned to your NVIDIA device is invalid"
- "BAR1 is 0M @ 0x0"
- "RmInitAdapter failed"
- in some attempts also:
  "fallen off the bus and is not responding to commands"

With NVIDIA Quadro P4000, the same general failure pattern was observed:
- the GPU is detected on the PCI bus,
- the NVIDIA kernel module loads,
- initialization fails,
- and the logs again point to invalid / missing BAR assignments and resource allocation problems behind the Thunderbolt PCIe bridge hierarchy.

For nouveau, the failure is visible as BAR/resource allocation failure as well, for example:
- "bar: one-time init failed, -12"
- "init failed with -12"
- "Device allocation failed: -12"

With AMD Radeon Vega 64 installed in the same enclosure on the same host, the same fundamental problem occurs:
- the enclosure is detected,
- the GPU enumerates,
- initialization fails,
- and the overall behavior indicates the same lower-level PCI resource / bridge window allocation issue.

This strongly suggests the issue is not NVIDIA-specific.

Thunderbolt state:
boltctl reports the Razer Core X Chroma enclosure as connected/authorized correctly, for example:
- type: peripheral
- generation: Thunderbolt 3
- status: authorized
- rx speed: 40 Gb/s
- tx speed: 40 Gb/s

Important conclusion:
Thunderbolt authorization itself appears to work.
PCI enumeration also works.
The failure happens later, at PCI resource assignment / bridge window sizing / BAR allocation time.

Key dmesg patterns observed:
- multiple "can't assign; no space" messages for PCI bridges and BARs
- BAR1 / BAR3 of the GPU not assigned
- bridge windows under the Thunderbolt hierarchy not large enough
- reassignment attempts that still leave required windows unassigned or invalid
- vendor driver subsequently failing device initialization

Examples of affected topology in logs:
- 0000:00:01.1
- 0000:05:00.0
- 0000:06:01.0
- 0000:0b:00.0
- in other boots, the same problem may appear under a different BDF address after Thunderbolt re-enumeration

Representative log excerpts:
- pci 0000:0b:00.0: BAR 1 [mem size 0x10000000 64bit pref]: can't assign; no space
- pci 0000:0b:00.0: BAR 3 [mem size 0x02000000 64bit pref]: can't assign; no space
- pci 0000:06:01.0: bridge window [mem size 0x10000000]: can't assign; no space
- pci 0000:05:00.0: bridge window [mem size 0x20200000]: can't assign; no space
- NVRM: BAR1 is 0M @ 0x0
- NVRM: RmInitAdapter failed
- NVRM: The NVIDIA GPU ... has fallen off the bus and is not responding to commands
- nouveau ... Device allocation failed: -12

Kernel command line options already tested:
The following boot parameters were tested in various combinations:
- pci=realloc=on
- pci=hpbussize=0x33,hpmemsize=256M,hpiosize=2M
- pcie_aspm=off
- pci=nommconf
- pci=noaer

These mitigations did not resolve the issue.

Why this looks platform/topology specific:
- The same Thunderbolt eGPU enclosure is recognized correctly.
- The same failure pattern appears with NVIDIA Quadro P400, NVIDIA Quadro P4000, and AMD Radeon Vega 64.
- The common denominator is Apple Mac mini 2018 + Thunderbolt PCIe topology under Linux.
- The failure mode is centered around PCI bridge window sizing / BAR placement, not around one vendor-specific driver alone.

Expected behavior:
The Thunderbolt eGPU should receive valid PCI bridge windows and valid BAR assignments so that the GPU driver can initialize the device successfully.

Actual behavior:
The GPU is visible on the bus, but bridge windows and BAR resources are not fully assigned, which leaves the device unusable and prevents the driver from completing initialization.

Request:
Please investigate PCI resource allocation / bridge window sizing for Thunderbolt eGPU topologies on Apple Mac mini 2018 under Linux, especially where large GPU BARs must be assigned behind multiple Thunderbolt / PCIe bridges.

Cross-vendor reproducibility on the same enclosure and same host strongly suggests that the root cause is in PCIe/Thunderbolt resource allocation on this platform rather than in a specific NVIDIA or AMD driver.