USB Behavior

Dual-PID Enumeration

The Facecam uses different USB Product IDs depending on the port speed:

Speed	PID	Product String	Video
USB 3.0 SuperSpeed	`0x0078`	`Elgato Facecam`	Full UVC
USB 2.0 High-Speed	`0x0077`	`USB3-REQUIRED-FOR-FACECAM`	None

This is a deliberate firmware behavior — the camera refuses to operate on USB 2.0 rather than producing a degraded experience.

Open/Close Lockup

The most impactful Linux-specific bug. After the first application closes the V4L2 device file descriptor, subsequent opens fail with EBUSY or produce no frames.

Root cause: Unknown — likely a firmware-side USB endpoint state management issue. The UVC driver's uvc_video_stop_streaming may leave the device in a state it can't recover from.

Workaround: The normalization daemon keeps the device open continuously. Consumer apps use the v4l2loopback virtual camera.

Recovery: USB sysfs authorized flag cycle forces kernel re-enumeration:

# Automated by facecam-ctl reset and the daemon's recovery logic
echo 0 > /sys/bus/usb/devices/10-2/authorized
sleep 0.5
echo 1 > /sys/bus/usb/devices/10-2/authorized

Startup Unreliability

The camera fails to start streaming approximately 50% of the time on first open. The daemon's retry_with_reset logic handles this by:

Attempting the operation
On failure, performing USB reset
Waiting for re-enumeration
Retrying (up to max_recovery_attempts times)

USB Transfer Modes

Firmware 3.00+ supports two transfer modes:

Mode	Description	Default
Bulk	Lower overhead, less error recovery	Linux default
Isochronous	Guaranteed bandwidth, better for real-time	Mac default

The mode is stored in device flash and can only be changed via Camera Hub. Switching to isochronous has been reported to resolve freezing on some systems.

Sysfs Path

The Facecam's sysfs path follows the pattern:

/sys/bus/usb/devices/<bus>-<port>

Example: /sys/bus/usb/devices/10-2 for bus 10, port 2.

Key sysfs files: