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[Tony <tony@xxxxxxxxxxxxxx>]

I'm hitting this exact same issue with a different device, it's a
development board that communicates over USB CDC (an Adafruit Bluefruit
Micro, a little bluetooth LE microcontroller board).  The device has
similar behavior as the mentioned MediaTek preloader where it implements
a very basic USB CDC endpoint and can abruptly disconnect at times if
for example it times out waiting for a response from the host computer.

Not sure what else I can add unfortunately to the discussion but your
debugging info is super helpful to confirm this appears to be the same
issue.  I'm not familiar with ModemManager either but is there any way
to rollback to an older version and pinpoint what change caused the
issue?

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https://bugs.launchpad.net/bugs/1473246

Title:
  MediaTek Preloader on a mobile device triggers cdc_acm kernel module
  usage by ModemManager

Status in modemmanager package in Ubuntu:
  Confirmed

Bug description:
  This is an interesting bug that is triggered under some circumstances
  if one of the official, MediaTek-SoC-based Ubuntu phones is connected
  to a Linux host via USB.

  Tested host:
  Ubuntu 15.04 Desktop, kernel 3.19.0-21-generic, package linux-image-generic 3.19.0.22.21 amd64, package modemmanager 1.4.0-1 

  Tested phones:
  Meizu MX4 (arale) and bq Aquaris E4.5 (krillin), image version irrelevant because the Preloader comes with all devices and cannot be changed

  How to reproduce, possibility 1:
  1. Disconnect the USB connection, if necessary.
  2. Turn off the phone.
  3. Connect the phone via USB. It will take a bit and then turn on, displaying the "charge animation".

  How to reproduce, possibility 2:
  1. Reboot the phone while it is still attached to the host.

  The kernel ringbuffer will contain the following log messages (example
  for krillin):

  [288774.244158] usb 1-1: new high-speed USB device number 22 using xhci_hcd
  [288774.373589] usb 1-1: New USB device found, idVendor=0e8d, idProduct=2000
  [288774.373593] usb 1-1: New USB device strings: Mfr=1, Product=2, SerialNumber=0
  [288774.373594] usb 1-1: Product: MT65xx Preloader
  [288774.373596] usb 1-1: Manufacturer: MediaTek
  [288774.435247] cdc_acm 1-1:1.1: ttyACM26: USB ACM device
  [288774.874607] usb 1-1: USB disconnect, device number 22
  [288774.874755] cdc_acm 1-1:1.1: failed to set dtr/rts

  And syslog will contain the following log messages:

  Jul 10 00:58:07 fire ModemManager[1103]: <info>  (tty/ttyACM27): released by modem /sys/devices/pci0000:00/0000:00:14.0/usb1/1-1
  Jul 10 00:58:10 fire ModemManager[1103]: <warn>  (Plugin Manager) (Iridium) [ttyACM27] error when checking support: '(Iridium) Missing port probe for port (tty/ttyACM27)'
  Jul 10 00:58:10 fire ModemManager[1103]: <warn>  (Plugin Manager) (Cinterion) [ttyACM27] error when checking support: '(Cinterion) Missing port probe for port (tty/ttyACM27)'
  Jul 10 00:58:10 fire ModemManager[1103]: <warn>  (Plugin Manager) (Nokia) [ttyACM27] error when checking support: '(Nokia) Missing port probe for port (tty/ttyACM27)'
  Jul 10 00:58:10 fire ModemManager[1103]: <warn>  (Plugin Manager) (Generic) [ttyACM27] error when checking support: '(Generic) Missing port probe for port (tty/ttyACM27)'
  Jul 10 00:58:10 fire ModemManager[1103]: <warn>  Couldn't find support for device at '/sys/devices/pci0000:00/0000:00:14.0/usb1/1-1': not supported by any plugin

  The allocated ttyACMx device is never freed, and after a couple of
  days developing on the phone (with lots of phone restarts) I've
  noticed that my machine has accumulated an usage count of 58:

  $ lsmod | grep cdc_acm
  cdc_acm                36864  58

  lsof show that ModemManager still holds the devices:

  # sudo lsof | grep ACM
  ModemMana  9214                   root    9u      CHR              166,1       0t0    7599355 /dev/ttyACM1 (deleted)
  ModemMana  9214                   root   10u      CHR              166,0       0t0    7596974 /dev/ttyACM0 (deleted)
  ...

  Restarting ModemManager immediately reduces the usage count to 0 again
  and frees all devices.

  Why does this happen? Luckily I've read the MediaTek Preloader source
  code. The Preloader is the first-stage bootloader on MediaTek SoCs and
  exposes a proprietary USB interface that can be used for debugging
  purposes and to download firmware to the device. The implementation of
  this interface is rudimentary and emulates a CDC ACM serial port,
  which probably doesn't fully conform to the standard and also just
  falls from the bus after a short amount of time if the host doesn't
  initiate the proprietary protocol. So I think the kernel and
  ModemManager immediately try to use the emulated device when it
  becomes available, but because the implementation in the Preloader is
  incomplete some bug (timeout?) is triggered and ModemManager never
  closes the device.

  Sadly I don't have the expertise to decide if this is a bug in
  ModemManager or the kernel or both, so I file it with ModemManager.

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