Revert Datamax to kernel usblp driver -- direct libusb bulk transfer times out on this Pi 3B's USB controller

Gemfile

@@ -6,6 +6,5 @@ gem 'puma'
 gem 'rqrcode'
 gem 'logger'
 gem 'ostruct'
-gem 'libusb'
 
 gem 'rackup', "~> 2.1"

app.rb

@@ -3,7 +3,6 @@ require 'sinatra'
 require "sinatra/reloader"
 require 'tmpdir'
 require 'net/http'
-require 'libusb'
 
 set :protection, :except => :frame_options
 set :environment, :production
@@ -14,14 +13,10 @@ if !File.exist?('config.json') || !File.readable?('config.json')
   raise RuntimeException, 'Could not read config.json'
 end
 
-# Addressed the same way as the Brother QL printers (usb://VID:PID/Serial via
-# brother_ql_web) instead of a udev symlink -- self-contained, no reliance on
-# OS device naming/ordering.
-DATAMAX_VENDOR_ID = 0x0b0b
-DATAMAX_PRODUCT_ID = 0x106e
-DATAMAX_SERIAL = '24624'
-DATAMAX_ENDPOINT = 0x02 # bulk OUT, interface 0
-
+# Direct libusb bulk transfer (bypassing the kernel usblp driver, matching how
+# brother_ql_web addresses its printers) times out on this Pi 3B's USB
+# controller -- back to the kernel driver via a stable udev symlink instead.
+PRINTER_DEVICE = '/dev/usb/lp_datamax' # see /etc/udev/rules.d/99-datamax.rules
 PRINTER_DPI = 203 # DatamaxONeil E4204B Mark III native head resolution
 PRINTER_DARKNESS = 8 # lower than factory default to stop thermal dot-gain from fusing barcode bars
 TEST_LABEL_PDF = File.join(File.dirname(__FILE__), 'test-label.pdf')
@@ -90,20 +85,7 @@ helpers do
   end
 
   def print_to_datamax(data)
-    usb = LIBUSB::Context.new
-    device = usb.devices(idVendor: DATAMAX_VENDOR_ID, idProduct: DATAMAX_PRODUCT_ID)
-                .find { |d| d.serial_number == DATAMAX_SERIAL }
-    unless device
-      raise format('Datamax printer not found (USB %04x:%04x/%s)',
-                    DATAMAX_VENDOR_ID, DATAMAX_PRODUCT_ID, DATAMAX_SERIAL)
-    end
-
-    device.open do |handle|
-      handle.auto_detach_kernel_driver = true # usblp kernel driver otherwise holds the interface
-      handle.claim_interface(0) do
-        handle.bulk_transfer(endpoint: DATAMAX_ENDPOINT, dataOut: data, timeout: 30_000) # USB 1.1, large ZPL payloads need more than the 1s default
-      end
-    end
+    File.open(PRINTER_DEVICE, 'wb') { |f| f.write(data) }
   end
 
   # Transparently reverse-proxies the current request to one of the