usb.py: defensive USB reset at open_dev() entry

Multiple users reported the 0xd51-family chips (138a:00ab, 06cb:00b7)
getting stuck after cold boot, unclean exit, or suspend/resume — the
chip accepts the bulk-OUT but never responds on bulk-IN, so the first
cleartext cmd (cmd 3e get_flash_info) times out and the daemon
restart-loops every 15 seconds. The workaround users were running
manually is a USB-level reset:

    sudo systemctl stop python3-validity open-fprintd
    sudo udevadm trigger --attr-match=idVendor=138a --attr-match=idProduct=00ab
    sudo systemctl start python3-validity open-fprintd

The reset call here is the in-driver equivalent. The chip's USB
address can shift after reset, so we re-find by vid/pid.

Reported by Killersparrow1 (issue uunicorn#238, Fedora 44, sensor vanishes
on reboot) and a separate Arch / ZBook G5 user (USBTimeoutError on
cmd 3e). The patch matches what the project memory has flagged for
the past two sessions as "kept local and not in PR" — turns out it
was the actually-load-bearing piece.

Locally confirmed: clean daemon restart, sudo matches in 1 retry,
no traceback, no "USB reset failed" warning.

Author: SimpleX-T

debian/changelog

@@ -1,3 +1,24 @@
+python-validity (0.16~hp7) noble; urgency=medium
+
+  * usb.py: defensive USB reset at the start of open_dev(). The 0xd51-
+    family chips (138a:00ab, 06cb:00b7) can be left in a "stuck"
+    protocol state across cold boot, unclean exit of the daemon, or
+    sudden suspend/resume — in that state the chip accepts the
+    bulk-OUT but never replies on bulk-IN, so the first cleartext
+    command (get_flash_info, cmd 3e) times out and the daemon enters
+    a 15-second restart loop. The reset is the in-driver equivalent
+    of the manual `udevadm trigger --attr-match=idVendor=...
+    --attr-match=idProduct=...` workaround multiple users were
+    running to recover after every reboot. Reported by Killersparrow1
+    (uunicorn/python-validity#238, Fedora 44, sensor vanishes on
+    reboot) and a separate Arch / ZBook G5 user (USBTimeoutError on
+    cmd 3e). Also observed intermittently on the maintainer's
+    machine.
+  * After reset, re-find the device by vid/pid because the USB
+    address can shift across the reset.
+
+ -- SimpleX-T <ntmark2004@gmail.com>  Sun, 25 May 2026 00:30:00 +0100
+
 python-validity (0.16~hp6) noble; urgency=medium
 
   * Diagnostic logging (no behavior change):

validitysensor/usb.py

@@ -1,5 +1,6 @@
 import errno
 import logging
+import time
 import typing
 from binascii import hexlify, unhexlify
 from enum import Enum
@@ -63,6 +64,34 @@ def open_dev(self, dev: ucore.Device):
         if dev is None:
             raise Exception('No matching devices found')
 
+        # Defensive USB reset on init.
+        #
+        # The 0xd51-family chips (HP 138a:00ab / 06cb:00b7) can be left in
+        # a "stuck" protocol state across a previous unclean exit of this
+        # daemon, a cold boot, or a sudden suspend/resume. In that state
+        # the chip accepts the bulk-OUT but never replies on bulk-IN, so
+        # the very first cleartext command (typically `cmd 3e`
+        # get_flash_info) times out — the daemon then restart-loops at
+        # 15s intervals and the sensor is "vanished" until a manual USB
+        # reset. This block is the in-driver equivalent of the manual
+        # `udevadm trigger --attr-match=idVendor=... --attr-match=idProduct=...`
+        # workaround users have been running to recover.
+        #
+        # Reported by Killersparrow1 (#238, Fedora 44, vanishes on reboot)
+        # and Maarten (Arch, ZBook G5, USBTimeoutError on first 3e). Also
+        # observed locally on the maintainer's machine (sensor prompts but
+        # doesn't detect after a while).
+        try:
+            vid, pid = dev.idVendor, dev.idProduct
+            dev.reset()
+            time.sleep(0.5)
+            # USB address may shift after reset; re-find by vid/pid.
+            dev = ucore.find(idVendor=vid, idProduct=pid)
+            if dev is None:
+                raise Exception('Device disappeared after USB reset')
+        except USBError as e:
+            logging.warning('open_dev: USB reset failed (often non-fatal): %s', e)
+
         self.dev = dev
         self.dev.default_timeout = 15000
         dev.set_configuration()