Troubleshooting the "Failed to Detach usblp" Error in Linux Printers

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Encountering the dreaded:

DEBUG: Failed to detach "usblp" module from <vendor>:<product>

When running CUPS’s USB backend, it can block all USB printing on modern Linux distributions. In this post, we’ll explore why the usblp kernel module causes conflicts, and then walk through a bullet-proof, step-by-step solution—including blacklisting, initramfs updates, and daemon removals—that permanently resolves the error and restores seamless USB printing via CUPS.

🔍 Understanding the Error

When you invoke the USB backend (/usr/lib/cups/backend/usb), CUPS attempts to detach the legacy usblp driver to claim the device via libusb instead. If usblp it remains bound—either because it reloaded from initramfs or was re-bound by another service—CUPS will log that detach failure and skip the device.

Blacklisting alone (blacklist usblp) is often insufficient, because modprobe may ignore blacklists without the “-b” flag, and an old copy in initramfs can reload a module on boot. Additionally, userspace daemons like ipp-usb their snap equivalent can re-bind USB printers ahead of CUPS and masquerade as, perpetuating the conflict.

✅ Step-by-Step Resolution

1. Repair Your Package Manager

First, ensure APT/Dpkg is in a clean state so you can remove conflicting packages:

sudo dpkg --configure -a
sudo apt update

This completes any interrupted installations and refreshes package metadata.

2. Block the usblp Kernel Module

Simply blacklisting may still allow indirect loads. Use both blacklist and install … /bin/false directives:

sudo tee /etc/modprobe.d/blacklist-usblp.conf <<EOF
blacklist usblp
install usblp /bin/false
EOF

This forces modprobe to refuse loading usblp under any circumstances.

3. Rebuild Initramfs and Reboot

A stale initramfs often contains an older modprobe configuration. Update it so your new blacklist is honored:

sudo update-initramfs -u
sudo reboot

After reboot, confirm the module is not loaded:

lsmod | grep usblp

No output indicates success.

4. Remove or Disable Conflicting USB/IPP Services

Daemonized IPP-over-USB services can rebind printers before CUPS’s backend runs. Purge or disable them:

sudo apt purge ipp-usb
sudo snap remove cups      # if you had a snap-based CUPS
sudo systemctl mask ipp-usb.service

This prevents any userspace reattachment of the device.

5. Validate the Fix in CUPS

Unload any stray module, then run the USB backend in debug mode:

sudo modprobe -r usblp
sudo /usr/lib/cups/backend/usb

Your printer(s) should list without any “Failed to detach” messages. Finally, check CUPS logs for errors:

sudo journalctl -u cups -e

🎯 Conclusion

By thoroughly blacklisting and blocking the usblp module, rebuilding initramfs, and removing IPP-over-USB daemons, you ensure that CUPS’s USB backend can reliably claim your printer via libusb—eliminating the “Failed to detach usblp” error once and for all. Enjoy hassle-free USB printing under CUPS!

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