[Technical Discussion] USB Hub Depth Limitation in BIOS Stage – Input from Recent User Feedback
Hi everyone,
We’ve recently received several user reports, and after internal testing, we observed a behavior that may be related to USB hub depth limitations at the BIOS/UEFI level. We’d like to share our findings and get broader input from the community.
Observed Behavior
Scenario A (through KVM):
- Keyboard and mouse connected to either the dedicated keyboard/mouse port or USB 3.0 ports on the KVM
- No input response in BIOS/UEFI
- Input works normally after entering the OS
Scenario B (direct connection):
- Keyboard connected directly to the motherboard USB port
- Works normally in BIOS/UEFI
Additional Testing
To isolate variables, we also tested without the KVM:
- When connecting the keyboard through a standard external USB hub (no KVM)
- We observed the same issue:
→ No keyboard input available in BIOS/UEFI
Hardware involved:
- Customer report: ASUS Z390
- Internal testing: Z490 platform
Platform Variability (Important)
This behavior does not appear to be universal across all systems.
In our lab testing:
- Several AMD B-series platforms
- As well as Windows laptops (e.g., Dell Latitude 5340)
…were not able to reproduce this issue. In these cases, keyboard and mouse input worked normally in BIOS/UEFI, even through multi-layer USB paths.
This suggests the limitation may depend on:
- BIOS/UEFI USB stack implementation
- Chipset / platform differences
- Or vendor-specific handling of USB enumeration
Preliminary Analysis
Based on current observations, there are two likely contributing factors:
1) Stricter USB hub depth limitation at BIOS level
- Windows supports up to 7-tier USB hub depth
- BIOS/UEFI implementations appear to have a much lower limit (possibly ~3 layers or fewer)
- KVM switches typically introduce a 3–4 layer hub topology, which may exceed what BIOS can enumerate
2) Limited handling of composite USB devices in BIOS
- KVMs and some USB hubs present composite device structures (HID + hub + control interfaces)
- BIOS-level USB stacks may have reduced parsing capability compared to OS drivers
- This can result in incomplete enumeration → no usable HID device at pre-boot stage
Discussion Goals
We’d like to better understand:
- How common this behavior is across different motherboard brands / chipsets / BIOS versions
- Whether users have seen similar issues even without a KVM (e.g., via simple USB hubs)
- Any configurations where multi-layer hubs still work reliably in BIOS
Why This Matters
For KVM users, BIOS-stage input is critical (e.g., boot selection, firmware updates).
Understanding these limitations helps us:
- Define clearer compatibility expectations
- Explore possible design optimizations
- Provide better setup guidance for pre-boot scenarios
If you’ve encountered similar behavior, please share your setup (motherboard model, BIOS version, connection topology). Real-world data here is extremely valuable.
Thanks in advance for your insights.
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