[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.