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October 31st, 2025 14:45
[BIOS 1.8.0 Bug] Pro Max 16 MC16250 - TB4 Dock Not Detected at Cold Boot (Confirmed by Kernel Maintainer)
System Configuration
Model: Dell Pro Max 16 MC16250
CPU: Intel Core Ultra 7 255H (Arrow Lake)
BIOS: 1.8.0 (09/10/2025) - latest available
OS: Kubuntu 24.04.3 LTS (kernel 6.11.0-9-generic)
Dock: Dell Thunderbolt 4 Dock (SD25TB4)
All firmwares: Up to date via fwupdmgr
Problem Description
When laptop is powered off and dock connected:
1. Dock powers on laptop (Power Delivery works)
2. System boots normally BUT peripherals don't work:
- External monitor: no signal
- USB keyboard: not detected
- USB mouse: not detected
- Ethernet: not detected
3. Unplug/replug dock → everything works instantly
4. Hotplug works perfectly as long as laptop stays powered on
This is a cold boot enumeration bug.
Bug confirmed by Mika Westerberg (mika.westerberg@linux.intel.com), official maintainer of Linux kernel Thunderbolt/USB4 subsystem.
Technical evidence:
- BIOS Connection Manager fails to detect dock at cold boot
- PORT_CS_18 register shows `Router Detected = 0x0` (should be `0x1`)
- All Thunderbolt lanes in `CLd` state (Closed/Link Down)
- No Thunderbolt device enumerated at boot (`/sys/bus/thunderbolt/devices/` empty)
- After hotplug: instant and correct enumeration
Kernel mailing list thread:
https://lore.kernel.org/linux-usb/?q=%5BBUG%5D+Dell+Thunderbolt+4+dock
Troubleshooting Already Done
BIOS Settings: Tested "Enable Thunderbolt Boot Support" ON/OFF - no effect
Firmwares: All up to date (BIOS 1.8.0, Dock 1.1.7.1, etc.) via fwupdmgr
Ports: Tested both USB-C ports - same bug
Hotplug: Works perfectly (proves dock + cables are functional)
Kernel logs: Full debug enabled, confirms BIOS doesn't enumerate dock
Hardware registers: Dumped via Intel tbtools, confirms no detection
Conclusion: This is NOT a configuration issue, firmware issue, or cable issue. This is a BIOS firmware bug.
Dell Support Response
Contacted Dell Support via chat (31/10/2025):
- Level 1 agent: Provided Windows .exe firmware files (not applicable)
- Escalated to Level 3: Refused assistance
- Reason given: "Linux not supported, contact Kubuntu support"
Why this response is problematic:
- This is a BIOS-level bug (pre-OS, happens before Linux loads)
What I'm Requesting
Escalation to Dell BIOS Engineering Team to investigate and fix this cold boot enumeration bug in BIOS 1.8.0.
This is affecting professional usage with a Dell-branded dock on a Dell-certified Ubuntu laptop. The diagnosis comes from an authoritative source (Linux kernel maintainer) with full technical evidence.
Additional Technical Details
BIOS: 1.8.0 (09/10/2025)
Dell Dock Package: 1.1.7.1
Dell Dock USB4 Controller: 44.83
Dell Dock PowerDelivery (PD): 1.36.0.0
Dell Dock LAN: 2.27
USB4 Retimer: 26.85
Dell Dock MST: 9.03.008
Is anyone else experiencing this issue with Dell Pro Max 16 + Thunderbolt 4 Dock?
DELL-ChrisM2
Community Manager
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367 Posts
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October 31st, 2025 15:13
What operating system shipped from the Dell factory on this Pro Max 16 MC16250?
Did you have the same cold boot issue with the operating system that shipped from Dell?
The Dell L3 response is both correct and incorrect at the same time. They could have stated something like,
The Pro Max 16 MC16250 Drivers & Downloads page shows the following supported Operating system =
Client Ubuntu 24.04 - U0
Enterprise Linux 9.4 - Dell Client
Windows 10, 64-bit
Windows 11
Kubuntu Linux is not on the list and therefore not supported by Dell. Contact Kubuntu support.
The SD25TB4 Dock Drivers & Downloads page shows the following supported Operating system =
Red Hat Enterprise Linux 9.2
Red Hat Linux 8.0
Ubuntu 22.04 LTS
Windows 10, 64-bit
Windows 11
Windows 11 ARM64
Windows 11 ARM64 S-Mode
Again, no mention of Kubuntu Linux.
user_62b031
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October 31st, 2025 15:23
Thank you for your response.
To answer your questions:
1. OS shipped from Dell: Windows 11 Pro
2. Did I test with Windows 11 Pro? No, I installed Kubuntu immediately after receiving the laptop.
You raise a valid point. I should have tested with the original Windows 11 Pro to rule out OS-specific issues.
However, I want to mention why I suspected this was BIOS-level:
- Hardware registers show Router Detected = 0x0 before OS loads
- Hotplug works perfectly once the system is running (same drivers, same OS)
- The Linux kernel Thunderbolt maintainer reviewed my diagnostics and suggested BIOS enumeration issue
My question for Dell Engineering:
Given that:
- All Dell firmwares are current (verified via fwupdmgr)
- Hardware registers indicate dock not detected at cold boot
- Hotplug works flawlessly
Could this be investigated as a potential BIOS bug that might affect multiple OS configurations?
I'm happy to provide full diagnostic data (kernel logs, hardware register dumps, etc.) if helpful.
Thank you for your time.
DELL-ChrisM2
Community Manager
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367 Posts
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October 31st, 2025 15:44
Before any issue can be escalated from our Forum, there would need to be 9 other Pro Max 16 MC16250 users stating the same, "Pro Max 16 MC16250 + SD25TB4, cold boot issue".